J 



174 FORMATIONS AND GUILDS [Part II 



atmospheric precipitations are necessary. During the resting season 

 grasses can endure great drought without injury. 



Grasses do not rise so high above the surface of the ground as dc 

 trees and consequently occupy damper atmospheric strata. Very drj 

 air, then, does them as little injury during the resting period as doe 

 the lack of rain. The lowest atmospheric strata are also the calmest 

 so that grasses suffer less than woody plants from the drying action 

 the wind. Winds that prevail during the dry seasons or during thi 

 winter, and that are so injurious to trees, are devoid of significance ii 

 relation to grasses. 



A good grassland climate is then composed of t/ie following elements :- 

 frequent, even if weak, atmospheric precipitations during the vegetativ 

 season, so that the superficial soil is kept in a moist condition, am 

 further a moderate degree of heat during the same period. 



Almost immaterial for grassland are the follozviiig : — Moisture in th 

 subsoil (except when the superficial soil has a great power of capillar 

 conduction), dryness of the air especialh' during resting periods (dr 

 season, winter), and winds. 



Hostile to grassland in the higher latitudes is drought in the chi 

 vegetative season of grasses (spring, early summer). 



A woodland climate leads to victory on the pari of the woodland, 

 grassland climate to victory on the part of the grassland. In transition. 

 climates edaphic infiioices decide the victory. Strong deviations from 

 zvoodland or grassland climate produce desert. 



That the interior of continents, especially outside the tropics, affori. 

 poorer tree-vegetation than districts near the coast, is due in the fir 

 place to the great dryness of the air that prevails there during certa 

 seasons and especially in winter. Middendorff adduces evidence of tl 

 unfavourable influence of a continental climate : — 



'At the same latitude, 58° N., in which at Yeniseisk I plunged into the fore; 

 of Siberia, at Sitka conifers that are so closely allied to the Siberian ones 

 to be distinguished from them specifically only by specialists, attain a height 

 160 feet with 7-10 feet diameter. . . . From Yeniseisk going northwards, according 

 eye-measurements taken in Livland, one could hardly ascribe to the forests 

 age of more than a half-centurj-, certainlj' not a century'. . . . Also beyond t 

 limits of the frozen soil, under the most favourable conditions, in Southern Siber 

 the species of trees growing there attain no considerable dimensions, never the 

 that they or their representatives attain in Europe ^. . . . At least 99 % of 

 apparently mature trees in the forest, even in the, favourable localities of Southe 

 Siberia, were not more than i foot to li feet in diameter. Three or four centur 

 appear, even in Southern Siberia, to be the extreme age-limit to which the bet' 

 trees in the forest attain on the average. The average life of the trees of a Siberia 



' Middendorft" op. cit. p. 631. - Id. p. 632. I 



