THE LEAF 



185 



same in that they are both to be referred to the property 

 inherent in protoplasm of responding to stimuH. There is 

 no indication, however, that hritability in the vegetable 

 kingdom is accompanied by anything like consciousness or 

 volition, or that plants possess any power of initiative. 

 While the movements in response to stimuli are in many 

 cases eminently adapted to a purpose, we have no evidence 

 of a controlling power behind them. The movement comes 

 automatically in response to the stimulus, whether the effect 

 at the moment be advantageous or the 

 reverse. 



202. Adjustments in relation to 

 moisture. — - These adjustments may 

 be — (1) To guard against excess of 

 moisture ; e.g. glands for excreting water 

 and salts ; scales, wax, down, etc., on 

 the surface of leaves. These may serve 

 also for protection against cold, insects, 

 excess of light and heat. (2) For the 

 conservation of moisture ; e.g. the rev- 

 olute leaf margins of grasses and sand plants growing along 

 the seashore ; the fleshy leaves of stonecrops and purselanes; 

 the hard epidermis of yuccas and aloes ; the scales, scurf, and 

 down, by which the moisture absorbed from the soil by plants 



growing in dry and bar- 

 ren places is prevented 

 from escaping too 

 rapidly through the 

 stomata ; the leaf cups 

 and holders sometimes 

 formed by winged 

 petioles and clasping 

 leaf bases for retaining 

 dew or rain water. 

 (3) For leaf drainage, 

 or the conduction of 



Fig. 251. — Cross sec- 

 tions of the leaf of sand 

 grass : a, unrolled in its or- 

 dinary position ; b and c, 

 rolled up to prevent too 

 rapid transpiration. 



Fig. 252.— 

 Winged petiole of 

 Polymnia. 



Fig. 253. — Water 

 cups of Silphium per- 

 folialum. 



