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 he 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 corn 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, 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 elastica) 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' 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 are many other adaptations for resisting the loss 

 of water, such as the rolling up of the leaves as m some of the 

 grasses, so as to prevent the under surface on which the stomata 

 Ire 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 «»£«^n 

 show how perfectly plants have succeeded m accommodating them- 

 selves to their physical environment, by preventing excessive trans- 



piration. 



