860 BOTANY: J. CLAUSEN PROC. N. A. S.

20 hr. The same would not be true, however, if both groups of cells were exposedto 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. Mlinimumcycle times were determined for root meristem cells. Temperatures of 300, 250,150, and 100C were used to obtain different minimum cycle times. The results ofchronic exposures indicated that (1) the minimum cycle time was not altered inPisum sativum until the total exposure per cycle exceeded about 440-450 r; (2)the percentage of cells showing damaged chromosomes at a stated daily exposurerate 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 upto 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 necessarystatistical analyses.

* Research was carried out at Brookhaven National Laboratory under the auspices of theU.S. Atomic Energy Commission.

I Sparrow, A. H., and H. J. Evans, in Fundamental Aspects of Radiosensitivity, BrookhavenSymposia 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 EVOLUTIONARYLIMITA TIONS*

BY JENS CLAUSENDEPARTMENT 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 canbe obtained through a survey of the location of tree lines in different parts of theworld. 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

VOL. 50, 1963 BOTANY: J. CLAUSEN 861

and cushion types, known as elfinwood or Krummholz. The tree line is where atree species disappears or changes into its elfinwood race.

Northern Hemisphere botanists usually have in mind a tree line of northernconifers. It has generally been accepted as a fact that the tree line rises towardthe low latitudes. This assumption is valid only until about mid-latitude, whereour northern conifers disappear. The germ plasm of the tree species is actuallya highly important factor in determining its limits of growth.

Factors That Influence Growth of Trees.-Data from all over the earth indicate that low shrubsand herbs have advanced farther toward high latitudes and high altitudes than have trees of thesame 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 theground, and the stresses increase with the height of the trees. The environment influences thewater balance of trees: uptake of water is slowed down with lower temperatures, and loss of waterthrough transpiration is increased at higher temperatures, in stronger wind, and under dry airconditions. 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 oceanshores, in arid regions, and in cold, wet ground at high latitudes and altitudes. Visible frostsymptoms 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, westernNorway, at 600 North latitude. During successive years 94% of seedlings from latitudes 470North in the state of Washington were damaged by frost, in contrast with only 7% from latitudes55-58° in southern Alaska. The introductions from intermediate British Columbia had inter-mediate frost susceptibility, only 64% of their seedlings being damaged at Bergen. From thisexperiment 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 (verylittle is known about such adjustments in trees). These four groups of adjustments relate tothe 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 withinthe favorable tropical regions, and only very few plant families have evolved trees adjusted tothe extremely cold climates at high latitudes and high altitudes.

Examples of Contrasts in Germ Plasms.-During 1953 the author was given anopportunity to compare low-latitude montane and alpine Brazilian vegetationswith 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 CulturalDivision of the Brazilian Ministry of Foreign Relations as a part of the Brazilianprogram for international exchange of scholars.) These two regions are botanicallyas different as any two on earth; in both, the montane ranges are forested, andalpine 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 1,950m 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,

862 BOTANY: J. CLAUSEN PROC. N. A. S.

TABLE 1\EGETATION OF SERRA DOS ORGXOS NATIONAL PARK, SOUTHEAST BRAZIL, AT 220 LATITUDE*

Numuber of SpeciesTotal,Woooy Total,

Family groups (orders), by Englerian numbers Trees plants all kindsConiferaeMonocotyledonae:

4. Glumiflorae: panicums, cortaderias (no Carex) 4 355. Principes: palms 6 6 68. Farinosae: bromeliads, Xyridaceae, eriocaulons - 599. Liliiflorae: amaryllis, vellozias, dioscoreas (no

lilies; no rushes) - 3 1511. 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 1813. Proteales: roupalas 7 7 718. Ranales: laurels, drimys, buttercups, etc. 11 49 5121. Rosales: saxifrages, cunonias, roses, legumes 40 57 7323. Geraniales: cocas, simarubes, mahoganies,

malpighias, euphorbias 21 53 8424. 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) - 1429. Myrtales: myrtles, melastomes 82 204 214

Dicotyledonae-Sympetalae:1. Ericales: clethras, heathers, blueberries 4 24 246. Tubiflorae: borrages, verbenas, labiates, night-

shades, figworts, bignonias 23 74 1718. Rubiales: camphor and coffee relatives 29 58 84

10. Campanulatae: lobelias to sunflowers 10 61 14221 orders, subtotal 289 714 1,38114 other orders 29 30 19435 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 lineat that latitude.

Serra dos Orgaos was visited in company with Dr. C. T. Rizzini of the Rio deJaneiro Botanical Garden, to whom I owe gratitude for many courtesies. Afterintensive field studies he published a complete list of the plants within the park in1954.29 Altitudes vary between 925 and 2,300 m, providing a rugged terrain inthe 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 alpinebush and grass vegetation at ca. 1,900-2,000 m.An analysis of the Organ Mountain vegetation has now been made (Table 1).

It has 286 species of ferns and allies and 1,575 species of flowering plants that belongto 129 families. Most of the orders to which the most important northern tem-perate forest trees belong are absent.

In common with other forests at low latitude, the Organ Mountain forests havea high percentage of tree species and woody climbing shrubs. There are 308species of trees in addition to 21 species of tree fern, but none of them go above the1,950-m level. As indicated by Table 1, these trees are members of tropical ordersand families. The myrtle relatives alone constitute 26 per cent of the trees.

VOL. 50, 1963 BOTANY: J. CLAUSEN 863

The alpine vegetation above the 1,950-m level is composed of grasses and lowbushes, such as a small bamboo, a holly, an acacia, a baccharis, and myrtle andheather relatives. These resemble alpines of the higher-latitude families but arerelated to the trees in the forest lower on the mountain.The last trees before the alpine vegetation are such as Drimys brasiliensis AMiers

of the Winteraceae, Belangera and Weinmannia of the Cunoniaceae, and Tern-stroemia of the Camellia family. They show the crippling effects of occasionallight frosts but nevertheless are among the hardiest of the local tree species. Theybelong to families that have not evolved trees within the temperate zones.

Climatic records are available from the 1,025-m level within the cloud forest(Table 2, left),29 indicating that the lowest temperatures (7-10° C) during thewinter months are well above the freezing point. The center columns indicate thetemperatures at 2,200 m on Mt. Itatia'a, 1 slightly above the cloud forests and withinthe alpine planalto vegetation; during the winter the monthly mean minimaltemperatures are 5-11O C, well above freezing, but slight night frosts and snowsoccur occasionally, as indicated by lowest temperatures. All the 308 tree speciesare absent above this point, although the temperatures compare with those in theCalifornia outer Coast Ranges, where such trees as Sequoia sempervirens are nativeand grow to giant size.

In sharp contrast with the Brazilian forests is the situation at the 3,000-m altitudein the Sierra Nevada of California. Three large tree species thrive here in aclimate (Table 2, right) where the monthly average minimal temperatures remainbelow the freezing point during eight months of the year, and frost can occur anyday during all seasons. The trees survive temperatures as low as -30° C, andmust possess germ plasms of a remarkable hardiness.These three California trees are the lodgepole pine, Pinus murrayana Balf., a

relative of the North European Scotch pine; the whitebark pine, P. albicaulisEngelm., a relative of Pinus cembra L., the tree-line tree of the Swiss Alps and theAltai Mountains; and the mountain hemlock, Tsuga mertensiana (Bong.) Sarg.,which has close relatives across northern North America and at high altitudes inJapan and in the Himalayas.

TABLE 2TEMPERATURES (IN 'C) IN REGIONS OF Two CONTRASTING VEGETATIONS

South: 220 latitude North: 380 latitudeSoutheast Brazil, Serra do Mar - California, Sierra Nevada

Altitude: 1,000 m 2,200 m 3,000 mPrecipitation: 318.3 cm 241.7 cm 78.3 cm

Cloud forest Alpine scrub (planalto) Subalpine forest308 tree No tree species 3 tree species

speciesMean Mean Mean Mean

Months Lowest Highest Lowest minima maxima Months Lowest minima maximaJuly 7.1 21.6 -6.0 5.1 13.0 Jan. -31.7 -11.8 1.7Aug. 8.7 23.4 -3.4 6.0 14.3 Feb. -27.8 -11.8 2.8Sept. 10.4 23.7 -5.0 8.0 15.7 March -26.1 -9.6 4.8Oct. 11.5 25.7 -1.1 8.0 15.2 April -23.4 -6.9 7.0Nov. 12.5 23.7 -0.3 8.8 15.9 May -20.0 -3.3 10.6Dec. 12.3 24.3 2.4 10.2 16.2 June -11.7 1.2 15.3Jan. 18.8 28.2 3.7 11.0 16.7 July -3.3 5.7 20.0Feb. 14.8 29.0 2.4 11.0 16.8 Aug. -1.7 5.4 19.7March 14.6 27.3 2.2 10.8 16.4 Sept. -11.1 1.6 15.6April 11.8 23.1 0.3 8.7 15.2 Oct. -19.4 -2.2 10.6May 9.6 23.0 -3.0 6.6 13.9 Nov. -22.2 -6.2 6.2June 9.5 r2 .7 -4,8 5.8 13.4 Dec, -27.2 -9.3 3.1

864 BOTANY: J. CLAUSEN PROC. N. A. S.

The three Sierra Nevada conifers have their tree limits at slightly differentaltitudes and exposures of 3,075-3,330 m. They develop giant trees almost totheir tree lines, although within 50-75 m below the tree limits, each species iscomposed of multitudes of interspersed biotypes that range from low, horizontalelfinwood cushions through many intermediates to fully erect, large trees. Onlythe low elfinwoods of each remain beyond the tree line. The whitebark pine has thehighest tree line of these three, at 3,300 m. Some of its elfinwood plants have aspread of up to 20 m with a height of only 1 m.

The Group of Most Tolerant Tree Species.-The striking difference in tolerancebetween the Brazilian low-latitude forest and the extremely tolerant tree speciesof the high Sierra Nevada prompted a search through regional floras and otherbotanical literature6' 8,15-17, 22, 24-26, 32, 33 to discover those species that provide thetree outposts toward the high latitudes and altitudes. This search producedsome rather unexpected facts, which are listed below:

(1) Only about 12 species complexes have been able to evolve trees for extremelycold climates; (2) a tree species occupying extreme habitats in one part of theearth is likely to have one or several close relatives under different names in similarextreme habitats of the same altitudinal zone; (3) if a tree species grows at highlatitudes, it can also be expected to grow at high altitudes farther south in thetemperate zone; and (4) climatically highly tolerant species belong to taxonomically"critical" species complexes. Many transitional forms, interpreted as beingnatural hybrids, tend to obscure the limits between the species. Controlled hy-bridization experiments7' 10-13, 18, 19, 21, 27, 28, 31, 35 have also shown that the speciesof most of the 12 complexes intercross fairly freely, so that each species complexrepresents a large gene pool.The twelve highly tolerant species complexes occupy a broad belt between the

latitudes of approximately 35-680 North or occasionally to the 70th latitude;toward their south limits these species ascend to 3,000-3,700 m altitudes of high

TABLE 3MOST TOLERANT TREES, 12 SPECIES COMPLEXES (NORTHERN HEMISPHERE, HIGH LATITUDES,

35-70 NORTH),~~Europe -- .Asia --North Ainerica -

Pinales, all n = 12:1. Larches: Larix decidua sibirica gmelini laricina2. Pines: Pinus silvestris contorta murrayana -banksiana

montana-mughu3. Pinus cembra -pumila albicaulis4. Spruces: Picea excelsa-obovata-alcocquiana sitchensis- glauca mariana

smithii5. Firs: Abies nordmanniana - veitchii lasiocarpa balsamea6. Hemlocks: Tsuga- -araragi mertensiana -,canadensis

brunoniana heterophyllaCupressales, n = 11:

7. Junipers: Juniperus communis communisSalicales, n = 19:

8. Poplare: Populus tremula tremuloides9. Willows: Salix phylicifolia-pyrifolia-pulchra pulchra -arbuscula

Fagales, n = multiples of 7 in these:10. Birches: Betula tortuosa-cajanderi-ermani kenaica-papyrifera11. Alders: Alnus incana -Jruticosa crispa-incana

Rosales, n = 17 in this complex:12. Mountain

ash: Sorbus aucuparia- --sambucifolia scopulina-americana decorasitchensis

VOL. 50, 1963 BOTANY: J. CLAUSEN 865

mountains. Table 3 lists the members of the 12 complexes with the Europeanmembers to the left, the Asian in the center, and the North American ones tothe right. The species names within each complex serve as guides to the literature.The gametic numbers of chromosomes' are indicated.

Six of the members of this highly tolerant group are pine relatives, all having auniform number of 12 pairs of chromosomes. Another conifer is the circumpolarJuniperus communis L., n = 11, which has both erect trees and elfinwood forms.Two complexes belong to the willow family: the aspens, Populus tremula-irem-uloides, n = 19, and relatives of Salix phylicifolia L., having multiples of n = 19.Two others belong to the birch family, a polyploid complex of birches, Betula, andthe alders, Alnus. Finally, there are the mountain ashes, members of the Sorbusaucuparia complex of the rose family, n = 17.These are all Northern Hemisphere plants. In the Southern Hemisphere no

trees exist at comparable combinations of altitudes and latitudes. At least 7 ofthe 12 complexes have at one point or another evolved elfinwood forms beyondtheir tree line.

Trees of Medium Tolerance.-A much larger group of a few thousand tree specieshave moderate tolerances and are natives between the latitudes of 20-50 , primarilyin the Northern Hemisphere (Table 4). This group is characterized by the hun-dreds of species of oaks, beeches, and chestnuts of the oak family,23 having uni-formly 12 pairs of chromosomes and prevalent hybridity. With the oaks areassociated the yellow pines and the cedars, equally hybridizing. Compared withthe Brazilian low-latitude group of trees, the oaks, yellow pines, and cedars haverespectable cold tolerance, but they are definitely more tender than the trees ofthe northernmost group.The highly tolerant and medium tolerant groups of trees overlap many latitudes.

In central California, for example, Quercus kelloggii Newb. marks the oak tree lineat 2,000 m in the Sierra Nevada; it is followed by an elfinwood bush species, Q.vaccinifolia Kell., which ceases at 2,600 m altitude. The oak tree line is not seenbecause the conifers of the hardy group go much higher.One single genus of the beeches, Nothofagus, reaches moderate altitudes and fairly

high latitudes in the Southern Hemisphere, forming the tree line in southern Chile,New Zealand, and Tasmania. The incense cedars, Libocedrus, n = 11, are asso-ciated with them in the Southern Hemisphere and have one species in North

TABLE 4MEDIuM-TOLERANT GROUP, EXAMPLES OF TREE SPECIES (LATITUDE 20-55°, BOTH HEMISPHERES)Coniferae:Podocarpaceae: Podocarpus nubigena, to 510 S. in PatagoniaCupressaceae: Libocedrus, Northern and Southern Hemispheres, n = 11Pinaceae, n = 12:Yellow pines: Pinus ponderosa-jeffreyi-scopulorum

hartwegii-arizonicaCedars: Cedrus atlantica-libani-deodara

Fagales, n = 12 in these:Oaks: Quercus kelloggii, oak tree line at 2,000 m in Sierra Nevada

vaccinifolia, elfinwood, to 2,600 mLithocarpus, tree line in Java at 2,700 m (elfinwood)

Beeches: Fagus silvatica, tree line in Italy at 1,830 m (elfinwood)Nothofagus, tree lines in Chile, New Zealand, Tasmania

Myrtales:Eucalyptus, n = 11, tree line in Australia at 2,000 m

866 BOTANY: J. CLAUSEN PROC. N. A. S.

America. Another conifer, Podocarpus nubigena Lindl., has trees until 510 Southlatitude in Patagonia.30

Similar to the oaks of the Northern Hemisphere, one exclusively Australiangenus of the myrtle family, Eucalyptus, n = 11, has evolved multitudes of treespecies in many environments. It provides the tree line in the Snowy Mountainsat the 35th South latitude at an altitude of 2,000 m, comparing with the tree lineof the Kellogg oak in California. Several Eucalyptus species have successfullybeen introduced to the high plateaus around 3,500 m of equatorial regions of othercontinents where trees are scarce.

Trees of Low Latitudes.-By far the largest number of tree species, probablymore than 50,000, occur within the tropics and appear to be "trapped" within thelatitudes of about 250 North and 250 South. A sample of this group of trees wasdiscussed under the Serra dos Orgaos vegetation of the southeast Brazilian plateau(Table 1). It would be impossible here to consider in detail the highly variedgroup of low-latitude trees, and only some of its more tolerant and wide-ranginggenera are listed in Table 5.Some of the more tolerant species of this group transgress to the milder climates

of higher latitudes, where they grow near sea level. The three latitudinal groupsof trees are obviously somewhat arbitrary and connect by intermediates, but theyaid in visualizing great differences in ranges of tolerance.

Multiple Tree Lines.-Because each tree species or tree genus has its own treeline, multiple tree lines can exist. The areas of most tree species are telescoped,however, so that the multiple tree lines are not obvious. Asia Minor has an oaktree line at 925 m, followed by steppe and bush oaks to 1,850 m, in turn replaced bya coniferous forest that produces a coniferous tree line at 2,450 m altitude (ref. 23,p. 405).

In the Argentinian Andes at the latitude of Tucuman,20 30 the subtropicalevergreen forest has its limits at 1,100 m, followed by a high Patagonian-typedeciduous forest of Nothofagus antarctica (Forst.) Oerst. that develops a deciduoustree line at 1,850 m. Between 1,850 and 2,750 m is the puna, an "alpine" meadowvegetation of grasses and scattered cushion plants in a region with regular wintersnow. Above 2,750 m follows, on the Chilean side, an open Polylepis forest with arosaceous tree line at 3,700 m. Above this "forest" is a second alpine vegetation,the paramo, consisting of low, woody cushions.

TABLE 5Low-TOLERANcE GROUP, EXAMPLES OF MOST TOLERANT TREES (Low LATITUDE: 250 N. to

250 S.)Coniferae:Podocarpaceae: Podocarpus-Dacrydium species: Peru,34 New Guinea, 3,600 Mr4Araucariaceae, n = 13: Araucaria and Agathis

Ranales: Drimys species: Patagonia;R0 Brazil, 1,950 m;29 New Guinea, 4,200 Mr4Rosales:

Cunoniaceae: Weinmannia: Brazil, 1,950 m;29 Andes,34 Hawaii, New ZealandRosaceae: Polylepis leptophylla: dry Andean tree line, 3,700 m2R 34Leguminosae: Sophora: Andes ;34 Hawaii at 3,050 m; New Zealand

Sapindales:Aquifoliaceae: Ilex: Brazil,29 trees in cloud forest; elfinwood above tree lineAnacardiaceae: Schinus molle, high-altitude Andes34

Parietales: Ternstroemia and Clusia: Brazil, tree line at 1,950 mRubiales: Cinchona: cloud forests, eastern Peruvian Andes, 3,700 M34

VOL. 50, 1963 BOTANY: J. CLAUSEN 867

Evolutionary Limitations within Groups of Species.-Only a few families of plants,such as the relatives of the nettles, roses, legumes, and the violets, have beenversatile enough to adjust to almost all latitudes from the tropics to the arctic andantarctic. In doing so, however, we find that the adjustment is accompanied bya change in growth form from trees to herbs.Most families have remained within certain latitudinal zones. The approximately

5,000 species of myrtle relatives and also many other plant families have remainedwithin the low latitudes, although their genera have crossed oceans and haveevolved species adjusted to many local niches within those latitudes.

Similarly, the early progenitors of the pine relatives existed in temperateEurope since early Cretaceous times. Pine relatives still cover the temperatenorthern latitudes (Table 3), occasionally "spilling over" slightly to lower latitudes,but generally they do not cross the equator. The physiologically adjusted hereditiesof pine and myrtle relatives have kept them confined within the wide belts of theirrespective environmental zones during the long geologic periods when they movedaround the earth.The families and orders were classified by taxonomists who used morphological

rather than physiological criteria. During the immense evolutionary periods thatthe tree groups have existed, the cohesive forces of heredity have been strongenough to maintain linkages between morphological and physiological characters.Moreover, mutations that would have radically changed the tolerance ranges wereeliminated or prevented.

Strong cohesive forces have been shown to exist within the germ plasms of races,species, genera, and families of plants.34 Each tree species is highly variable,but its variability does not permit it to be successful in an entirely different zone.A given germ plasm has therefore definite evolutionary limitations beyond whichit has not been able to move.Summary.-The most important factors determining tree growth in various

climates are inherited differences in the germ plasm. Each tree species has its owntree line, and some have evolved low elfinwood races beyond the tree limits. Themost widely distributed trees belong to species complexes that are known for theirability to hybridize.The tolerance groups of trees follow major systematic orders. These are based

on morphological characters, which apparently are associated with physiologicalcharacters of adaptive importance.

* Reference is made to regional floras throughout this paper.lAnonymous, Normais climaticas (Rio de Janeiro: Servico de Informacion Agricola, Metero-

logia, Ministerio da Agricultura, 1941).2 Clausen, Jens, "Evolutionary differentiation at tropical latitudes" in Carnegie Institute of

Washington Year Book, No. 53 (1954), pp. 162-164.3Clausen, Jens, and Wm. M. Hiesey, in Carnegie Inst. Wash. Publ., p. 615 (1958).4Clausen, Jens, and Wm. M. Hiesey, these PROCEEDINGS, 46, 494-506 (1960).5Darlington, C. D., and A. P. Wylie, Chromosome Atlas of Flowering Plants (New York:

Macmillan Co., 1956).6 Daubenmire, R. F., Botan. Rev., 9, 323-393 (1943).7Duffield, J. W., and F. I. Righter, in Forest Res. Notes (Calif. Forest and Range Exp. Sta.,

1953), Bull. 86.8 Graham, Edward H., "Botanical studies in the Uinta Basin of Utah and Colorado," Ann.

Carnegie Museum, vol. 26 (1937).

868 BOTANY: J. CLAUSEN PROC. N. A. S.

9 Hagem, Oscar, Medd. Vestlandets Forst. Fors#ksst., 4, 217 pp. (1931).10 H~kansson, Artur, Hereditas, 18, 199-214 (1933) (chromos. conjugation in Salix hybrids).11 Helms, Anna, and C. A. Jorgensen, Bot. Tidsskr., 39, 57-133 (1925) (Betula hybrids).12 Heribert Nilsson, Nils, Lunds Univ. rs88kr. N. F. Avd. 2, vol. 14, no. 28 (1918).18 Ibid., vol. 27, no. 4, (1930).14 Hoogland, R. D., "The alpine flora of Mount Wilhelm (New Guinea)," Blumea, suppl. 4,

pp. 220-238 (1958).15 Hulten, Eric, Flora of Kamtschatka and the Adjacent Islands, Kgl. Svenska Vetenskapsakad.

Handi. Ser. III, vol. 5, nos. 1-2; vol. 8, nos. 1-2 (1927-1933).16 Hulten, Eric, Outline of the History of Arctic and Boreal Biota during the Quaternary Period

(Stockholm: Bokforl. AB. Thule, 1937).17 Hulten, Eric, Flora of Alaska and Yukon (Lund: C. W. K. Gleerup, 1941-1950).18 Johnson, H., "Interspecific hybridization within the genus Betula," Hereditas, 31, 163 (1945).19 Ibid., 35, 115-135 (1949).20 Krapovickas, Antonio, personal communication.21 Larsen, C. Syrach, The Employment of Species, Types and Individuals in Forestry (Copenhagen:

C. A. Reitzel, 1937).22 Ledebour, Carolus F., Flora Altaica (Berlin: G. Reimer, 1829-1833), vols. I-IV.23 Gersted, A. S., Kgl. Danske Videnskab. Selskab., 9, 336-506 (1871). (Monograph on oak

family around the world; information about vegetation belts and tree lines; Danish.)24 Ostenfeld, C., and C. S. Larsen, Kgl. Danske Videnskab. Selskab, Biol. Medd., vol. 9, (1930).26 Porsild, A. Erling, Natl. Mus. Canada, Bull. 101 (1945).26Ibid., Bull. 135 (1955).27 Righter, F., and J. Duffield, "Interspecific hybrids in pines," J. Heredity, 42, 75 (1951).28 Righter, F., and P. Stockwell, "The fertile species hybrid, Pinus murraybanksiana," Madrono,

10, 65-69 (1949).29 Rizzini, Carlos T., "Flora Organensis," Arquiv. Jard. Bot. Rio de Janeiro, 13, 117-246 (1945).30 Skottsberg, Carl, "Die Vegetationsverh~ltnisse langs der Cordillera de los Andes S. von 410

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