53 



Particular attention stould be directed to the way irt 

 which the leaf is folded. Some of the xerophytic forms are- 

 permanently folded, e.g., Danthonia, some of the more meso- 

 phytic ones are permanently flat, e.g., Anthistiria, Digitaria„. 

 etc., but intermediate between those are a large number which 

 have their leaves flat and unfolded under moist conditions,^ 

 and folded on the midrib (conduplicate), or rolled in from the. 

 margins (convolute), in dry weather. In such leaves some of ' 

 the cells of the upper epidermis (either those above the mid- 

 rib, or those in each of the hollows between the ridges) are- 

 larger and thinner walled. When they are full of water and 

 turgid, as is the case when their water supply is not restricted, 

 they keep the leaf unfolded or the ridges apart. In dry 

 weather, however, they lose water and more or less collapse, 

 which has the effect of drawing the two halves of the leaf: 

 blade together or causing it to roll inwards from the margin. 

 The experiment may be tried with Eragrostis curvula, which 

 is so common along the roadsides in Spring. Pick a few 

 leaves of it when they are flat and expanded and allow them 

 to lie for a few minutes in the sun. They very soon roll up 

 from the margins. In such grasses the thin-walled cells. 

 referred to are known as "motor cells." Similar thin-walled 

 cells occur on the leaves of Andropogon, Digitaria, Setaria, 

 which do not, at any rate readily, fold or roll up. In this, 

 case they do not function so much as motor cells, but rather 

 as water storage cells. An examination of the figures will 

 show that in such caises the water storage cells are more 

 numerous, covering in some cases most of the upper surface, 

 and there are no prominent ridges. There are other more- 

 minute points of structure itf grass leaves which are fairly 

 constant, and are useful for the purpose of distinguishing- 

 species by their leaves, but it is not my purpose to discuss these 

 at the present time. The chief species will now be dealt with 

 individually, the genera again for the sake of convenience of 

 reference being arranged alphabetically. 



Achneria- (See Fig. l.D.) A temperate genus found 

 chiefly in the South Western region. It differs from Penta— 

 schistis in being awnless. A. capensis is an important species 

 in the early stages of the xerosere. It invades moss carpets; 

 on bare sandstone or granite rocks, and remains for a short time 

 dominant. A. ecklonii and A. ampla favour marshy or wet 

 sandy soils, and belong rather to the hydrosere. The others, 

 occur sparsely scattered among the Macchia shrubs of the - 

 S. West, and are of little importance in the plant succession^ 



