479 
foliage, the leaves being generally large and thin, with numerous 
stomata, and sometimes provided with long dripping points, by 
means of which the water is rapidly drained off. In addition to their 
very numerous stomata, which as we have seen so materially assist 
in the process of transpiration, hygrophytes are often furnished with 
special epidermal organs — hydathodes — for excreting water. Resi- 
dents in the tropics will have noticed that the forests are often 
dripping with moisture during the night and the early hours of the 
day, even in a comparatively dry season : this moisture which is 
generally believed to be dew, owes its origin, in part, to these special 
organs of evaporation ; the excretion being most active during the 
night when, owing to the saturated state of the atmosphere, which is 
therefore unfavourable to transpiration, the plant becomes over- 
charged with moisture. These organs are commonly present on the 
leaves of aroids, and the blades of the Indian com and other 
grasses. 
Dry region plants, or xerophytes, are characteristic of regions 
where the external conditions are such as to limit the supply of 
water, or to accelerate its exit from the plant. The plants consti- 
tuting the flora of such regions, have developed all manner of devices 
for effectually checking the loss of water, with the result that few 
districts are so dry as to be entirely destitute of vegetation. 
The leaves of such plants are often few 7 , and are generally ex- 
tremely small, or, as in the case of the Cacti and some Euphorbias, 
entirely absent ; thus diminishing the evaporating surface, and re- 
ducing the loss of water to a minimum : in such cases the stems are 
generally green and enormously swollen, and perform the functions 
of leaves. When present the leaves are often very thick and fleshy 
as in the Agave, or tough and leathery in texture as in many epiphy- 
tic orchids, or in the ‘Rambong’ {Ficus' elastic a) where transpira- 
tion is impeded owing to the increased thickness of the epidermis of 
the leaves, which is rendered almost imprevious to water. 
Sometimes the leaves are covered with a dense felt of hairs as in 
the Castilloa, and the £ silver tree J of South Africa ( Leucadendron 
argenteum ) this covering protects the leaf against the too powerful 
rays of the sun, and to its presence the characteristic grey colouring 
of many desert plants is due. Many species of Acacias inhabiting 
the hot and dry regions of tropical Africa and Australia are provided 
with pinnate leaves ; the leaflets being mobile and capable of closing 
together during the hotter part of the day when transpiration is 
greatest. There atfe many other adaptations for resisting the loss 
of water, such as the rolling up of the leaves as in some of the 
grasses, as to prevent the under surface on which the stomata 
are absent to the rays of the sun ; and the placing of the leaves 
edgewise as in the case of the Eucalyptus of Australia, so as to 
minimise the effect of the sun’s rays • but the foregoing examples will 
show how perfectly plants have succeeded in accommodating them- 
selves to their physical environment, by preventing excessive trans- 
piration. 
