ill-adapted to withstand the vicissitudes 

 of winter. During the cold period, about 

 the only parts of a tree that are exposed 

 are the buds and bark, organs which 

 are readily seen to be well protected. A 

 bud like that of the horse-chestnut is 

 covered on the outside by thick bud 

 scales, which are impervious to water, 

 and hence well adapted to check evapora- 

 tion ; these scales also delay the ingress 

 of heat or cold. The delicate buds within 

 are still further protected from the ele- 

 ments by means of copious quantities of 

 resin, and by a dense coat of hairs. While 

 all winter buds are not so fully protected 

 as are horse-chestnut buds, one or more 

 of the things found there is commonly 

 present. In small winter buds, which 

 develop late, the devices for winter pro- 

 tection are less complete in most in- 

 stances. An examination of bark dis- 

 closes the fact that it has properties sim- 

 ilar to those found in buds. In particu- 

 lar, one finds a strong development of 

 cork or some other tissue which is rela- 

 tively impervious to water, and hence 

 adapted to reduce the winter transpira- 

 tion. Cork is one of the most impermea- 

 ble of known plant substances, which 

 accounts for its common use as bottle- 

 stoppers. Bark is doubtless of impor- 

 tance also in moderating the effect of 

 temperature changes outside the tree. It 

 is instructive to compare tropical trees 

 with those of temperate regions. Within 

 the rainy tropics, trees commonly have 

 but little bark, and little or nothing com- 

 parable to winter buds, since bud scales 

 are usually wanting. In tropical deserts, 

 on the other hand, trees and shrubs pos- 

 sess a considerable amount of bark, and 

 their buds have bud scales. In this lat- 

 ter case, it is obvious that the advantage 

 of bark and bud scales is in checking the 

 loss of an undue amount of water, since 

 cold temperatures are unknown or at 

 least uncommon. Not all trees, even in 

 cold climates, shed their leaves in winter. 

 All are familiar with pines, firs, and other 

 cone-bearing trees, and it may be asked 

 how such trees are protected in cold 

 weather, since they are obviously alike 

 at all seasons. To properly appreciate 

 the situation here it is necessary to make 

 a microscopic study of the leaf. A study- 

 such as this shows that the leaves of these 

 evergreens are remarkably well protected 



from outside dangers. In the first place 

 the skin is unusually thick, as compared 

 with other leaves. The outer part of this 

 skin, the so-called cuticle, is relatively 

 impermeable to water, and is other- 

 wise protective against external dangers. 

 Within there are other thick-walled ele- 

 ments, also protective. These characters 

 are somewhat evident without the micro- 

 scope, since they cause the rigidity of the 

 evergreen leaf. Evergreen leaves then, 

 such as those of the pine, are adequately 

 adapted to the severities of winter, as 

 indeed is evident at the outset from the 

 mere fact that they stand the severest 

 winters without injury. 



We are now prepared to consider the 

 matter of compensation to which allusion 

 has previously been made. While it is 

 doubtless true that trees are as well pro- 

 tected as other plants, the cost, as meas- 

 ured in terms of energy expenditure, 

 must be enormous. Plants with under- 

 ground stems are well protected during 

 winter both from cold and loss of water, 

 by reason of the fact that all aerial and 

 hence exposed parts have died down. 

 Plants with the winter habits of the ordi- 

 nary grasses are similarly though less 

 protected, inasmuch as they remain close 

 to the soil, partaking of its heat ; they are 

 usually more or less covered also by snow 

 or dead leaves. Plants such as these are 

 not obliged to expend extra energy in 

 building up protective tissues, but can 

 devote all their strength to the develop- 

 ment of leaves, stems, flowers, and fruit. 

 Trees, however, must expend a great 

 amount of energy in the construction of 

 bark layers over every limb, and pro- 

 tective structures over every bud. It is 

 then generally true among plants that 

 those which are best fitted to receive a 

 maximum of light are also most exposed 

 to climatic vicissitudes, and are obliged 

 to be most prodigal in the development of 

 protective tissues and organs. On the 

 contrary, those plants which are most 

 unsatisfactorily related to light are the 

 very plants which are fully protected 

 with the least expenditure of constructive 

 energy 7 . 



There are a great many more interest- 

 ing relations between trees and the phys- 

 ical world, but it is perhaps wiser to 

 devote attention now to the social rela- 

 tions of trees, that is the relations of trees 



43 



