860 BOTANY: J. CLAUSEN PROC. N. A. S. 20 hr. The same would not be true, however, if both groups of cells were exposed to 200 r/day. Summary.-Experiments were performed with seedlings of Pisum sativum to test the hypothesis that radiosensitivity with respect to chromosome damage in- creases during chronic irradiation with increased mitotic cycle time. Mlinimum cycle times were determined for root meristem cells. Temperatures of 300, 250, 150, and 100C were used to obtain different minimum cycle times. The results of chronic exposures indicated that (1) the minimum cycle time was not altered in Pisum sativum until the total exposure per cycle exceeded about 440-450 r; (2) the percentage of cells showing damaged chromosomes at a stated daily exposure rate increased with decreasing temperatures if mitotic cycle time was not con- sidered; but (3) when the cycle time is taken into account, the production of dam- aged cells is a linear function of the amount of total exposure per mitotic cycle up to the critical value 440-450 r/cycle. The authors wish to thank Dr. D. Roy Davies and Mrs. Rhoda C. Sparrow for discussions and suggestions concerning this manuscript, and Dr. Keith Thompson for performing the necessary statistical analyses. * Research was carried out at Brookhaven National Laboratory under the auspices of the U.S. Atomic Energy Commission. I Sparrow, A. H., and H. J. Evans, in Fundamental Aspects of Radiosensitivity, Brookhaven Symposia in Biology, No. 14 (1961), p. 76. 2 Sparrow, A. H., R. L. Cuany, J. P. Miksche, and L. A. Schairer, Radiation Botany, 1, 10 (1961). 3 Van't Hof, J., and A. H. Sparrow, Radiation Botany, in press. 4Van't Hof, J., G. B. Wilson, and A. Colon, Chromosoma, 11, 313 (1960). 5 Van't Hof, J., and G. B. Wilson, Chromosoma, 13, 39 (1962). 6 Brown, R., J. Exptl. Botany, 2, 96 (1951). 7 Sax, K., and E. V. Enzmann, these PROCEEDINGS, 25, 397 (1939). 8 Giles, N. H., A. V. Beatty, and H. P. Riley, Genetics, 36, 552 (1951). 9 Beatty, A. V., J. W. Beatty, and C. Collins, Am. J. Botany, 43, 328 (1956). 10 Evans, H. J., Intern. Rev. Cytol., 13, 221 (1962). 11 Wimber, D. E., Abstracts, Second Annual Meeting of the American Society for Cell Biology, San Francisco, 1962, p. 198. 12 Humphrey, R. M., W. C. Dewey, and Ann Cork, Radiation Res., 19, 247 (1963). 13 Evans, H. J., and A. H. Sparrow, in Fundamental Aspects of Radiosensitivity, Brookhaven Symposia in Biology, No. 14 (1961), p. 101. TREE LINES AND GERM PLASM-A STUDY IN EVOLUTIONARY LIMITA TIONS* BY JENS CLAUSEN DEPARTMENT OF PLANT BIOLOGY, CARNEGIE INSTITUTION OF WASHINGTON, STANFORD, CALIFORNIA Read before the Academy, Apiil 22, 1963 A rough estimate of the capacity of plants to adjust to extreme environments can be obtained through a survey of the location of tree lines in different parts of the world. This is because trees are subject to severe stresses from climate. Definition of Tree Line.-Tree lines and alpine vegetations are contiguous. Where the trees disappear, the alpines begin, rich in growth forms of the low-bush Downloaded by guest on September 25, 2021 VOL. 50, 1963 BOTANY: J. CLAUSEN 861 and cushion types, known as elfinwood or Krummholz. The tree line is where a tree species disappears or changes into its elfinwood race. Northern Hemisphere botanists usually have in mind a tree line of northern conifers. It has generally been accepted as a fact that the tree line rises toward the low latitudes. This assumption is valid only until about mid-latitude, where our northern conifers disappear. The germ plasm of the tree species is actually a highly important factor in determining its limits of growth. Factors That Influence Growth of Trees.-Data from all over the earth indicate that low shrubs and herbs have advanced farther toward high latitudes and high altitudes than have trees of the same genus or species. This fact suggests that unfavorable climates impose more severe limita- tions upon trees than upon shrubs and herbs. Trees face physical and physiological stresses in their upward transport of water from the ground, and the stresses increase with the height of the trees. The environment influences the water balance of trees: uptake of water is slowed down with lower temperatures, and loss of water through transpiration is increased at higher temperatures, in stronger wind, and under dry air conditions. Low temperatures can produce drying effect&, similar to those caused by drought; trees native to tropical latitudes, such as the coffee tree, may therefore show frost symptoms be- fore the freezing point has been reached. Dry, dead tops of trees are common at windy ocean shores, in arid regions, and in cold, wet ground at high latitudes and altitudes. Visible frost symptoms may therefore result from many causes. Heredity influences the degree of tolerance to frost. Hagem9 tested seedlings of Picea sitchensis (Bong.) Carr from coastal areas of western North America in outdoor sowings near Bergen, western Norway, at 600 North latitude. During successive years 94% of seedlings from latitudes 470 North in the state of Washington were damaged by frost, in contrast with only 7% from latitudes 55-58° in southern Alaska. The introductions from intermediate British Columbia had inter- mediate frost susceptibility, only 64% of their seedlings being damaged at Bergen. From this experiment it is obvious that Sitka spruce is composed of climatic races that differ in their 3us- ceptibility to frost. Inherited Responses That Mitigate Water Stresses in Trees.-Trees buffer the stresses that ex- treme environments impose upon their water balances' by developing adjustments such as: (1) low growth forms (elfinwood), effectively reducing evaporation through snow covering; (2) re- duced leaf surfaces, as in most conifers; (3) dropping of the leaves during the unfavorable season, accomplished through complicated growth processes; and (4) physiological compensations (very little is known about such adjustments in trees). These four groups of adjustments relate to the germ plasm and are regulated by genes. Combinations of such responses mitigate the stresses but do not enable a tree to grow every- where on the earth. By far the greater percentage of tree species have therefore remained within the favorable tropical regions, and only very few plant families have evolved trees adjusted to the extremely cold climates at high latitudes and high altitudes. Examples of Contrasts in Germ Plasms.-During 1953 the author was given an opportunity to compare low-latitude montane and alpine Brazilian vegetations with similar ones at higher latitudes and altitudes in the Sierra Nevada of Cali- fornia.2 (This visit was made possible by a cordial invitation from the Cultural Division of the Brazilian Ministry of Foreign Relations as a part of the Brazilian program for international exchange of scholars.) These two regions are botanically as different as any two on earth; in both, the montane ranges are forested, and alpine vegetations have evolved from local sources. At the 220 South latitude in Serra do Mar of the state of Rio de Janeiro, the tran- sition from the forest to the alpine planalto occurs at approximately 1,950 m altitude. This change was observed at two places: in the 2,300-m-high Serra dos Orgdos (Organ Mountains) and 100 km farther eastward at Mt. Itatiafa (highest in Brazil, Downloaded by guest on September 25, 2021 862 BOTANY: J. CLAUSEN PROC. N. A. S. TABLE 1 \EGETATION OF SERRA DOS ORGXOS NATIONAL PARK, SOUTHEAST BRAZIL, AT 220 LATITUDE* Numuber of Species Total, Woooy Total, Family groups (orders), by Englerian numbers Trees plants all kinds Coniferae Monocotyledonae: 4. Glumiflorae: panicums, cortaderias (no Carex) 4 35 5. Principes: palms 6 6 6 8. Farinosae: bromeliads, Xyridaceae, eriocaulons - 59 9. Liliiflorae: amaryllis, vellozias, dioscoreas (no lilies; no rushes) - 3 15 11. Microspermae: orchids, burmannias - 226 Dicotyledonae-Chloripetalae: 2. Piperales: pepper relatives 1 27 28 3-11. Willows, walnuts, birches, oaks, beeches, chest- nuts 12. Urticales: figs, mulberry, nettle, elm relatives 12 13 18 13. Proteales: roupalas 7 7 7 18. Ranales: laurels, drimys, buttercups, etc. 11 49 51 21. Rosales: saxifrages, cunonias, roses, legumes 40 57 73 23. Geraniales: cocas, simarubes, mahoganies, malpighias, euphorbias 21 53 84 24. Sapindales: hollies, cupanias (no maples) 16 26 28 25-26. Rhamnales, Malvales: gouanias, sloaneas, mal- lows 5 10 22 27. Parietales: camellias, guttifers, violets, fla- courtias, begonias 22 38 80 28. Opuntiales: cacti (predominantly epiphytes) - 14 29. Myrtales: myrtles, melastomes 82 204 214 Dicotyledonae-Sympetalae: 1. Ericales: clethras, heathers, blueberries 4 24 24 6. Tubiflorae: borrages, verbenas, labiates, night- shades, figworts, bignonias 23 74 171 8. Rubiales: camphor and coffee relatives 29 58 84 10. Campanulatae: lobelias to sunflowers 10 61 142 21 orders, subtotal 289 714 1,381 14 other orders 29 30 194 35 orders, 129 families, total 308 744 1,575 19.5% 47.2%/c * Altitudes, 925-2,300 in. Tree line at 1,950 nm. 2,800 m). To a Northern Hemisphere botanist this is an incredibly low tree line at that latitude. Serra dos Orgaos was visited in company with Dr. C. T. Rizzini of the Rio de Janeiro Botanical Garden, to whom I owe gratitude for many courtesies. After intensive field studies he published a complete list of the plants within the park in 1954.29 Altitudes vary between 925 and 2,300 m, providing a rugged terrain in the area composed of gneiss and granite estimated to be approximately 160 km2. The lower slopes are covered by dense cloud forests that are replaced by an alpine bush and grass vegetation at ca. 1,900-2,000 m. An analysis of the Organ Mountain vegetation has now been made (Table 1).