Chap. I] THE FORMATIONS 173 



a warm vegetative season, a continuously moist subsoil, damp and calm 

 air especially in winter. 



Immaterial for woodland is : — whether the moisture in the soil is sup- 

 plied by rain or by telluric water, whether the rain falls frequently or 

 rarely, or whether the rain falls during the period of activity or of rest. 

 To the optimum of the woodland climate corresponds the hygrophilous 

 tree, and to the lower grades of woodland climate, in descending order, 

 correspond the tropophilous tree, the xerophilous tree, and brushwood. 



Hostile to woodland in the higher latitudes is a climate with dry 

 winters, because, during the winter, trees cannot replace the water lost 

 in transpiration. 



iii. GRASSLAND CLIMATE. 



The demands that grassland makes on climate are quite different from 

 those of woodland. As already stated, the grassy covering is the 

 controlling feature of grassland. Formations of perennial herbs partially 

 or entirely devoid of grasses cannot successfully contend against woodland 

 growth, and are found only in deserts, as in the so-called Artemesia-steppes, 

 possibly in most cases mingled with brushwood. 



Herbaceous grasses are in the main inhabitants of temperate lands. 

 In the tropics, we find luxuriant grassland, rich in grass, chiefly on the 

 high plateaux that are periodically cool, for example in the interior of 

 Brazil and in Central Africa, and a scanty growth of grass only where 

 the temperature during the vegetative season constantly exceeds 30 C. 

 in the daytime. It cannot be decided at present why high temperatures 

 do not suit most herbaceous grasses. During the season of rest, the 

 highest temperatures that occur naturally do not injure the parched carpet 

 of grass. 



When compared with woody plants and many perennial herbs, the 

 grasses composing the covering of grassland are shallotv-rooted plants, 

 and therefore suffer in prolonged drought during the vegetative season. 

 It has been already mentioned what disastrous effects the dry vegetative 

 season of the year 1893 had on grasses in contrast to woody plants 

 and deeply rooted perennial herbs, and Woeikof has also cited similar 

 observations. A climate in which the dry and hot seasons are 

 synchronous, like that of the countries bordering on the Mediterranean, 

 is consequently unfavourable to the growth of grass, and therefore also 

 to grassland. Thus in August, 1886, near Lisbon I saw the grass and 

 most herbaceous perennials quite dried up, while deeply rooted thistles 

 were blossoming gaily and trees displayed their foliage unimpaired. 



Alois tn re in tie subsoil las little influence on the covering of grass; 

 only moisture in tie superficial soil is important to it. As the latter 

 is soon lost owing to evaporation and filtration, frequent, even if weak, 



