51 



HI.— Ecological Notes on the Principal 

 Species in each Genus. 



In this section it is niy purpose to set forth the principal 

 iacts that have been ascertained, regarding the part played 

 by all the more important species in the grassland plant suc- 

 cession, and also by means of selected examples, to illustrate 

 the general differences, which are shown in morphological 

 -characters, and particularly in leaf anatomy. Much may be 

 learned by studying simple transverse sections of the leaves 

 of grasses. It is much less laborious, and probably even more 

 useful on the whole than elaborate chemical analyses of the 

 .herbage, for these without doubt vary greatly according to 

 the time of the year, and even according to the state of the 

 weather. Leaves which after a spell of dry sunny weather 

 may be found full of storage food, may, after a few weeks of 

 rainy, cloudy weather, contain relatively little starch. Farmers 

 have repeatedly expressed the opinion to me that during the 

 past season of exceptional rains, and cloudiness with low tem- 

 peratures, the veld grasses, though they grew well, especially 

 in the drier low veld, were less nutritious than usual. 

 'Chemical analyses therefore to be of value should be repeated 

 at intervals throughout the season, and this, so far as I am 

 aware, has seldom been done. 



In attempting to gauge the feeding value of a grass from 

 its ecological behaviour and its morphological appearance, it 

 should be remembered that there is a very close connection 

 between the two processes of transpiration (or loss of water) 

 and carbon assimilation, since the stomata are involved in 

 both processes. Water is lost through the stomata and carbon 

 dioxide is taken in through the stomata. Now a grass which is 

 forced to protect itself against excessive transpiration (i.e., 

 a xerophytic grass) must at the same time be hampered in its 

 •carbon assimilation. Xerophytic grasses therefore cannot grow 

 so quickly, and are not so valuable for pasturage as the more 

 mesophytic types. There are few general rules of this kind 

 which do not admit of exceptions, however, and some xero- 

 phytic grasses, e.g., Danthonia purpurea (see Fig. 9), though 

 iheir leaf surfaces are greatly reduced, tend to become some- 



