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 do 

 trees and consequently occupy damper atmospheric strata. Very dry 

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

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

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

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

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

 relation to grasses. 



A good grassland climate is then composed of the following elements: — 

 frequent, even if weak, atmospheric precipitations during the vegetative 

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

 further a moderate degree of heat during the same period. 



Almost immaterial for grassland arc the following'. — Moisture in the 

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

 conduction), dryness of the air especially during resting periods (dry 

 season, winter), and winds. 



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

 vegetative season of grasses (spring, early summer). 



A woodland climate leads to victory on the part of the woodland, a 

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

 climates cdapliic influences decide the victory. Strong deviations from a 

 woodland or grassland climate produce desert. 



That the interior of continents, especially outside the tropics, affords 

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

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

 seasons and especially in winter. Middendorff adduces evidence of the 

 unfavourable influence of a continental climate : — 



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

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

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

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

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

 age of more than a half-century, certainly not a century 1 . . . . Also beyond the 

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

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

 that they or their representatives attain in Europe 2 . ... At least 99% of all 

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

 Siberia, were not more than 1 foot to \\ feet in diameter. Three or four centuries 

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

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



1 Middendorff, op. cit. p. 631. 2 Id. p. 632. 



