28 



THE PLANT CELL. 



ii. The respiratory cavity, which lies deeper than the guard-cells, 

 and is surrounded by the thin-walled cells of the raeeophyll, or, in the case 

 of herbaceous stems, by the outermost cortical cells (see Fig. 15, b). 



From this preliminary examination of the structure of a 

 stoma, it is possible to deduce its function. If the mesophyll 

 cells* of a leaf are studied, it will be found that they conform in 

 structural characters to the type of thin-walled assimilating-cell 

 which was examined in Chapter i. In each cell there is seen a 

 layer of peripheral protoplasm, in which are suspended chloro- 

 plasts and nucleus ; moreover, a large amount of watery ceU-sap 

 is present in the central vacuole, and during the daytime aqueous 

 vapour is being constantly given off through the thin walls into 



\ 



Fig. 14.— 'A Stojia from the Leai' of Sedum, showing the subsidiary cells 

 (1,2, and 3 show the order of formation of the walls of these latter). 



the respiratory cavity of the stoma. This process is known as 

 transpiration, and a current, the transpiration current, is kept up 

 by the evaporation of moisture through the stomata, so that 

 water is drawn up from the stem and root to replace that 

 evaporated from the mesophyll cells. Transpiration is readily 

 demonstrated by placing a leafy plant under a bell-jar, in the 

 sunlight, when the moisture evaporated through the stomata will 

 condense upon the inner surface of the bell-jar. (For further 

 details of transpiration see Chap. x.). 



In some plants there are contrivances (hairs) in connection 



* Sometimes called the spongy parenchyma. 



