PHYSIOLOGY -205 



The recent investigations agree, however, in concluding that THE 



TRANSPIRATION CURRENT ASCENDS IN THE CAVITIES OF THE WOOD 

 THROUGH THE VESSELS AND TRACHEIDES. 



4. SUCTION-FOIICE OF TKANSPiiiiSG SHOOTS. A shoot, the cut end of which is 

 placed in water, shows by remaining fresh that it must be able to draw up water to 

 its extreme tips. The force of suction exerted by such a transpiring leafy shoot may 

 be demonstrated, by fitting the cut end in a long glass tube filled with water in 

 such a manner that it shall be air-tight. Thus arranged, the shoot will be able to 

 sustain and raise a column of water 2 metres high. If the lower end of the tube 

 be inserted in mercury, it will be found that even the heavy mercury will be lifted 

 by the transpiring shoot to a considerable height. Vigorous coniferous shoots 

 absorb water through the cut end with a force of suction equal to one atmosphere, 

 and are thus able to raise the mercury to a height equal to the barometric pressure 

 (760 mm.), and owing to the cohesion of the water column even beyond this 

 (920 mm.). The complete exclusion of the external atmosphere is absolutely 

 requisite for the existence of such a suction-force, and this condition is actually 

 fulfilled in the water-courses of plants^ 27 ). 



The Giving- off of Water. The requisite amount and proper 

 concentration of the nutrient water supplied by the transpiration 

 current are maintained only by the constant discharge of the 

 accumulating water. This may occur in two ways, either by 

 the evaporation of the water through the cell walls and the 

 stomata in the form of vapour that is, by transpiration or less 

 copiously and also less frequently by the actual exudation of drops 

 of water. 



1. TRANSPIRATION. In their outer covering of cork, cuticle, and 

 wax, plants possess a protection from a too rapid loss of water. A 

 pumpkin, with its thick cuticle and outer coating of wax, even after 

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

 great loss of water. A potato or an apple is similarly protected by a 

 thin layer of cork from loss of water by evaporation. The green 

 organs of plants, o'n the other hand, which must be able to get rid 

 of the surplus water in order to secure the concentration of the 

 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 evaporated, even from uninjured epidermal 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 RENDERS THEM ADMIRABLY ADAPTED FOR THE WORK OF 



TRANSPIRATION. Evaporation is also greatly promoted by the 

 numerous STOMATA (AIR-PORES) which perforate the epidermis, and 

 give the air, saturated with watery vapour, an opportunity to 

 escape from the intercellular spaces. Although the stomata are so 



