Vol. X, No. 2.] Leaf Structure of Zoysia pungens, Willd. 61 



[N.8.] 



Motor cells. — These bands of cells, as their name indicates, 

 serve the purpose of bringing about movement. They are 

 generally found in grass leaves at the base of each groove, if 

 the leaf is a ribbed one, or by the midrib, if the leaf is non-ribbed. 

 The leaf of Zoysia pungens is very faintly ribbed. We find the 

 motor cells here lying in layers at the bas j of each groove. They 

 differ from the ordinary epidermal cells in their greater depth. 

 A3 pointed out before, the leaves fold or roll themselves, when 

 dryness prevails. This folding or rolling is brought about by an 

 alteration in the turgUity of the motor cells, causing them to 

 contract and thus shortening the upper surface of the leaf. 

 When the motor cells contract, the ridges are brought closer 



and the whole leaf becomes a little more closely rolled. In 

 this manner, the upper epidermis is completely shut off from 

 the surrounding air. This is clearly an adaptation to prevent 

 excessive loss of water by transpiration. 



Stomata. — These are the respiratory organs of the plant 

 and are usually arranged in grasses in longitudinal rows 

 interspersed with the epidermal cells. Normally, stomata are 

 found on both sides of gra-s leaves. But in Zoysia pungens 

 they are entirely confined to the upper epidermis. This, in 

 itself, is a very powerful adaptation against excessive trans- 

 piration, as we have already found the upper epidermis to be 

 always protected. Even on the upper epidermis, they occupy 

 good sheltered spots, for instance, on the flanks of the 

 feeble ridges. We have already seen that these flanks contain 

 a number of papillae. Their action is to hinder excessive 

 transpiration by preventing a free circulation of air in the 

 groove — the sides of which contain the stomata and represent 

 the transpiring surface. Consequently, vigorous transpiration 

 is checked. We may then say that in the epidermis there exist 

 several protective devices purposely adapted to prevent excessive 

 light and excessive transpiration. 



Vascular bundles. — The vascular bundles of Zoysia pungens, 

 in common with other grasses, enter the leaf separately and 

 run parallel from base to apex of the blade along the ridges. 

 Those met with in this leaf are of two kinds— one with two large 

 lateral vessels which are characteristic of the monocotyle- 

 donous type, the other without them. In both types there 

 are no intercellular spaces. In spite of the smallness of the 

 leaf, there are a good number of vessels. Probably these are 

 useful in conveying large quantities of water rapidly when 

 there is an occasional increase of water-supply (e. g. after a 

 sudden fall of rain) and thus fill the water cells which may 

 afterwards yield their contents to the assimilating cells when 

 the supply of water is reduced. 



Bundle- sheaths. — In nearly all leaf-sections of grasses, each 

 vascular bundle is seen to possess at least two conspicuous 

 sheaths. The inner one is always thickwalled and the outer 



