284 MEANS OF ACCELERATING TRANSPIRATION. 



2. EEGULATION OF TRANSPIEATION. 



Means of accelerating transpiration. — Maintenance of a free passage for aqueous vapour. 



MEANS OF ACCELEEATING TRANSPIRATION. 



Aquatic plants do not transpire; therefore they do not require either vascular 

 bundles or stomata. Neither trees nor shrubs grow under water, and even the 

 largest Floridese and the most gigantic sea-wracks have no wood nor stomata. 

 These structures are on the other hand very important for land plants, and in these 

 they are developed in extraordinary variety. When one considers how much the 

 humidity and temperature of the air, those very conditions which influence the 

 transpiration of plants, are continually changing, this diversity is not really 

 surprising. What endless series of gradations there are between the damp air of 

 a tropical estuary, and the arid wastes in the interior of large continents ! What 

 varieties of temperature in the different zones and regions of the earth, and in the 

 changing seasons; what differences, even in a narrow space in a single small valley, 

 between the conditions of moisture of the air and ground in the depths of a shady 

 glen, and on the sunny, rocky slopes! In the one place the air is so saturated with 

 water- vapour that even evaporation cannot take place from exposed pieces of water, 

 much less then from plants; in the other it is so dry and the sun is so strong that 

 plants can hardly suck up enough from the ground to compensate for the water 

 evaporated from their surface. In the former case contrivances must be devised 

 which will promote transpiration as much as possible; in the latter, however, it is 

 important that too much evaporation, which would cause the drying up and death 

 of the plant, should be prevented. 



One of the most important ways of increasing transpiration consists in the 

 development of many cells whose surface is in contact to the greatest possible 

 extent with the atmospheric air, and which are so organized that water in the form 

 of vapour can be exhaled from them. Fui ther, it is of importance that the access 

 of air to these cells is not rendered difiicult, and that as great a portion as possible 

 of these cell-groups, which help in transpiration, are reached by the rays of the sun. 

 It is only in the delicate-leaved mosses, which have no stomata, that the whole of 

 the cells of a leaf, in contact with the air, give off unlimited water, in the form of 

 vapour, directly to the atmosphere. In plants possessing leaves provided with 

 stomata, the outer walls of the epidermal cells, which are directly in contact with 

 the air, are almost always rather thicker than the inner and side walls; moreover, 

 the outer wall is overlaid by the already repeatedly mentioned covering, termed 

 " cuticle ", through which water- vapour can pass only with difiiculty. In tropical 

 ferns, especially in the tree-ferns, which grow in narrow wind-sheltered ravines, 

 traversed by streams of water, and which spread out their fronds in the still, damp, 

 warm air, the outer walls are so thin and delicate, and are covered by a cuticle of 



