100 



Water-Pores ^his excess water is often got rid of by exudation from the 

 ordinary stomata, or from special openings in the leaves called 

 water- pores, such as are found for instance at the tips of the 

 leaves of Colocasia antiquorum. Such pores are usually 

 larger than ordinary stomata, and, as their guard-cells contain 

 no living protoplasm and cannot therefore alter their shape, 

 the pores are always open. In the early morning drops of water 

 may often be seen exuding from the leaves of grasses and other 

 plants which have become surcharged with water by the 

 actively absorbing roots during the night. In such cases 

 the escaping water carries with it salts in solution, whereas 

 in transpiration only the water escapes in the form of vapour, 

 the salts remaining behind. As has already been noted, plants 

 can to a certain extent control transpiration by opening or 

 closing their stomata and this is effected mainly by the guard- 

 cells ; when the latter are turgid the stomata open and vice 

 versa. The guard-cells become turgid when active transpira- 

 tion is advantageous, in strong sunlight when plenty of water 

 is available, or when the surrounding air is moist, while they 

 become flaccid and thus close the stomata when active trans- 

 piration is likely to be injurious owing to the failure of the 

 water supply, or other cause. In dry localities where roots 

 can obtain very little water from the soil, only those plants 

 can exist which possess very efficient means of preventing 

 rapid transpiration, or which get over the difficulty in some 

 other way. Thus in desert plants the total area of the leaf 

 surface is usually greatly reduced and leaves in many species 

 may be entirely absent, the living, chlorophyll- containing 

 cells being situated in the thick stems and protected by a 

 more or less impervious epidermis, instead of being arranged 

 in large thin leaves provided with numerous stomata. 



In many localities the water difficulty only becomes acute 

 during certain seasons of the year and plants are able to exist 

 provided they can complete their development during the 

 period when the soil around their roots is sufficiently moist 

 to supply their needs. Thus in the plains of India, most 

 grasses and herbs complete their development during the rainy 

 season and die down in the hot weather. Deciduous trees 

 also lose their leaves and remain inactive during the seasons 

 when #n account of either the low temperature of the soil, 

 or the scarcity of water, the roots would find difficulty in 

 obtaining their supplies. Evergreen trees on the other hand 

 may be able to exist through such seasons owing to the 

 effective checks on transpiration possessed by them in the 



