392 LEAF-ANATOMY OF THE GKNUS ANDROPOGON, 



Ecological cha7'acte7'istics. 



Tlie morphological structure of the leaves of A. re/ractits and 

 A hombycinus shows well marked xerophytic characteristics. The 

 well dexeloped sclerenchymatous tissue, besides aiding in the 

 mechanical strength of the leaf, also has a connection with the 

 dry environment {see Warming, Ecology of Plants). The cuticle 

 is also extremely thick on both the superior and inferior surfaces, 

 thus depressing transpiration. 



The presence of strata of water-storing cells, on the superior 

 surface, is also a v/ell marked xerophytic characteristic. Whether 

 the bottom layer functions as hinge-cells, curling up the leaves on 

 losing their turgescence, is uncertain. It has been pointed out 

 by Warming and others, that, wdien hinge-cells occur and func- 

 tion in rolling the leaf when diminished transpiration is neces- 

 sary, the stomata mainly occur on the superior surface, and are 

 thus protected . The stomata, in the two species in question, are 

 mostly confined to the inferior surface; and it is difficult to see 

 what advantage the leaf would have, so far as the stomata are 

 concerned, if it were rolled up. The stomata, being arranged 

 mostly in grooves, are well protected, when the leaf is in the flat 

 position. This xerophytic stomatal arrangement is also referred 

 to by T. Holm* in discussing some of the lowland species of the 

 Rocky Mountains, Colorado, 



The presence of "girdle-canals " in A. homhyciniis is also pro- 

 bably a xerophytic characteristic, designed to depress transpira- 

 tion. These are referred to by Giltay, as occurring in some 

 arenicolous grasses. 



The habitat of these two species corresponds, to a very large 

 extent, with their xerophytic characteristics. A. refractus is 

 distributed over a great part of JS'ew South Wales, and is com- 

 monly foumd in sandy and rocky situations A. hombycinus is 

 confined mostly to the interior, and is often found on sand-hills. 



In A. ischcemum, the xerophytic characteristics are not as well 

 marked as are those of A. rejractus or A. hombycinus, but it may 

 be considered more xerophytic than A. intei'medius, A. affinis, 



* Botanical Gazette, Vol. xlvi. 



