IAWA Journal, Vol. 14 (3), 1993: 289-298

COMPARATIVE ANATOMY OF THE SECONDARY PHLOEM OF TEN SPECIES OF ROSACEAE

by

Uu Donghua1 and Gao Xinzeng2

Summary Materials and Methods The anatomy of the secondary phloem of The barks used for this investigation were species belonging to four genera in Rosaceae obtained from the trees grown on the campus is described. The three genera of the Maloi­ of Peking University, Peking, China. The deae studied are more or less similar in their collection of materials began September 15, phloem anatomy; tangential bands of fibre­ 1991. All the sampies were taken from trunks sclereids alternate with bands of sieve ele­ (3 trees each species) at 1 m height of appar­ ments, companion cells and parenchyma ently healthy trees over 10 years in age. Sci­ cells; superficially, the nonconducting and entific names of the trees studied are shown conducting phloem are not distinct from one in Table 1. another; sieve plates are compound and there Small pieces of about 5 mm 3 were re­ are conspicuous sieve areas on lateral walls; moved from bark sampies (15 mm3 ), fixed rays are uniseriate and multiseriate, and homo­ in FAA, and then embedded in paraffin. From cellular. In the five species of Prunus (Prun­ representative bark of each specimen, trans­ oideae) studied, there are no fibre-sclereids in verse, radial and tangential seetions of 10-12 the conducting phloem, end walls bearing sim­ 11m thickness were obtained with a sliding ple sieve plates are oblique to nearly horizon­ microtome. The seetions were stained with tal; and rays are uniseriate and multiseriate, safranine and fast green. homocellular. Macerations were obtained with a solution of 1 part 10% chromic acid and 1 part 10% Key words: Rosaceae, secondary phloem, nitric acid, and then stained with star blue. sieve elements, rays, phloem anatomy, Quantitative data were obtained from 30 cells crystals, fibre-sclereids. of each species.

Introduction Results From an anatomical point of view, both The results show that the genera of Maloi­ phloem and xylem are considered important deae are similar to each other in the structure and useful for identification and evaluating of their secondary phloem and that there are the taxonomie position of plant species. Many differences between Prunus and the genera of studies have been published on phloem anat­ Maloideae (Figs. 1-6). omy (e.g., Esau 1939, 1950, 1979; Evert 1960, 1961, 1963a, b; Davis & Evert 1968, Sieve elements and companion cells 1970; Derr & Evert 1967; Furuno 1990). Sieve plates are divided into two types, This study is aimed at increasing our un­ compound sieve plates and simple sieve derstanding of the anatomical features of the plates, according to the degree of specialisa­ secondary phloem in ten species in four gen­ tion of the end walls of sieve elements. The era of the Rosaceae. three genera of the Maloideae show the fol-

1) Departrnent of Biology, Tianjin Normal University, Tianjin 300 074, People's Republic of China. 2) Department of Biology, Peking University, Peking 100 871, People's Republic of China.

Downloaded from Brill.com09/27/2021 08:11:32PM via free access IV'0 Table 1. Anatomical characters ofthe secondary phloem of 10 species ofthe family Rosaceae from China. 0

Sieve elements Ray parenchyma cells Parenchyma cells Sclerenchyma I I I I cells

Uniseriate rays Multiseriate rays Fibre-sclereids 11

S- ~ .:; 'il ~ E S- -;;- oS .:; I -a ~ ~ .:; I ~ ,,~ I !l > 1l I .., .., I '" ~ ~ ] j !l oS -a " ~ .!I .... -5 -5 ...." (;' co .~ 0 " !;j ~ .~ .;"f~:9 0 e ~ ::~~ Ii & .0 Jj '6 " ..c: ci 1 JJ '6 o " S ~ " .., a'. a'.~8 ! " .~ '"~ :2 ~ "~ .* .ll.~~ "~ 1l " e'O -5 " I ~ " :l ~ e ~ ~ ~ ~ " ~ ~ "0 § .E ~ ~ s : .~ " . ] ~ " .!2 1 I0 > "> 0 > ~ .t:: > .( .( CI) .( .( .( u iE. < :I:

Prunus armeniaca L. 272 35 + 144 2-12 11 670 5-62 67 2-6 + + 7.3 + + + 862 15 Prunus cerasifera Ehrh. var. pissardii Koehn. 461 26 + 153 2-16 12 638 7-78 63 2-5 + 9.8 + + + 714 16 262 24 138 2-17 15 599 8-80 53 2-6 8.3 1097 16 Prunus davidiana Franch. + + + + + + ..... Prunus persica Batch. 243 33 + 159 2-17 20 562 5-60 101 2-7 + + 5.6 + + + 740 24 > Downloaded fromBrill.com09/27/2021 08:11:32PM var. rubro-plena Schneid. 268 24 + 145 1- 9 18 579 6-55 111 2-4 + 5.4 + + + 845 16 ~ ..... 0 Maloideae $::: ::I Malus pumiUa Mill. 409 23 + 12.2 90 1- 8 12 189 3-21 27 2-3 + 10.8 + + 419 18 ~ <: Malus spectabilis Borkh. 476 25 + 12.5 84 2-12 17 208 4-37 32 2-3 + 9.3 + + 812 17 ~ Pyrus betulaefolia Bunge 470 20 + 12.5 128 2-17 12 184 4-24 21 2-3 + 10.3 + + 460 19 + ...... />0. Pyrus bretschneideri Rehd. 424 30 + 12.2 147 2-25 16 299 4-31 32 2-3 + 10.7 + + 621 22 ,-... ~ Crataegus pinnatifida Bunge 316 24 + 11.4 123 2-18 14 244 8-40 28 2-3 + 12.5 + + 290 19 + via freeaccess

I~ Liu Donghua & Gao Xinzeng - Phloem of Rosaceae 291 lowing features: the length of sieve elements parenchyma cells occur in more or less regu­ is over 316 Jlm on average (Table 1); the end lar tangential bands one celllayer in depth, walls, which bear compound sieve plates, are altemating with uniseriate layers of sieve ele­ extremely inclined and can hardly be distin­ ments and their companion cells (Fig. 5). The guished from lateral walls so that the cells are parenchyma cells in Prunus are irregular in truly fusiform; the sieve plates bear many arrangement (Fig. 9). sieve areas, ranging from 11.4 to 12.5 Jlm on With the cessation of function of the sieve average (Table 1); the lateral sieve areas are elements and the companion cells, the paren­ conspicuous, rounded or elliptical in appear­ chyma cells continue to increase in size both ance, and scalariformly arranged (Fig. 7); and radially and tangentially in nonconducting the sieve elements of the conducting phloem phloem. Some parenchyma cells develop into are squarish or nearly rounded in transection fibre-scJereids. In the species of Maloideae, (Figs. 2, 4, 5), while those of the noncon­ the parenchyma cells and the rays form a net­ ducting phloem become irregular in outline as like pattern in transection (Fig. 5). their radial walls gradually collapse (Fig. 4). Crystal-conraining cells - Crystal-con­ The sieve elements in the Prunus species taining cells were found in all the sampies in­ are mostly cylindrical and the end walls, which vestigated. There are some regularities in their bear simple sieve plates, are oblique to nearly morphology, arrangement and distribution. horizontal, and rare compound sieve plates Prismatic and styloid crystal-containing cells are found in P. cerasifera var. pissardii and occur in the three Maloideae genera, but druse P. persica. The lateral sieve areas are incon­ crystal-containing cells characterise the Pru­ spicuous (Fig. 8). The average length of sieve nus species. The prismatic and styloid crystal­ elements is much shorter than that of the sieve containing cells are distributed in two ways: elements of the species of Maloideae, except I) linked in strands where each chamber con­ that of P. cerasifera var. pissardii (Table 1). tains a solitary prismatic crystal which is The sieve elements of the conducting phloem slightly longer than wide and parallel to the appear almost rounded in transection (Fig. 9). sieve elements in arrangement, as seen in ra­ The sieve tubes of the nonconducting phloem dial section (Fig. 10); 2) spatially associated collapse and are obliterated soon after becom­ with the tangential bands of phloem fibre­ ing nonfunctional and the nonconducting scJereids (Fig. 11). The druse crystal-con­ phloem contains large intercellular spaces taining cells appear scattered in transection. (Figs. 6, 9). Ray parenchyma cells - Phloem rays are The companion cells are shorter than the mostly uniseriate and multiseriate and homo­ sieve elements in all species examined. They cellular in all species investigated. There are appear nearly triangular in transection and are differences in height and width of rays among located on one side of the sieve elements. the genera (Figs. 12, 13). Multiseriate rays in Generally , each sieve element has one to two the three genera of Maloideae range from 21 companion cells in species of the Maloideae, Jlm to 32 Jlm in width (2- or 3-seriate) and or two to three companion cells are arranged 184 Jlm to 299 Jlffi (21 to 40 cells) in height, in strands in Prunus. The companion cells but those in the Prunus species are much die when the sieve element protoplasts are larger (Table 1). Uniseriate rays in Prunus disorganised and the walls common to both are composed of procumbent cells. Druse cells collapse. crystal-containing cells were found only in the multiseriate rays of Prunus. In addition, Parenchyma in Pyrus some ray parenchyma cells adjacent Axial parenchyma cells - In the three gen­ to the fibre bands are sclerified Pits are clear­ era of Maloideae the phloem parenchyma cells ly discernible in the thickened cell walls. The in the conducting phloem appear rounded in rays do not become dilated in the nonconduct­ transection. The phloem parenchyma cells ing phloem, but rather become curved to that are relatively dark in appearance occur in varying degrees. The rays of the Maloideae the late phloem, and those with relatively light curve slightly (Fig. 4) and those of Prunus contents are located in the early phloem. The strongly (Fig. 6).

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Sc/erenchyma lern) consists of more celllayers (Fig. 6); the The fibre-sclereids in the secondary phlo­ phellern cells are flat in the radial direction as em of Maloideae are grouped in tangential seen in transection. The cells are arranged bands which alternate with bands of sieve closely and contain dark materials. In the elements, companion cells and parenchyma Maloideae studied, the phellern consists of 4 cells (Fig. 2). In Pyrus betulaefolia and Cra­ to 8 layers of radially flattened cork cells. The taegus pinnatifida the bands of fibre-scle­ celllumina are filled with tanniniferous mate­ reids, crystal-containing cells and sclereids rial (Malus pumilla). The phelloderm in all alternate with the bands of sieve elements, the species studied is composed of 2 or 3 cell companion cells and parenchyma cells (Fig. layers in which protoplasm is dense and nuclei 5). In the Prunus species, fibre-sclereids are can be seen clearly. absent in the conducting phloem and sparse (P. persica and P. persica var. rubro-plena), Discussion and grouped in the nonconducting phloem. No sclereids were observed in Prunus. As Comparison 0/ secondary phloem can be seen from Table 1, the average length The sieve elements are considered to be of fibre-~clereids in the Prunus species is the most important phloem components to greater than that in the Maloideae. understand phylogenetic trends, although other elements such as parenchyma and scle­ Periderm renchyma are generally present as weIl. Some All the species studied have periderm. The research workers (Cheadle & Whitford 1941; periderm in Malus spectabilis, Pyrus bret­ Cheadle 1948; Cheadle & Uhl 1948; Esau schneideri and Crataegus pinnatifida arises 1950; Zahur 1959) stated that the primitive from the cortex (Fig. 5) and in the rest origi­ characters include long sieve elements, very nates from phloem parenchyma cells in the oblique sieve plates and bear ten or more sieve secondary phloem (Fig. 6). In Prunus the areas, that the medium-evolution features periderm is thicker and the cork layer (phel- (text continued on page 297)

Legends 0/ Figures 1-13:

Lettering used on the photographs: c = cortex; cy = crystal; cz = cambial zone; fs =fibre-scle­ reids; np = nonconducting phloem; p = parenchyma cells; per = periderm; r = ray; se = sieve elements.·

Figs. 1 & 2. Light micrographs of successive growth increments in the bark of Malus pumilla, showing cambial zone, secondary phloem and periderm, as seen in transection; x 280.

Figs. 3 & 4. Light micrographs of successive growth increments in the bark of Pyrus bret­ schneideri, showing cambial zone, secondary phloem, cortex and periderm, as seen in tran sec­ tion; x 260.

Figs. 5 & 6. Light micrographs of transections of barks, showing cambial zone, secondary phloem and periderm; x 260. - 5: Crataegus pinnatifida. - 6: Prunus persica var. rubro-plena.

Figs. 7-13. Showing secondary phloem in transverse, radial and tangential sections. -7: Pyrus betulaefolia in radial section; x 200. - 8: Prunus persica in radial section; x 550. - 9: Prunus da­ vidiana in transverse section; x 510. - 10: Polarised micrograph, showing crystals in parenchyma strands in radial section (Malus pumilla); x 530. - 11: Polarised micrograph, showing crystal­ containing cells adjacent to fibres in radial section (Crataegus pinnatifida); x 600. - 12: Crataegus pinnatifida in tangential section; x 260. - 13: Prunus armeniaca in tangential section; x 260.

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Downloaded from Brill.com09/27/2021 08:11:32PM via free access Liu Donghua & Gao Xinzeng - Phloem of Rosaceae 297 include sieve elements of medium length, and subsequently differentiate in nonfunction­ oblique sieve plates and bear two to ten sieve al phloem. We observed that the sclerenchyma areas, and that advanced characters include cells in the species studied are fibre-sclereids short sieve elements, slightly oblique to trans­ which develop in the nonconducting phloem verse sieve plates and bear one sieve area. which is in agreement with the early findings Evert (1963) indicated that the sieve elements of Evert (1960, 1963). of Pomoideae (Maloideae) might be charac­ terised as long, slender cells with greatly in­ Relationship of anatomical features between clined end walls bearing much-compounded phloem and xylem sieve plates and that the sieve elements of Many researchers (Cheadle & Whitford Pomoideae (Maloideae) were relatively prim­ 1941; Cheadle 1948; Esau et al. 1953; Zahur itive among angiosperm sieve elements, after 1959) suggested that there is evolutionary ad­ he investigated Pyrus communis (1960) and vancement in sieve-tube members in the an­ Pyrus malus (1963). giosperms, e. g., a change from oblique to The investigated speeies of Rosaceae in more horizontal end walls, associated with an the present study can be clearly separated into increase in speeialisation of sieve areas on end two groups. The sieve elements with simple walls and a decrease in their development on plates in horizontal end walls occur in the side walls. Bailey (1953) and Carlquist (1975) Prunus speeies, and those with compound found that the change from compound to plates in oblique end walls belong to the gen­ simple sieve areas paralleis the trends for tra­ era of Maloideae. As for the anatomical fea­ cheary elements in the xylem. The results of tures of sieve elements in the Maloideae, this the present study, incidentally, agree to some is in agreement with early findings of Evert extent with the recent findings of Zhang and (1960, 1963). Zahur (1959) classified the Baas (1992) and Zhang (1992), in which distributional patterns of the parenchyma cells wood anatomy of the Rosaceae was investi­ in secondary phloem as banded (primitive) gated. For example, in Prunus advanced sieve and irregular (advanced). However, Roth elements with simple sieve plates correlate (1981) suggested that an irregular arrangement with somewhat specialised xylem fibres with of the parenchyma and of other cell types is their pits confined to the radial walls, whilst relatively primitive. The results in this experi­ the compound sieve plates of the Maloideae ment show that the parenchyma cells in Prunus are associated with fibre-tracheids with dis­ are irregular in arrangement and those in the tinctly bordered pits in the radial and tangen­ three genera of Maloideae are banded. We are tial walls. Xylem vessel perforations are pre­ not sure which type is advanced or primitive. dominantly simple in all Rosaceae, so there is· Supplying a great number of concrete ex­ no correlation between the morphology of amples (including Rosoideae: Rubus) where conducting cells in the xylem and the phloem. fibre development in the functioning phloem The type and distribution of crystals in the could be observed, Zahur (1959) indicated secondary phloem of the species studied are that true fihres develop generally in function­ more or less similar to the findings of Zhang ing phloem. Evert (1963) stated that the scle­ and Baas (1992) for the secondary xylem. renchyma cells should be divided into three For instance, druses were found in the spe­ types: 1) phloem fibres, which derive directly eies of Prunus, and prismatic crystals in the from fusiform cambial initials, 2) fibre-scle­ species of Maloideae. reids, arising from parenchyma cells in non­ From what is indicated above, we suggest functional phloem, and 3) sclereids develop­ that phloem anatomy is as important and valu­ ing in either functional or nonfunctional able as wood anatomy in exploring diversity phloem and arising from parenchyma cells patterns of systematic and evolutionary sig­ which may or may not become variously nificance. branched. Evert (1963) also indicated that the sclerenchyma cells in apple are fibre-sclereids. Aclmowledgmnents They develop from parenchyma cells which This project was supported by the Nation­ overwinter on the margin of the cambial zone al Science Foundation of China.

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