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Agricultural Experiment Station Technical Bulletins SDSU Agricultural Experiment Station

1971 The iS berian Elm: Slippery Elm Hybrid Paul E. Collins

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Recommended Citation Collins, Paul E., "The iS berian Elm: Slippery Elm Hybrid" (1971). Agricultural Experiment Station Technical Bulletins. 47. http://openprairie.sdstate.edu/agexperimentsta_tb/47

This Article is brought to you for free and open access by the SDSU Agricultural Experiment Station at Open PRAIRIE: Open Public Research Access Institutional Repository and Information Exchange. It has been accepted for inclusion in Agricultural Experiment Station Technical Bulletins by an authorized administrator of Open PRAIRIE: Open Public Research Access Institutional Repository and Information Exchange. For more information, please contact [email protected]. ~~ Technical Bulletin 39 November 1971 The Siberian Elm Slippery Elm Hybrid

HUf~/~ LIBRAR

Agricultural Experiment Station South Dakota State University, Brookings Abstract Controlled crosses between sev­ crosses has been measured annual­ eral species of elm were initiated in ly. In 1970, the F 1 hybrids averaged 1953. Progeny of the early crosses 3 to 4 feet taller than progeny of between Siberian elm ( Ulmus the Siberian elm parent. Growth pumila L.) and slippery elm ( U. cessation of the various hybrids at rubra Muhl. ) and of back crosses th end of the growing season did were compared with parent trees not differ from progeny of either in several leaf, fruit, Hower and parent species. Fruit collected from veg tative bud characteristics. slippery elm trees growing near Progeny of a cross between two F 1 Siberian elm trees tend d to have hybrid elm trees ( Slippery x Siber­ a high percentage of hybrid ian elm) were also compared by the embryos. Fruit from slippery elm ame morphological characteristics trees containing hybrid embryos with the parent species. The hybrids germinated promptly while those t nded to have intermediate char­ having slippery elm embryos requir­ acteristics between the parents in ed cool stratification before ger­ most cases. Height growth of 1964 mination took place.

2 The Siberian Elm Slippery Elm Hybrid

By Paul E. Collins, associate professor Horticulture-Forestry Department

The information reported is based on a dissertation submitted to the Grad­ uate School and School of Forestry, University of Minnesota in partial ful­ fillment of the requirement for the Ph.D. degree.

Agricultural Experiment Station South Dakota State University, Brookings 3 Contents Introduction ------5 Parent Species ------6 Hybridization Stu dies ______------7 Previous Work ------7 Controlled Crosses in Elm ------8 Selfing Trials ------9 Siberian-Slippery Elm Hybrids ------10 Growth ------~- 11 Growth Cessation ------15 Seed G rmination ------16 Production of Hybrid Seed ------16 Morphological Characteristics ------18 Leaves ------18 Fruits ------22 Flowers and Buds ------24 Summary ------27 Literature Cited ------29

4 The Siberian Elm Slippery Elm Hybrid

By Paul E. Collins, associate professor Horticulture-Forestry Department

The Northern Great Plains Re­ species is relatively limited, plant gion of the Unit d States is a natur­ introductions from other areas of al grassland area. Native tree the United States and from other growth is limited to stream courses, continents have become an impor­ lake shores, hillside breaks and tant part of plains windbreak plant­ mountainous areas where soil-mois­ ings. Some of the befter introduc­ ture relationships are favorable for tions have come from Russia, Siber­ trees. For most of the area paucity ia and Northern China where the of precipitation, heavy soils and trees have developed under nvir­ low air humidity favors grasses over onmental conditions similar to the tre s. Average annual rainfall over Great Plains. Among those now much of the area is 20 inches or less. commonly used are Russian-olive Summer t mp ratures over 100° F. ( Elaeagnus angustifolia L.), Sibe­ are not uncommon, and winter tem­ rian peashrub ( Caragana arbares­ peratures often drop to -20° F. or cens Lam. ) , Tatarian honeysuckle lower in January and February. ( Lonicera tatarica L. ) , common The first settlers who established lilac ( Syring a vulgaris I,.), and homes in the region sought to im­ Siberian elm ( Ulmus pumila L.). prove the harsh environment by Although the introductions in­ planting trees for shade, wind pro­ crease the number of species avail­ tection and aesthetic purposes. At able for tree planting, tree improve­ first, wildings were dug from native ment continues. Many tree species stands and transplanted to home are beset by prob! ms that limit sites. Later trees were purchased their usefulness. The sub-humid to from commercial sources. By trial semi-arid climate of the plains des­ and error and by organized tree tines trees to grow under condi­ planting trials at experiment sta­ tions of moisture stress during most tions, knowledge has been accumu­ of the growing season except where lated on adapted species and on cul­ ground water is near the surface. tural practices for successful estab­ Consequently, many insect and dis­ lishment. Since the choice of native sease problems which normally 5 would be of secondary importance the hybrids. Since an improved Si­ act as primary destructive agents to berian elm seed source had been further weaken or kill the trees. identified, hybrids having the im­ ~Iodern herbicides pose a serious proved source as one of its parents threat to the native boxelder ( Acer should demonstrate reliable winter negunclo L.) and other species. hardine s and drought resistance. Dutch elm disease ( Ceratocystis ulmi l Buism. I C. Moreau threat­ Parent Species ens to restrict or eliminate the use Slippery elm is a native species to of American elm ( Ulmus ameri­ all states east of the Great Plains in­ cana L. ) . Thus, there is a constant cluding favorable sites in Eastern need to seek out better species, South Dakota ( Harlow and Harrar, strains, and superior trees through 1968 ). Siberian elm is an Asiatic introduction, seed source evalua­ elm of Eastern Siberia, North China tion and selection. Tree hybridiza­ and Turkestan ( Rehder, 1940). tion, both inter- and intra-specific, Siberian elm was first introduced offers an opportunity to up-grade into this country in 1905 ( Wyman, the trees available for tree plant­ 1951 ). Test plantings at various ex­ ings. periment stations showed satisfac­ tory performance and soon farm One introduction, Siberian elm, and town plantings were made in found wide acceptance in the the prairie areas of the United Northern Plains; fast growth, trans­ States. The number of trees planted planting ease, drought resistance reached large-scale proportions by and an acceptable mature height the early 1930s. Wholesale importa­ combine to give it this standing. tion of seed was necessary to pro­ Fast growth is especially useful in duce enough planting stock. Ac­ farmstead windbreak planting cording to Webb ( 1948) most of where it provides protection from the seed came from parent trees wind and snow just a few years after growing near anking, China, the planting. By the time the Siberian same latitude as Ft. Worth, Texas. elm rows b gin to die out, slower This seed source proved to be un­ growing, but longer-lived trees, are suitable when the 1940 Armistice tall enough to provide the protec­ Day freeze seriously damaged or tion needed. killed a high percentage of Siberian Many plantings of Siberian elm elm trees in the N othern and Cen­ were planted in close proximity to tral Great Plains ( Engstrom and the native slippery elm ( Ulmus Matthews, 1942 ). rubra Muhl.). Natural hybridiza­ Some identifiable seed sources of tion occurred between the two elms, Siberian elm were not injured by and clones of the F 1 have been ex­ the sudden freeze. Seedlings from ploited by the nursery trade ( Anon., these sources were planted in a trial 1950). at the South Dakota Stat Agricul­ This study was initiated to fur­ tural Experiment Station. One of ther explore the hybridization pat­ the sources, the Harbin seed, exhib­ terns between these two species and ited a relatively early cessation of to document useful growth data on growth, early enough to avoid in-

6 jury from fall frosts ( Maxon, 1951). bud and vegetative bud are only The Harbin source came from sparsely pubescent. The samara seed collected in the vicinity of fruit of slippery elm is densely pube­ Harbin, Manchuria, where the cli­ scent over the seed cavity; the Sib­ mate is characteristically continent­ erian elm samara is glabrous. al north temperate. Winters are long and cold, summers are short Hybridization Studies and warm. The latitude of Harbin Previous work closely parallels the North Dakota­ The first report of artificially pro­ South Dakota border. duced hybrids of forest trees was in In 1952, the Harbin source was Germany in 1845, when two species named Chinkota elm and released each of pine, oak, elm and alder under state certification standards were crossed ( Larsen, 1956). Early by the Experiment Station. Chin­ tree hybridization in the United kota elm seedlings were planted in States has been reported by Schrei­ rows adjacent to a common com­ ner ( 1937 ) in oaks, chestnuts and mercial source in an experimental poplars. windbreak at Brookings in 1952. An Much of the early breeding work early October freeze that same fall, was initiated in response to dis­ injured or killed 80%of the commer­ eases that threatened important tree cial trees, while 90% of the Chinkota species. Af!1ong the most important elms were alive to the tips or only diseases for which disease-resistant slightly injured the following spring trees have been sought are ch stnut ( Collins, 1955 ). Selected trees of blight ( Endothia parasitica the original foundation stock of [Murr. l A.S.A. ), white pine blister Chinkota elm were used as parents rust ( C ornartium rihicola Fisher) in this study. and Dutch elm disease. Richens The slippery elm parent used in ( 1945 ), Graves ( 1948), Clapper most crosses is a campus tree about ( 1952 ), and Gerhold et al. ( 1966) 50 years old. Other slippery elm have described these early tree tr es were used as parents in some breeding programs. phases of the hybridization study. The identification of Dutch elm Slippery elm and Siberian elm disease in The Netherlands in 1919 are distinctly different in a number ( Beattie, 1937), gave impetus to an of morphological characteristics. Im breeding program in that coun­ Slippery elm is relatively large-leav­ try. Went ( 1938 ) summarized the ed with few branched stout twigs early program of testing elm species and large buds. The upper leaf sur­ and varieties collected from many face and young twigs are scabrous, parts of the world. More recent hy­ and the elongate flower and vege­ bridization and selection work has tative buds are densely pubescent. been reported by Went ( 1954) and In contrast, Siberian elm is small­ Heybroek ( 1962 ); Gerhold et al., leaved and develops a profusion of ( 1966 ). Accounts of the impact of slender twigs with small buds. The Dutch elm disease and the programs upper leaf surface is smooth, and initiated to solve the problem have the young twigs are finely pubescent been reported from other countries to glabrous. The spherical flower including England ( Melville, 1944; 7 Peace, 1960; Anderson, 1961), usually resulted in failure. All at­ Sweden ( Ehrenberg, 1954), Italy tempts to cross American elm with ( Goidanich, 1938) and Canada Siberian elm or slippery elm at the (Johnson, 1939; Anon., 1954). In South Dakota Agricultural Experi­ the United States, progress in elm ment Station have failed. Probably tree breeding has been reported by a major barrier to successful inter­ Smucker ( 1944), Graves ( 1948), specific crosses with American elm Swingle et al. ( 1949), Clapper and is the chromosome number. The Miller ( 1949 ), Clapper ( 1952), and basic number in elms is x==l4 and Gerhold et al. ( 1966). most elm species are diploid ( 2n== Studies have shown that resist­ 28 ). However, American elm is a ance to Dutch elm disease exists in tetraploid ( 4n== 56) ( Sax 19~3; Asiatic elms. European elms are Darlington and Wylie, 1956). Der­ generally susceptible, but some men and May ( 1966) and others species and varieties have shown have isolated apparent tetraploid varying degrees of resistance. Amer­ Siberian elm seedlings after colchi­ ican elms have proven to be the most cine treatment and plan to use these susceptible. However, Smalley and trees in an attempt to obtain Siber­ Kais ( Gerhold et al., 1966) and ian-American elm hybrids. other workers have found that a few sources of American elm have some Controlled Crosses in Elm resistance to severe crown damage The first controlled crosses be­ and a few have even recovered from tween Siberian elm and slippery the infection. They also noted that elm were made in 1953 and 1954 as some slippery elm seedlings show shown in Table 1. Crosses also in­ resistance to inoculations in some­ cluded American elm trees and two what the same manner as Ulmus x F 1 hybrid trees ( Siberian x slippery hollandica vegeta (Loud.) Rehd., elm). Pollinations were made by a variety of Dutch elm. introducing pollen-bearing flowers Several selections of smoothleaf into parchment bagged flowers on elm ( Ulmus carpinifolia Gled­ the parent trees, and then the itsch. ), resistant to Dutch elm dis­ branch with the bagged flowers was ease, have been released in Holland shaken. A small population of F 1 ( Heybroek, 1962). More recently hybrids was obtained where slip­ the cultivar Ulmus x hollandica pery elm was the seed parent. The 'Groeneveld' was released to grow­ reciprocal cross gave only one plant. ers in Holland. This resistant clone Backcross progeny were obtained was the result of a cross between from both of the parent species, Ulmus glabra Huds. ( Scotch elm) though the number was quite small and U. carpinifolia ( Heybroek, in the slippery elm backcross. The 1963). General hybridization pat­ F 1 hybrid elm tree produced a good terns within the genus Ulmus were population of F 2 when crossed with reported by Britwum ( 1961). another F 1 hybrid. Controlled crosses between most When F 2 populations are refer­ elm species have not been difficult; red to in this report, it means the however, attempts to cross Amer­ progeny of a cross between two dif­ ican elm with other elm species has ferent F 1 hybrid trees. The self-in- 8 Table 1. Inter-and Intra-Specific Uhnus Crosses and Selfings Made in 1953 and 1954 Female Parent Pollen Parent Siberian 1 Slippery American Hybrid' Elm Elm Elm Elm Selfs B F s B F s B F s B F s B F s Siberian Elm1 ___ 13 8 1 5 0 0 3 91 51 12 5 1 Slippery Elm _ 14 204 106 5 52 1 6 3 0 3 59 10 13 15 0 American Elm 4 0 0 4 0 0 10 177 21 2 1 0 H brid Elm2 5 255 155 3 37 8 10 184 96 4 1 0 B-number of bags; F-number of apparently- filled seeds; S-actual number of seedlings ob­ tained. 1"Chinkota" ~eed ~ource. 2Slippery x Siberian elm (parent trees unknown).

compatible nature of F 1 trees used Table 2. Selfing Frequency of Individ- in this study precluded production ual Trees in Slippery Ehn of typical F 2. No. Prag ni s from controlled crosses No. No. of No. of Per cent Tree of Rower of filled filled were transplant d in adjac nt rows No. Bags Clu ters Fruit Seeds Seeds in 1954 and 1955 since the popula­ tions in all crosses were insufficient 1 ·-·- 2 10 70 2 3 for a replicated trial. The trees pro­ 2 ---- 2 17 149 5 3 3 ---- 2 19 175 1 .6 vided a source of leaves, flowers, 4 - 2 26 270 3 1 fruits and twigs for morphological 5 2 19 142 77 54 measurements and gave some indi­ 6 ---· 2 14 129 7 5 cation of growth rate and form. 7 ---- 2 9 10 2 20 Selfing Trials ·--- 2 28 157 0 0 Johnson (1946), Johnson and 9 ---- 2 23 162 1 6 2 30 230 0 0 Heimburger ( 1946) and Went 10 - 11 -- 2 21 65 1 2 ( 1955) have reported a high incid­ 12 ---- 1 7 105 0 0 ence of self-st rility in elms. How­ - ever, Went (1954) noted that in­ dividual trees varied in this respect. He found that a few hybrid elm of slippery Im produced 20% or tr s were highly self-fertile. more appar ntly filled fruits out of Since attempts to obtain viable the total crop. The F 1 hybrid trees ed by elfing were not successful ( lippery x Siberian elm) produced in these trials, several trees of four no filled fruits. elm sp cies were t sted for self­ Self-incompatible tr es for con­ compatability in 1959. Flower-bud trolled crosses are advantageous bearing twigs on these tr es were because the perfect flower of elm is bagged prior to flowering and left very small, and emasculation r - until the fruit had matured. The re- quires magnification and painstak­ ults of the selfing trials are pre­ ing care to remove all stamens with­ ented in Tables 2, 3, 4, and 5. Prac­ out injury to the pistil. Some in­ tically all trees tested showed high dividual elm trees show marked self-incompatability. Only two trees protogyny, permitting artificial pol- 9 Table 3. Selfing Frequency of Individ­ Table 4. Selfing Frequency of Individ­ ual Trees in Siberian Elm ual Trees in American Elm

No. Per No. Per No. No. of No. of cent No. No. of No. of cent Tree of flower of filled filled Tree of flower of filled filled No. Bags Clusters Fruit Seeds Seeds No. Bags Clusters Fruit Seeds Seeds 1 ------1 21 300 1 3 1 ------2 11 245 5 2 2 ------3 106 590 0 0 2 ------2 7 110 0 0 3 ---- 2 58 530 0 0 3 -- 2 17 260 0 0 4 ---- 2 90 950 22 2 4 2 8 135 0 0 5 ------2 57 580 4 .7 5 2 10 175 1 .6 6 _---- 2 61 760 0 0 7 ------3 73 950 15 2 8 ---- 3 101 1000 4 .4 Table 5. Selfing Frequency of lndivid- 9 -- 2 73 640 5 ual Trees in Slippery Elm x Siberian 10 ---- 2 69 800 0 0 Elm 11 -·---- 3 89 800 0 0 12 --- 3 49 725 3 .4 No. Per 2 57 475 0 0 No. No.of No. of cent 13 -·-· Tree of flower of filled filled No. Bags Clusters Fruit Seeds Seeds lination before anthers dehisce. 1 -- 2 44 725 0 0 2 _ ·- 2 5 825 0 0 This is a most useful feature in 3 -- 2 23 225 0 0 those trees not having a high degree 4 -- _ 2 53 650 0 0 of self-sterility. Figures 1, 2, 3, and 5 ---- 2 44 575 0 0 4 illustrate such flowers. 6 ---- 2 53 800 0 0 7 ------2 42 500 0 0 Siberian-Slippery Elm Hybrids 8 _ 2 22 148 0 0 An effort was made in 1964, to 9 2 32 300 0 0 produce F 1, F 2 and backcross popu­ 10 2 3 116 0 0 lations to c.ompare with seedlings of the parent species. Most of the crosses were made on trees, but ad- The branches were re-bagged im­ ditional pollinations were made on mediately after the storm, but som cut branches and bottle-grafted Chinkota elm flower buds were trees in the greenhouse. Unfortun- damaged and some pollen contami­ ately, high temperatures in the nation may have occurr d. greenhouse caused most of the dev- The trees were pollinated over a eloping fruits to drop prematurely. four-day period by forcing pollen Figure 5 shows a bottl -grafted tree into the bags with a syringe. By on which a fruit dust r is nearing May 22, the fruit had ripened and maturity. Figure 6 shows a 9-year- wa collected. The filled fruits were old bottle-graft of slippery elm with s parated from empty fruits and its prominent flower buds. c.ounted. They were then stored in The field trees were bagged with air tight containers under refrigera­ parchment bags the first week of tion. Results of the fruit yield are April. A severe windstorm on April presented in Table 6. All crosses 13, destroyed over half of the bags. yielded ample quantities of fruit 10 Figure I. Protogynous ( maturing of pistils before stamens shed pollen) Flowers of Siberian Elm.

except those between F 1 hybrid trees. This cross was repeated in 1965, with the same result. The F 1 trees used in these pollinations were from 1953-54 crosses, which were now old enough to bear fruit. Growth Exceptional growth and vigor have been reported in progenies of Figure 2. Protogynous Flowers of interspecific crosses in several tree Slippery Elm. genera. For instance, Stockw 11 and Righter ( 1947) suggest d that pine hybrids would increase volume two to three times over natural stands. Comparable data in elm are some­ what fragmentary. Aljbenskii ( 1951; 1956) reported that hybrids of Siberian elm and Europ an white elm ( Ulmus laevis Pall.) grew taller and had larger diamet­ ers than the parent species. These hybrids also showed good drought and soil salinity resistance. Hartley Figure 3. Protogynous Flowers of ( 1927 ) report d that the Hunting­ Slippery Elm x Siberian Elm. don elm ( U. glabra x U. montana) grew twice as fast in height as other elms in the same plantation. Rock­ well ( 1945) stated that in South Da­ kota hybrid elms ( U. pumila x U. rubra) gr w as fast in height as Si­ berian elm. By 1966, the F 1 hybrids 11 .Figure 4. Protogynous Flowers of American Elm.

of Siberian and slippery elm which were produced in 1954, averaged 35 feet in height-the tallest tree was 40 feet. In the same planting Chin­ kota elm averaged 33 feet. The 1964 seed was sown in green­ house flats on July 2. Prior to sowing all seeds were soaked in water, and the slippery elm seeds were strati­ fied for 23 days at 41 ° F. The ger­ minating seedlings were transplant­ ed into peat pots after 12 days and kept in the greenhouse for about two weeks. They were then placed outside to harden for a few days. On August 3, the seedlings were planted into the field site in a ran­ domized complete block design in Figure 5. Bottle-Graft of Scotch four tree plots and 10 replications. Elm on a Siberian Elm Seedling The seedlings were planted one Showing Samaras on the Scotch foot apart in rows three feet apart. Elm Nearing Maturity. 12 Figure 6. A Nine-Year-Old Bottle-Graft of Slippery Elm on a Siberian Elm Seedling Root. ( Prominent buds on the slippery elm top are Hower buds.) 13 100 94 t, 90 , .. ,A 84 4 80 P-!7XR-I, ,-"--...---+--... -- f ,, 70 I I c I e 60 I ai I a. I I I "'O a, I a, 50 1/) t R-1 X P·I 7 ,,,,.. 49/. "'O a, I ,....A- - -.-....-,;..... 471. 0 , ..-- ,>------...... §C 40 ,' /"'' R-/ X R-20 a, t:> .30 ! /l

20 ! /1 I , I

I 0 //J ,','/ _/ /

2 3 4 5 6 7 8 9 10 II 12 15 Days ofter Sowing

P- Siberion R- Sl ippery Elm

Figure 7. Germination of Seeds of Siberian Elm, Slippery Elm and their Hybrids ( 80 seeds per lot).

The trees were watered at planting Seeds from the controlled cross and subsequently as needed. Insec­ betw en two slippery elm trees ticides wer applied to control leaf failed to germinate promptly. Con­ defoliators and root feeders. The sequently, op n-pollinated seed­ area was fenced to pr vent rabbit lings of slippery elm were planted injury. Trees that died in the plant­ as slippery elm trees. After a month ing were replaced by transplanting of growth, it was apparent that supply plants of the same age grow­ open-pollinated seedlings of slip­ ing in adjacent rows. Replication pery elm were actually natural hy­ number 10 was lost to residual ac­ brids with Sib rian elm. To rectify tion of simazine which had been ap­ th situation, seeds of slippery elm plied four years earlier in an unre­ x slippery elm, which had been lated experiment. placed in stratification in June, 14 90

80

70

~ 60 . ,_p_c~ ii 1 : ------G------I :s 50 ) I ! i ... -~-~---r····+------~ ,-1 40 12---!--+-+----­ .:1..c.e_1-+--.-- - ~ 30 --+---·p I I :' I l 20

50 60 70 80 90 100 110 120 130 140 150 160 170 Leaf length - mm

P-Siberian R- Slippery Elm BCP- Backcross to Siberian Elm BCR- Backcross lo Slippery Elm

Figure 8. Ranges, Means and Two Standard Errors of Leaf Width and Leaf Length of Siberian Elm, Slippery Elm and their Hybrids. were sown in August. They germin­ significant by the analysis of varir ated satisfactorily and were kept in ance method. ( See Table 8 for 1970 the greenhouse until natural growth height growth data.) The Duncan cessation occurred in October. Af­ multiple range test applied to the ter a chilling treatment they were 1970 data shows the superiority of forced into growth in the green­ F 1 hybrids in height growth as com­ house in March, 1965, and trans­ pared to plants of the parent species planted into the study plot in May, and backcrosses. The tallest tree at replacing one of the slippery elm the end of the 1970 growing season open pollinated lots ( F 1). was an F 1 hybrid which had attain­ ed a height of 27 feet. In the spring of 1966, all trees in the planting were undercut, lifted, Growth Cessation and replanted in a new area. The In mid-August, 1965, every tree trees were spaced 8'xl2' and planted in the test planting was staked with in a randomized complete block de­ a 4-foot bamboo pole. The leading sign with 4 tree p,lots and 9 replica­ shoot was tied loosely to the stake. tions. Height growth data and other Beginning on August 30, increase in observations have been taken since height growth was marked on the that time. Average height growth stake at two- to three-day intervals. for each year is given in Table 7. A reference mark at the base of the In all years measured, the progeny stake and the tree insured constant group differences have been highly alignment. The date of cessation of 15 height growth was recorded for Table 6. Hybridizatiqns between Siberian Elm and Slippery Elm and the F 1 Hybrid in 1964. each tree. , -~ ;i Seed Parent Pollen Source Results are given in Table 9. The Female Pm P-17 F~,H R-20 H-1 H-17 P-30 P-32 P-35 date on which most trees stopped Tree B TF F B TF F B TF F B TF F B TF F B TF F B TF F B TF F B TF F growing is underlined. For most 300 2006 500 progeny lots this occurred on Sep­ P-17 2 79 13 300 5 68 1 tember 20. The most variable in 600 1000 4QO 1300 400 growth cessation was slippery elm R-1 7 400 6 750 0 10 1000 7 3 and the backcross to slippery elm. 100 The least variable was Chinkota 160 00 10 R-20 7 61 7 • 79 1 0 4 62 elm.Generally there were no mark­ 10,000 .. ,.4000 ·' 10,000 10,000 ed differences between progenies of 8 ;r ( the two seed parents or their hy­ H-1 10 1 0 12 0 15 0 brids. 400 400 I 10,000 H-17 1 0 8 0 Seed Germination I Eighty seeds each of Chinkota 1000 500 300 elm (P-170P), Chinkota elm x slip­ P-30 ------6 81 3 88 3 20 pery elm (P-17 x R-1), slippery elm ,socs 5,000 300 x Chinkota elm (R-1 x P-17) and P-32 14 c 15 8 172 3 0 -- slippery elm ( R-1 x R-20) were sown in greenhouse Hats in June 500 400 500 3 ,y T ,1 0 2 130 5 0 1965, to obtain seedlings for mount­ P-35 ------·- ing and measurement. Daily germi­ B-number of bags; TF-total number of fruits (above 2p 0 estimated to nearest 1000) F-filled fruits nation dates were recorded. Slip­ P-Siberian "Chinkota" elm pery elm seeds were stratified three R-Slippery elm months before sowing. The germi­ H-F1 hybrid (slippery x Siberian) nation pattern of these seed lots is Pm-mixture of pollen from several "Chinkota" trees illustrated in Figure 7.

The influence of the seed parent Production of Hybrid Seed linated fruit was collected from In June, the remammg seeds on rate of germination was rather Siberian elm and slippery elm slippery elm trees which ripened were removed from stratification striking. Chinkota elm seeds germ­ have flowering dates that overlap. ample quantities of samaras. In Jan­ ( 142 days) and sown in flats. These inated rapidly and reached germi­ Usually, Siberian elm flowers slight­ uary 1965, 300 stored seeds from were also grown to the same size native capacity in 7 days. Seeds of ly earlier and is shedding pollen at one slippery elm parent were plac­ and were mounted. P-17 x R-1 began germination a day the time the protogynous flowers of ed in cold stratification at 40° F. By The seedlings were classified as later, but they germinated rapidly slippery elm are receptive. This of­ April, after 46 days of stratification, hybrids ( slippery elm x Siberian in a pattern similar to Chinkota fers an opportunity for natural hy­ 90 sprouted seeds were removed elm ) or as slippery elm on the basis elm. F 1 hybrid seeds from the slip­ bridization to occur with slippery and sown in Hats. These seeds had of leaf shape, presence of long hairs pery elm seed. parent and slippery elm as the seed parent. In 1952, a little or no dormancy and had be­ on the upper leaf surface, degree of elm seeds were markedly sluggish planting was made in the Brookings gun to grow slowly at the low strat­ sunkenness of main veins and seed­ in their germination pattern. Only area to explore this possibility. Chin­ ification temperature. The resulting ling vigor. All seedlings that devel­ half of the latter two seed lots had kota elm and slippery elm trees seedlings were left in flats until at oped from early germinating seeds filled embryos as verified by check­ were planted side by side in an iso­ least 4 true leaves had formed, at were hybrids; all but five seedlings ing the non-germinating seeds after lated area. First flowering and fruit­ which time they were pulled, press­ from late germinating seeds were the test was completed. ing occurred in 1964, and open pol- ed and mounted. slippery elm which is consistent 16 17 with stratification requirements of that elm leaves developed different that species. shapes depending upon the part of Approximately 45% of the seed­ the crown and upon the kind of lings from 300 seeds were hybrids. shoot on which they were growing. Furthermore, all seeds that had lit­ Leaves formed on short lateral shoots differed from those formed tle or no dormancy produced F 1 hy­ on a leading shoot, a proleptic brid seedlings. Dormant seeds pro­ shoot, an epicormic shoot or a suck­ duced almost all slippery elm seed­ er. He recommended that sample lings. leaves be taken from short lateral This test verified that natural hy­ shoots. Later he ( 1960) recom­ bridization does occur when slip­ mended that the third leaf from the apex on short lateral branches be pery elm grows in close proximity taken for samples since these leaves to Siberian elm. It also suggests a are the least variable. method by which commercial pro­ duction of hybrid seed is possible. Leaves If selected slippery elm and Siber­ In this study the third leaf from ian elm trees are planted in adja­ the apex on a short determinate lat­ cent rows or alternated in the row, eral branchlet on the south side of seed can be collected from the slip­ the mid-crown was collected from pery elm trees and sown in rows several trees of the parent species without prior stratification. Seeds and from the hybrids of the 1953 that germinate promptly will pro­ and 1954 crosses. When the third duce F 1 hybrids; dormant seeds, leaf was not usable because of mal­ most of which are slippery elm, will formation or mechanical injury, the probably fail to germinate or fourth leaf was sampled. All leaves germinate too late to cause any were dried, pressed and measured. problem in the lifting and grading process. All sample leaves were measured for leaf length and width ( at widest Morphological Characteristics dimension) , length of leaf from the An extensive study of the taxo­ widest point to the base, length of nomic characteristics of elms has petiole, number of pinnate veins on been undertaken by Richens ( 1955, the longest side of the leaf, number 1956, 1958, 1959, 196la,, 1961b) for of bristles and/ or hairs per 80~m2 the purpose of identifying elm on the upper leaf surface, number species and hybrids growing in of hairs along 8mm of length of the England. He measured several leaf longest vein on the leaf undersur­ characteristics including length, re­ face and leaf weight. Averages and lative width (width/ length), rela­ standard errors of these measure­ tive petiole length ( petiole length/ ments are presented in Table 10. leaf length), basal asymmetry, Figure 8 illustrates the ranges, num her of teeth, a set of measure­ means and two standard errors of ments on the marginal tooth, and two leaf characteristics measured. the degree of scabrousness of the Sample leaves of each progeny leaf surface. Melville ( 1937) noted group are shown in Figure 9. 18 Table 7. Average Height Growth of Various Elm Progeny from 1965 through 1970. -- - -- 1965 1966 1967 1968 1969 19701 Progeny inches inches feet feet feet feet P x R . ---- 39 57 10.9 11.7 16.8 21.7a RxP ---- 49 66 11.5 12.2 16.6 20.7a R OP ______40 54 9.4 10.9 14.9 18.8 b P ------34 48 7.3 9.5 13.4 17.1 C BCR ______26 36 7.0 7.8 13.2 16.9 cd R ------22 44 7.3 7.6 11.8 15.4 de BCP _ ---- 32 43 6.9 8.6 11.9 14.9 e ------P- Chinkota elm; R-slippery elm; RxP, PxR, R OP-F1; BCR, BCP-backcrosses. 1 Any means not followed by the same letter are significantly different at the 5 percent level.

Table 8. Analysis of Variance of Height Growth of 7-year-old Elm Progeny. Source of variation JF ss MS F

Progenies ------·- ____ 6 1441.25 240.20 22.61 H Replications ------8 1 0.71 22.58 2.11 • Experimental error 48 509.52 10.62 .99 Sampling error ______189 2023.69 10.70 Total .. __ ------·-·· 251 4155.17 ..Si gnificant at 1% level. •Significant at 5% level.

Table 9. Growth Cessation of Individual Trees within Each Ulmus Progeny Group by Frequency and Date of Cessation (1965)

Progeny Groups P-17 P-17 P-17 P-17 R-1 R-20 R-1 H-1 OP X X X X X OP Date H-1 R-1 P-17 H-1 R-20 Aug. 30 ______1 Sept. 1 ______1 1 3 ------2 2 2 1 8 ------3 1 2 1 1 2 1 10 ------3 3 3 1 1 2 3 1 13 ------9 8 3 2 7 6 4 15 ------7 9 5 3 7 3 11 17 ----··---- 1 1 1 1 2 1 20 ------3 10 17 16 16 2 11 12 22 ------3 2 5 3 3 3 24 ------1 7 5 1 1 27 ------1 2 4 2 3 2 29 ------1 1 1 Oct. 1 ______3 2 1 1 4 ------1 2 1 8 ------1 P-Siberian elm ("Chinkota"); R-Slippery elm; H- Siberian x slippery elm 19 R p

BCP

R-Slippery Elm P-Siberian BCP-Backcross to Siberian Elm BCR-Backcross to Slippery Elm Figure 9. Typical Leaves of Siberian Elm, Slippery Elm and their Hybrids ( 5/ 16 natural size).

The F 1 hybrid showed intermedi­ the F 1 and the parent species in leaf ate characteristics between the two length and width, number of veins parents in leaf length and width, per leaf and hairiness and to some number of veins per leaf, hairiness extent in leaf weight. ( both Rner hairs and bristles ) and The F2 population tended to have weight. The F 1 was similar to Sib­ mean characteristics similar to the erian elm in leaf width to length F 1 and the backcross to Siberian ratio, in leaf shape and in petiole to elm. The 68 trees surviving at the blade length ratio. The petiole time of leaf collection were numer­ length of the F 1 exceeded both par­ ically insufficient to produce the ents. full array of segregates expected. Backcrosses also showed inter­ Seedling leaves of the parent mediate characteristics between species, of the Chinkota elm x Slip- 20 Table 10. Means and Standard Errors of Ten Leaf Characteristics of Siberian and Slippery Elm and their Hybrids Progeny Group Leaf Characteristics Blade Blade Width/ Petiole Leaf Length Width Length Length Weight mm mm ratio E/L mm P/ L' Pub' grams P ------69.0 30.4 .44 .44 8.6 .12 12.3 0.0 0.7 .1424 Std. Err. _ 1.44 .64 .009 .009 .33 .004 .26 .25 .007 BCP ______76.4 36.2 .47 .43 7.8 .10 12.4 8.5 8.0 .1425 Std. Err. 1.63 .91 .007 .008 .42 .005 .32 2.09 . 3 .0094 F1 ---·-- _ 97.1 44.9 .46 .44 10.4 .11 14.0 52.9 1 .2 .2609 td. Err. 1.84 1.39 .003 .005 .28 .003 .19 9.85 1.23 .0150 BCR ------·- 120.6 58.5 .4 .47 8. .07 16.3 220.1 35.6 .4369 Std. Err. 3.16 1.69 .01 .008 .40 .004 .40 24.09 2.16 .0252 R ______126.5 70.1 .55 .47 7.7 .06 16.6 264.1 4 .7 .6201 Std. Err. _ 4.35 1. 0 .01 .009 .41 .002 .35 14.49 2.30 .03 8

F2 5.7 40.5 .47 .45 7.2 .0 13.l 132.6 16. .226 Std. Err. 1.54 .75 .006 .005 .25 .003 .24 18.11 1.32 .007

P-Siberian elm; BCP-Backcro ~ to iberian elm; BCR-Backcros~ to Slippery elm. R-Slippery elm 1E/L-Ratio of length from base of blade to widest part tn total length. 2P/L-Ratio of petiole length to blade length. 3V-Number of main pinnate veins on longest side of blade. 'Pub-Number of hairs on a O mm2 area on upper surface of blade. 0HV-Number of hairs along mm length of longest pinnate vein on under surface. p ry elm hybrid, and of the recipro­ Table 11. Means and Standard Errors of cal cross were also compared. The Four Leaf Characteristics of the First first two true leaves formed showed True Leaf of Seedling Elms the 1 ast variability and wer the Length Width most useful as indicators of type of mm mm E/L' W/L' progeny. Leaf length and width and the distance from the widest P-17 15.6 6.5 .57 .42 Standard portion of the leaf to the leaf base Error .30 .14 .003 .010 were mea ured on one leaf of each P-17 x R-1 16.6 .4 .48 .50 seedling. Standard The Chinkota elm seedling leaf Error .25 .14 .007 .007 was typically narrow and distinct­ R-1 x Pm 19.l 10.2 .48 .54 ly obovate. The slippery elm leaf Standard was much wider than the Chinkota Error ___ .50 .35 .007 .013 elm leaf and gen rally had a r ti­ R-1 x R-20 ---- 16.2 9.3 .48 .58 culate appearance on the upper Standard surface due to sunken main and Error .48 .32 .012 .021 side veins. Pubescence was present P- iberian elm ("Chinkota") on the upper surface of seedling R-Slippery elm I aves of both sp cies, though some Pm-Mixed pollen of "Chinkota" elm 1E/L-Ratio of length of blade from base to Chinkota elm leaves were glabrous. widest part to length of blade Hairs were much longer on leaves 2W /L-Ratio of blade width to length 21 p RX p R

I J (' \

P- Siberion R- Sl ippery Elm Figur.e 10. Typical Seedlings of Siberian Elm, Slippery Elm and their Hybrids. of lippery elm seedlings than on percentage of the trees matured leaves of Chinkota elm. fruit. Each samara was measured in length, width, length from the base F 1 hybrid seedlings had leaves to the point of greatest width, length that averaged longer and wider of fruit stalk ( after freeing from the than leaves of Chinkota elm seed­ residual calyx ), depth of the apical lings. This was most striking where notch and length of the free portion slipp ry elm was the seed parent. of one of the residual stigma lobes The F 1 hybrid se dlings grew more at the apical nd. In addition, the rapidly than slippery elm seedlings. numb r of hairs on the seed cavity The latter tended to grow so slowly and on the main vein from the base that the fir t four leaves often show­ of the samara to th~ seed cavity was ed a whorl-like arrangement; on the counted. Each samara was weigh­ other hand, hybrid seedlings show­ ed. ed definite elongation of the int r­ nodal region between the first and The means and standard errors second pairs of leaves. Averages of nine samara characteristics are and standard errors of seedling leaf presented in Table 12. The most measurements ar given in Table obvious differences betw en popu­ ~l.. A sample. of each seedling type lations were in pubescence counts, 1s illustrated m Figure 10. though the F 1 and F 2 samaras were somewhat more sparsely pubes­ Fruits c nt than anticipated. Apparently Four samaras were collected the glabrous condition of Siberian from each fruiting tree of the vari­ Im was somewhat dominant. Inter­ ous hybrid elm types and from the mediate characteristics in the F 1 parent species in 1961, when a high were also noted in the depth of the 22 Table 12. Means and Standard Errors of Nine Samara Characteristics of Siberian Elm, Slippery Elm and their Hybrids

Progeny Group Characteristics Length Width Fruit stalk Samara mm mm E/L' N' Length mm RS 3 SC4 VH5 Width gms. P ------12.4 12.0 .55 2.6 1.4 .3 0 0 .0069 td. Edd. .27 .34 .009 .13 .06 .03 .0003 BCP ____ 12.0 11.6 .57 1.9 1.5 .5 1 1 .005 Std. Err. .23 .23 .006 .09 .02 .03 .30 .08 .0003 F1 ------12.6 12.5 .59 1.4 1.8 .9 22 8 .0073 Std. Err. .19 .17 .008 .05 .06 .04 1.7 .7 .0002 BCR ______13.7 11.9 .57 1.6 2.4 1.0 134 29 .0086 Std. Err. .32 .26 .009 .08 .03 .04 8.2 2.5 .0003 R ------_ 13.2 11.0 .60 . 0 2.4 1.2 459 63 .009 Std. Err. .17 .25 .010 .04 .08 .04 4.3 3.1 .0003 F:z ------11.7 10.7 .57 1.5 1.4 .5 18 5 .007 Std. Err. .12 .13 .004 .04 .05 .02 2.1 .5 .0002 P-Siberian elm; BCP-Backcross to iberian elm; BCR-Backcross to slippery elm; R-slippery elm 1E/L-Length from base to widest part to total length ratio 2N-Depth of apical notch in mm 3RS-Length of free portion of residual stigma 'SC-Number of hairs on the seed cavity -VH-Number of hairs on main vein from base of ~eed cavity to ba~e of ~amara

Table 13. Means and Standard Errors of Buds and Flower Characteristics of Siberian Elm, Slippery Elm and their Hybrids

Progeny Group Characteristics Vegetative Flower Bud Bud Floret Floret No. of Length Length Length Width Florets/ mm mm mm mm inflorescence p ------·--·----- 1.9 3.4 2.2 1.5 15.1 Std. Err. ______.16 .13 .04 .04 .54 BCP ------2.0 4.3 19.4 Std. Err. ______.05 .03 .94 F1 ______-----·---- 3.1 5.0 3.3 1.6 24.6 Std. Err. ______.02 .04 .04 .05 .76 BCR ------3.6 5.2 21.6 Std. Err. ______.02 .07 .87 R ------5.7 7.3 4.4 2.3 17.9 Std. Err. ______.13 .13 .02 .04 .65 F2 ______2.6 4.2 19.4 Std. Err. ______.04 .02 .44 P-Siberian elm; BCP-Backcro s to Siberian elm; BCR-Backcro s to slippery elm R-Slippery elm 23 p ... ._...,...; .. - BCR BCP

..:

R-Slippery Elm P-Siberian BCR-Backcross to Slippery Elm BCP- Backcross to Siberian Elm

Figure 11. Typical Samaras of Siberian Elm, Slippery Elm and their Hybrids ( about i natural size.)

apical notch, fruit stalk length and cation for counts of the number of frµit weight and length. F 2 samaras stamens in each floret. Only 5% of were very similar to the backcross the F :! population fruited and sev­ to Siberian elm. Figure 11 is a pho­ eral of those trees had poorly dev l­ tograph of samples of each progeny oped buds. Most trees in the other group. progeny groups bore flower buds. Flowers and Buds Later in the season, inflorescences One twig which bore several were collected at the time of initia­ flower buds was collected from each tion of anthesis and were stored in tree of the various progenies and alcohol for later measurement. parent trees. Every flower bud was Means and standard errors of flo­ dissected to determine the number ret lengtli and width as well as ve­ of florets in each inflorescence. Ten getative bud and fl ower bud per cent of the florets from each lengths are recorded in Table 13. twig were dissected under magnifi- The F 1 floret was intermediate in 24 BO- 76% 7 0 ...

6 0 ... 56%

5 0 47%

~ 42% 42% ~ Q) 40 ... CJ 1-1 ~ 3 0 ...

20

IO - 8%

0 I p BCP BCR R

P-Siberian R-Slippery Elm BCP-Backcross to Siberian Elm BCR-Backcross to Slippery Elm Figure 12. Percent of Inflorescences Having 20 or More Florets in Siberian Elm, Slippery Elm and their Hybrids. length between the two parent ncies shows that they differ at a species, but the width was similar highly significant level. Some of the to Siberian elm. F 1 inflorescenc shad over 40 florets The inflorescence of the F 1 con­ and 19 percent had 30 or more. tained mor florets than either par­ Eight percent of the F 2 flower ent, apparently an expression of clusters had 30 or more florets, and hybrid vigor. Th frequency with on flower bud contained 56 florets, which inflorescences having twenty the maximum counted in any or more florets pres nt in each of inflorescence. the progeny groups is shown in Fig­ The individual florets of each of ure 12. The chi-square test of in­ the progeny groups also differed in dependence applied to these frequ- the number of stamens present. In 25 R BCR BCP p

(

Figure 13. Typical Twigs of Siberian Elm, Slippery Elm and their Hybrids Showing both Vegetative and Flower Buds (about 1/4 natural size).

Siberian elm, four stamens per flo­ surements of F 1, backcrosses, and ret were most common. In slippery F :.! w r intermediate betw n the elm, most florets had s v n stamens parental types. The e measurements but the number varied from 5 to 8. were made on th first vegetative th r prog ny group had stamen bud below the p udoterminal bud count that were int rmediate. The and on the Br t flower bud from the number of tam n in F 2 florets ap x on ten twigs from each tr e. varied from 3 to 7 ( Table 14). Typical twig with both kinds of Vegetative and flow r bud mea- buds ar illustrated in Figure 13. 26 Table 14. Percent of Occurrence of Stamens per Floret in Siberian Elm, Slippery Elm and their Hybrids

No. of stamens per floret Progeny 3 4 5 6 7 8 Percent of florets sampled Siberian elm __ _ _ 1 92 7 Backcross to Siberian elm 3 63 33 1 F 1 Hybrid ------·- ______--·- 37 60 3 F~ Hybrid 21 55 21 2 Backcross to slippery elm __ 5 53 42 Slippery elm _ -· 2 28 51 19

Summary The primary objective of this cal cross. American elm failed to study was to produce hybrids be­ cross with ither of these species. tw en Siberian elm and slippery Backc.ross progenies were obtained elm using a hardy seed source of from the F 1 hybrid on both parent Siberian ( Chinkota) elm as one speci s, and a progeny re embling par nt. Hybrids were backcrossed an F ~ population was obtained by to the parents, and two F 1 hybrids crossing the two F 1 hybrid trees. wer crossed to obtain a popula­ The progeny were out-planted for tion similar to an F 2 population. morphological studies and g neral Progenies resulting from these growth performance. crosses were planted-out for growth The s If-compatibility of several performance and for morphological slippery, Siberian, American and F 1 comparisons. hybrid elm trees was tested in 1959. The first hybridization trials Selfing occurred in some of the in­ were made in 1953 and 1954, be­ dividual trees of each species, but tween slippery, Siberian, American th per cent of filled seeds was less and hybrid ( slippery elm x Siberian than 6% in all but two trees of slip­ elm) elm trees located in the Brook­ pery elm. one of the F 1 hybrid ings ar a in South Dakota. Small tre s yielded any viable fruit. populations were obtained from the In 1964, a randomized complete cross between slippery and Siber­ block design growth study was ian elm trees and from the recipro- planted from crosses made in the 27 spring. Included in the planting promptly germinating seeds. Seeds were progenies of both parents, of of the same collection which requir­ controlled crosses ( slippery elm x ed a cold strafication period before Chinkota elm and reciprocal), and germination developed into seed­ of backcrosses to both parents. A lings that were mostly slippery elm. natural F 1 hybrid population which This suggests that quantity pro­ came from seed collected from a duction of F 1 hybrid seedlings slippery elm tree in close proximity could be accomplished by establish­ to Chinkota Im tree was also in­ ing an isolated slippery and Chinko­ cluded in the planting. Height ta elm planting and collecting seed growth measurements were deter­ from the slippery elm trees. The mined annually. s quence of flowering in these Highly significant height growth species in the Brookings area is so differences between the various timed that many of the protogynous progeny types were found by analy­ flowers of slippery elm are recep­ ses of variance after each growing tive at the time Chinkota elm trees season. At the end of the 1970 sea­ are shedding pollen. son, th average heights of the three Morphological comparisons of F 1 progeni s were significantly dif­ leaves, flowers, fruit and buds of the ferent from the other types at the parent species, bac.kcrosses, F 1 and 5% level ( Duncan's multiple range F 2 were studi d in progenies of the test ) . Some of the F 1 trees had an 1953, 1954 crosses. Since these vari­ average annual height growth of ous structures contrasted sharply almost 4 feet; mean height growth between the parent species, it was for the best F 1 averaged more than anticipated that the F 1 would show three feet per year. A severe wind­ several interm diate characteristics. storm in 1968 resulted in major This was true with respect to leaf stem breakage that affected the size, number of veins, hairiness and height growth ranking of progeny weight. Th F 1 samara was inter­ groups. Damage was particularly mediate in amount of pubescence, severe among the taller F 1 hybrid depth of apical notch, stalk length trees. and fruit width. The floret of the Height growth cessation of seed­ F1 was intermediate in length and lings in 8 progeny groups was ob­ in the number of stamens. The num­ served in the fall of 1965. Only min­ ber of florets in a single inflores­ or differences were noted between cence of the F 1 exceeded those of progeni s, though the range of ces­ either parent. Lengths of flower and sation varied and was greatest in vegetative buds in F 1 trees were in­ slippery elm. termediate in size to those of the Seeds having Chinkota elm as the two parents. Similarly flower and seed parent germinated more rapid­ vegetative bud sizes in the back­ ly than seeds having slippery elm as crosses and in the F 2 population the seed parent. w re intermediate between the par­ Seed from an open-pollinated ental types though the F 2 popula­ slippery elm tree growing adjacent tion was too small to show the full to Chinkota elm trees produced array of segregates. F 1 hybrid seed­ only F 1 hybrid seedlings from ling 1 aves averaged longer than 28 similar leaves of the parent species. fast growing tries for windbreaks On the basis of height growth, F 1 and shelterb Its in South Dakota. hybrids between Chinkota elm and Life span, maximum height and re­ slippery elm have shown superiority action to environmental pressures to the parental species. Since quan­ such as drought, diseases and in­ tity production of hybrid seed is sects remain to be ascertained as feasible, the F 1 off rs a source of test plantings of this hybrid are ob­ fast growth and improved height of served through the years.

Literature Cited --- 1950. Sixty-third Annual Sta­ Collins, Paul E. 1955. Chinkota tion Report, Agricultural Experi­ Elm. South Dakota Farm and ment Station, South Dakota State Home Res. 7:14-16. College. Darlington, C. D. and A. P. Wylie. --- 1954. Forest Tree Breeding in 1956. Chromosome Atlas of Flower­ Canada. J. of For. 52:682-5. ing Plants. Th MacMillian Com­ Aljbenskii, A. V. 1951. ( Results of pany, New York, N. Y. Hybridization of Larch, Maple, Dermen, H. and C. May. 1966. ColL Elm and Ash Trees). Trud. Inst. chiploidy of Ulmus pumila and Its Lesa. 8:88-94, 205. PBA 25:558. Possible Use in Hybridization with ( Original not seen). U. americana. For. Sci. 12:140-6. --- 1956. (New Hybrids of Trees). Les. Hoz. (Forestry) 9:15- Ehrenberg, C. E. 1954. Almsjukan 19. PBA 27: 2260. ( Original not Historik och Atgardn for Framstall­ seen.) ning av mot Sjukdomen Resistenta Alman. ( Elm disease. History and Anderson, M. L. Elm Disease. Na­ measures for the production of elms ture Land. 189:886-7. resistant to the disease.) Svenska Beatti , R. K. 1937. The Dutch Elm SkogsvForen. Tidskr. 52:35-41. Disease in Europ . Am. Forests PBA 14: 3384. ( Original not seen. ) 43: 159-161, 201. Engstrom, Harold E. and Lewis S. Britwum, S. P. K. 1961. Artificial Matthew. 1942. Effects of the 1940 Hybridization in the Genus Ulmus. Armistice Day Freeze on Siberian Proc. 8th Ntheast. For. Tre lmpr. Elm in the Plains Country. J. of For. Cof., New Haven, Conn. 1960, pp. 40:704. 43-7. Gerhold, H. D., E. J. Schreiner, R. Clapper, R. B. 1952. Breeding and E. McDermott and J. A. Winieski. Establishing New Tr es Resistant 1966. Breeding Pest - Resistant to Disease. Econ. Bot. 6:271-93. Tre s. Pergamon Press. Oxford, Eng. --- and J. M. Miller. 1949. Breed­ ing and Selecting Pest-resistant Goidanich, G. 1938. ( Notes on Re­ Trees. Yearbk. Agri. U. S. Dept. searches in the Selection of Elms Agric. pp. 465-71. Resistant to Elm Disease.) Ital. 29 Agric. 75:69-74. PBA 8: 1347. ( Orig­ --- 1944. The British Elm Flora. inal not seen. ) Nature, Land. 153: 198-9. Graves, A. H. 1948. Forest-Tree --- 1960. A Metrical Study of Breeding. Econ. Bot. 2: 284-305. Leaf-Shape in Hybrids: I. The Leaf­ shape of some F 1 Hybrids and Their Harlow, W. M. and E. S. Harrar. Parents. Kew Bull. 14:88-102. 1968. Textbook of Dendrology. 5 ed. McGraw-Hill Book Co., New Peace, T. R. 1939. The Resistance of York, N. Y. Elms to the Disease Caused by Oph­ Hartley, E. P. 1927. Forest Genetics iostoma ( C eratostomella) ulmi. Inst. with Particular Reference to Dis­ Leaflet No. 2, pp. 4. Imperial For. ease Resistance. J. of For. 25:667- Insti., Oxford, Eng. 86. Rehder, Alfred. 1940. Manual of Heybroek, H. M. 1962. Elm Breed­ Cultivated Trees and Shrubs. 2nd ing in The Netherlands. Silvae Ed. The MacMillan Company, New Gent. 6: 112-7. York, N. Y. --- 1963. ( The "Groeneveld" Hichens, R. H. 1945. Forest Tree Elm.) Ned. Bosh. Tijdschr. 35:370- Breeding and Genetics. Joint Pub. 4. ( Trans. in Plant Dis. Reptr. 48: Imp. Agric. Bur. No. 8. 187.) --- 1955. Studies on Ulmus I. The Johnson, L. P. V. 1939. The Breed­ Range of Variation of East Anglian ing of Forest Trees. For. Chron. 15: Elms. Watsonia 3: 138-53. 139-51. --- 1956. Elms. New Biol. 20:7-29. --- 1946. Fertilization in Ulmus with Special Reference to Hybridi­ --- 1958. Studies on Ulmus: II. zation Procedure. Canad. J. Res. 24 The Village Elms of Southern Cam­ ( Sect. C) 1-3. bridgeshire. For. 31: 132-46. --- and C. Heimburger. 1946. --- 1959. Studies on Ulmus: III. Preliminary Report on Interspecific The Village Elms of Hertfordshire. Hybridization in Forest Trees. Can­ For. 32: 138-54. ad. J. Res. 24 ( Sect. C) 308-12. --- 1961a. Studies on Ulmus: IV. Larsen, C. Syrach. 1956. Genetics in The Village Elms of Huntingdon­ Silviculture. Essential Books, Inc. shire and a New Method for Ex­ Fair Lawn, N. J. ploring Taxonomic Discontinuity. For. 34:47-64. Maxon, Marcus A. 1951. Siberian Elm. South Dakota Farm and Home --- 1961b. Studies on Ulmus: V. Res. 2:28-30. The Village Elms of Bedfordshire. For. 34: 181-200. Melville, R. 1937. The Accurate Definition of Leaf Shapes by Rect­ Hackwell, F. I. 1945. What Kind of angular Coordinates. Ann. of Bot. Trees Shall I Plant? N. & S. Dak. N Series 1: 673. Hort. 18:04. 30 Sax, Karl. 1933. Chromosome Num­ Went, J. E. 1938. Compilation of bers in Ulmus and Related Genera. the Investigations on the Suscepti­ J. of Arn. Arb. 14:82-84. bility of Different Elms to C erato­ Schreiner, E. J. 1937. Improvement stomella ulmi Buis. in The Nether­ of Forest Trees. U. S. Dept. Agric. lands. Phytopath. Zeit. 11:181-201. Yearbk. pp. 1241-79. ---1954. The Dutch Elm Disease. Smucker, S. J. 1941. Comparison of Tijdschr. o. PlZiekt. 60: 109-27. Susceptibility of the American Elm and Several Exotic Elms to C erato­ --- 1955. Verslag van de onder­ stomella ulmi. Phytopath. 31:758-9. zoekingen over de iepenziekte en andere boomziekten, uitgevoerd op Stockwell, P. and F. I. Righter. 1947. bet Phytopathologisch Laborator­ Hybrid Forest Trees. U. S. Dept. ium "Willie Commelin Scholten" te Agric. Yearbk., 1943-1947, pp. 465- Baarn, gedurende 1952 en 1953. 72. ( Report on investigations on Elm Swingle,R. U., B.S. Meyer and Cur­ disease and other diseases at the tis May. 1945. Phloem Necrosis Re­ "Willie Commelin Scholten" Phy­ search during '1944. Phytopath. 35: topathological Laboratory, Baarn, 489. 1952-53). Meded. Com. Ziekt. Boomsoorten 48. PBA 28:4752. ( Or­ Webb, W. E. 1948. A Report on Ul­ iginal not seen.) mus pumila in the Great Plains Re­ gion of the United States. J. of For. Wyman, Donald. 1951. Elms Grown 46:274-78. in America. Arnoldia 11: 79-93.

31