PHYSIOLOGY 189 



has been separated from its parent plant for months, suffers no great 

 loss of water. A potato is similarly protected by a thin layer of 

 cork from loss of water through evaporation. The green organs of 

 plants, on the other hand, as they are active in the processes of nutri- 

 tion, and must be able to get rid of their surplus water in order to 

 secure the proper concentration of their nutrient salts, make little use 

 of such protective coverings. On the contrary, they are provided with 

 special contrivances for promoting evaporation. The cell walls of all 

 living organs are saturated with water, and, when the cuticle of the 

 epidermis is not too strongly developed, water is constantly evapor- 

 ated, even from uninjured cells, in amounts varying with the area of 

 the exposed surfaces. From this point of view, it will be seen that 



THE FLAT EXPANSION OF FOLIAGE LEAVES KENDERS THEM ADMIRABLY 



adapted FOR THE work of TRANSPIRATION. Evaporation is also 

 promoted by the numerous STOMATA (air-pores) which penetrate the 

 epidermis, and which give the air, saturated with watery vapour, an 

 opportunity to escape from the intercellular spaces. Although the 

 stomata are so small that neither dust nor water can pass through 

 them into the plant, they are usually present in such enormous 

 numbers (p. 94) that their united action compensates for their minute- 

 ness. When it is taken into consideration that a medium -sized 

 cabbage leaf (Brassica oleracea) is provided with about eleven million, 

 and a Sunflower leaf with about thirteen million air-pores, it is possible 

 to estimate how greatly evaporation must be promoted by these fine 

 sieve-like perforations of the epidermis. 



The stomata also afford plants a means of regulating evapora- 

 tion. The pores, which are the mouths of intercellular spaces, are 

 surrounded by GUARD-CELLS. As the term guard-cell suggests, these 

 cells have the power of closing the pore. The closing and opening of 



THE STOMATA ARE ACCOMPLISHED THROUGH A CHANGE IN THE TURGIDITY 



IN THE guard-cells. In consequence of their peculiar wall thicken- 

 ings, elasticity, and lateral attachment, a change of turgidity affects the 

 size and shape of the guard-cells in such a way that, by diminished 

 turgidity, they become flatter and close the air-passage, while an 

 increase of turgidity has the contrary effect and opens them (Figs. 

 176, 177). 



In many plants the so-called accessory cells (p. 94) participate in various ways 

 and degrees in these processes, depending upon the special structure of the whole 

 apparatus. The opening and closing of the stomata may be effected by either 

 external or internal stimuli ; but such stimuli affect different plants in a different 

 manner. Generally speaking, the stomata begin to close on the diminution of the 

 water-supply ; they open, on the other hand, when active transpiration is advan- 

 tageous (in light, in moist air, etc.). The quantity and quality of the sub- 

 stances held in solution in the nutrient water react in a remarkable manner 

 upon the stomata. The size of their opening is decreased, and the quantity 

 of water evaporated is therefore lessened when more than the usual amount of 



