

62 Journal of the Asiatic Society of Bengal. [February, 1914. 



thinwalled. In Zoysia pungens, two sheaths can be easily 

 recognized. The inner sheath is not continuous in the smaller 

 bundles but surrounds only the phloem. In the bigger ones, 

 however, it is continuous. The function of this thiekwalled 

 inner sheath is simply that of mechanical protection to the 

 soft-walled phloem. 



The cells of the outer sheath however are very large 

 and contain chlorophyll to some extent. In places where 

 the inner sheath is joined to the epidermis by a scleren- 

 chyma band, the outer sheath is nob continuous. It then 

 presents a V-shaped appearance. The function of this outer 

 sheath in grasses gener illy, is supposed by some botanists 

 to be conduction of water, serving as an auxiliary conduct- 

 ing system. But this view is negatived in Zoysia pungens 

 by the fact that there is no necessity for an auxiliary con- 

 ducting system. The vessels in themselves are more than 

 enough for this purpose. Consequently, I am disposed to regard 

 this sheath, which sometimes is called by the name of transfusion- 

 tissue, as a special sort of water reservoir. It is I think in 

 these cells that water is stored, which is sooner or later made 

 use of by the abutting assimlating cells. Here again, we notice 

 another adaptation in the direction of economizino- the water- 

 supply. 



Mechanical tissue.— The mechanical tissue, or stereome. 

 consists in grasses, as a general rule, of sclerenchyma fibres. 

 These fibres are known to withstand wonderfully the strain 

 of flexure, traction or pressure. The sole function of these 

 fibres is support. Let us now consider how these band 

 of stereome are disposed in the leaf to the best advantage. On 

 both ends near the margins the two big vascular bundles are 

 girdered to the upper and lower epidermis by strong bands. 

 It is the lower band that is larger and stronger. Along 

 the two margins we also find a C3rtaiu amount of °mechanical 

 tissue. Moreover three small bundles are girdered to the 

 lower epidermis only. Thus the majority of the stereome is on 

 the lower side. It will be noticed then that there is a marked 

 tendency for the mechanical tissue to be stronger on the 

 lower than on the upper side. What is the purpose of this 

 arrangement ? These maritime sand grasses are, as already 

 stated above, constantly exposed to terrific storms and 

 winds. Therefore, a firm support is needed and this is supplied 

 by this greit development of mechanical tissue. I say ■ great ' 

 in comparison with the smallness of the leaf. Under dr\ 

 conditions, which is almost always the case with these plants, 

 the leaves remain rolled up and become cylindrical organs. 

 Physical experiments have shown us that the mechanically 

 best arrangement for supporting a cylindrical structure is to 

 have a strengthening band at the periphery. This is exactly 

 the case here. The bands of stereome are just below the lower 



