ch. n] General Biology 21 



plant food materials they need (Fig. 1). Their adventitious 

 origin at the lower nodes of the stem, together with the pro- 

 duction of rhizomes or stolons in many species, enable the plants 

 to spread rapidly and cover large areas. 



Leaves. The leaves have two principal functions to perform, 

 and for both of these they show signs of great adaptation. The 

 assimilation of carbon from the carbon- dioxide of the air and the 

 formation of starches and sugars from the elements carbon, hydro- 

 gen and oxygen (photo-synthesis) is the work of the chlorophyll 

 corpuscles. The thin, long, blades of grasses probably expose 

 a larger area of chlorophyll granules to the sunlight than any 

 other class of plants if the proportional dry weight of the plants 

 be taken into consideration. 



We find even greater adaptation for the control of the rate 

 of transpiration. Many species are markedly adapted to xero- 

 phytic conditions, i.e. to grow under circumstances which allow 

 only the minimum moisture supply. 



In such xerophytic species the motor cells (p. 9) are well 

 developed and serve to regulate the rate of transpiration. Their 

 thin elastic walls enable them to take up or part with excess of 

 moisture readily. When moisture is plentiful they become turgid, 

 and, owing to their situation between the ribs, the blades are 

 kept fully expanded. When the moisture supply fails these 

 cells become flaccid, and as they shrink they permit the ribs to 

 come together. In this way they cause the blade to roll or fold up, 

 and since the stomata are chiefly or wholly upon the upper surface 

 and between the ribs, the transpiration is very much reduced. 

 The lower and exposed surface moreover is protected by hard 

 tissue, and in extreme cases the blade itself may be reduced in 

 size, e.g. Air a flexuosa and Sheep's Fescue. If the motor cells 

 are confined to the sides of the mid-rib the blade folds under 

 dry conditions; but if they are developed between all the ribs 

 the blade rolls up. The presence of the hard tissue at the top of 

 the ribs and on the lower surface, together with the girders joining 

 them up to the vascular bundles, all assist in the closing up or 

 expansion of the blades and in maintaining the required position. 

 In many species the presence of hairs also assists in the regulation 

 of the rate of transpiration. In the case of shade species the 



