QUEKETT MICROSCOPICAL CLUB. 447 



one of the reasons for the success of the family. The leaf con- 

 sists of a Siieath, a ligule, which is a prolongation of the sheath^ 

 and helps to prevent water from running down between the 

 sheath and the stem, and a blade. The function of the blade is 

 to carry on the interchange of gases between the plant and the 

 air, from which the plant obtains raw carbonaceous food-stufit, 

 and to give off in the form of vapour the surplus water which has 

 served as a carrier for the raw nitrogenous food-stuff absorbed by 

 the roots and sent up to the leaves. When plants live in dry places 

 or are exposed to dry winds there is a danger of excessive evap- 

 oration and consequent withering of the leaves. This is guarded 

 against in various ways. In Echmocactus, for instance, the 

 whole structure is altered for this purpose. The leaves disappear, 

 the branches become spines, and the plant becomes a green 

 wax-coated water- reservoir. The grass-leaf effects this without 

 any expensive modification of form or structure by simply rolling 

 up its leaf-surface. Hence the character of the blade often 

 reflects that of the environment, being thin and flat in shade 

 grasses, and narrow, or often rolled, in those exposed to sun, 

 wind or drought. The rolling up is often caused by special 

 motor cells. Some of the cells of the upper epidermis, either 

 next the central rib or betvv^een the other ribs, are large and 

 thin-walled and full of sap when distended. These cells are so 

 placed that as they lose water by evaporation they contract and 

 draw together the halves of the blade, or each ribbed longitudinal 

 segment, thus causing infolding or inrolling and protection of the 

 stomata on the concave surface of the leaf. These stomata are 

 often situated in crevices running up the length of the leaf, and 

 the opening of the crevice may be protected further by hairs 

 which interlock when the leaf rolls up. The outer side of the 

 leaf, which is left exposed, is protected by harder tissues. The 

 President promised to deal v/ith the flowers of grasses on a sub- 

 sequent occasion, and he was heartily thanked for his interesting 

 address. 



There will be an excursion to the grounds of Sion House on 

 the 22nd inst. At the next Ordinary Meeting, which will be on 

 October 8th, the first meeting of the new session, Mr. N. E. 

 Brown, A.L.S., will give an address on " The Fertilisation of th& 

 Fig." 



