THE LEAF 185 
same in that they are both to be referred to the property 
inherent in protoplasm of responding to stimuli. There is 
no indication, however, that irritability 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 ; ¢.g. glands for excreting water 
and salts; scales, wax, down, etc., on 
Fie. 251.—Cross sec- 
the surface of leaves. These may serve tions of the leaf of sand 
also for protection against cold, insects, rss: © unrolled in its or- 
inary position; 6 and «, 
excess of light and heat. (2) For the rolled up to prevent too 
conservation of moisture; e.g. the rev- "Pd transpiration. 
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. 
Fie. 252.— Fic. 253. — Water (3) For leaf drainage, 
Winged petiole of cups of Stlphium per- : 
Polymnia. foliatum. or the conduction of 
