TRANSPIRATION 



37 



time, as i,ooo, the evaporation from an equal surface of a dilute solution of 

 gum was 843, of malic acid 837, of glucose 773. An Opuntia, on the other 

 hand, gave off only ten units of water per unit of time. A special factor must, 

 however, be taken into consideration, viz. the cuticle, already described in 

 speaking of the absorption of water, whose influence is in the direction of 

 retarding transpiration. Since the cuticle imbibes little or no water, it acts 

 like a film of oil spread over an aqueous surface. The differences between 

 varieties of cuticle have been already referred to, and these are of the greatest 

 importance in relation both to the absorption and giving off of water. The 

 thin and gelatinous external walls of the root and of submerged plants are very 

 permeable to water, so that these parts readily dry up and wither when exposed to 

 air, and between this condition and the other extreme, where the cuticle is thick 

 and practically impervious to moisture, as in hard leathery leaves, every possible 

 transition occurs. Some numerical idea of the action of the cuticle may be 

 obtained from a study of some of Boussingault's results (1878). He experi- 

 mented on apples which were in part provided with a normal cuticle and in part 

 had the cuticle removed. 

 A square centimetre of 

 normal apple surface lost 

 0-005 g- of water per 

 hour, whilst the skinned 

 apple lost 0-277 g-» or 

 fifty-five times as much. 

 Such investigations, 

 however, take for granted 

 that the cuticle over the 

 exposed part is a con- 

 tinuous layer and desti- 

 tute of all apertures, but 

 this is by no means true of 

 all cuticles. In very many 

 cases the cuticle is pierced 

 by microscopically mi- 

 nute but extremely numerous holes ; the otherwise continuous epidermis is 

 interrupted by special organs, the stomata. Each stoma (Fig. 7) consists of two 

 cells (guard-cells) which differ from other epidermal cells in their curved 

 form. Owing to the fact that these cells have their concave sides turned toward 

 each other, a small slit is left between them, opening on the one side to the air and 

 on the other into a large intercellular space, known as the ' respiratory cavity ' 

 (see Fig. 7, B), standing in direct communication with the general intercellular 

 space-system in the body of the plant. The spaces found between the cells in 

 the plant's interior are not, however, completely shut off from each other, but 

 form an intercommunicating system of chambers and canals, constituting the 

 aeriferous system of the plant. By means of the stomata this system is put 

 in direct communication with the atmosphere. 



The stomata, the exits of the aeriferous system, permit gases of all kinds 

 to enter the plant as well as to pass out, and direct gaseous exchange may, by 

 this means, take place between cells deep in the interior of the plant and the 

 air. We can easily convince ourselves of the value of the stomata and inter- 

 cellular space-system for such exchange by placing a leaf from an appropriate 

 plant in water and arranging that the atmospheric pressure on the end of the 

 submerged petiole is less than that on the leaf blade. As a result of a quite 

 insignificant difference of pressure — mouth suction is often sufficient for the 

 purpose — a continuous stream of air-bubbles may be seen escaping from 

 the petiole. It can be at once demonstrated that this air has entered by the 



Fig. 7. Lower epidermis of Tradescantia virffinica. A, from above ; 

 two guard-cells centrally placed : B, in section, showing below the guard- 

 cells the respiratory cavity, on which the chlorophyll-bearing parenchyma 

 abuts. X 240. (From the Bonn Textbook.) 



