472 VENTILATING SYSTEM 



and roof are often formed by the subsidiary cells. In some Eestiaceae 

 it is the subsidiary cells that send forth the thick-walled processes 

 which enclose the internal air-chamber, while in Tradcscantia viridis 

 the same cells are responsible for the occlusion of the stomata which 

 takes place under certain conditions. 



Benecke has drawn attention to the fact that the presence of two 

 or more subsidiary cells is particularly characteristic of succulent plants, 

 and of xerophytes in general.' 231 He suggests that these cells help 

 to preserve the guard-cells from the injurious effects of the tensions 

 that inevitably result from the shrinkage which leaves undergo through 

 loss of water. Where the outer and inner tangential walls of the 

 subsidiary cells are thin, they will undoubtedly undergo deformation 

 when the epidermis contracts, and will thus protect the guard-cells 

 from compression to a certain extent. Their action in this respect is 

 essentially similar to their function as hinge-areas ; in either case their 

 effect is to preserve the stoma from being deformed owing to the 

 rigidity of the epidermal walls. 



7. Distribution and location of stomal a. 



The fact that the stomata serve to place the intercellular air- 

 spaces of the plant in communication with the external atmosphere, 

 sufficiently explains why these structures are restricted to aerial organs, . 

 and why they occur mainly where the subjacent tissue is at least 

 moderately well provided with intercellular spaces, but are rare or 

 altogether wanting on portions of the epidermis which are underlain 

 by sclerenchyma or collenchyma. 



The root-system is probably the only portion of the plant-body 

 which never bears stomata. These structures are, however, usually 

 present in the greatest numbers upon foliage leaves, in accordance 

 with the fact that the leaves are generally the special photosynthetic 

 organs, and also the most actively transpiring portions of the plant. 

 On an average, the number of stomata borne on 1 sq. mm. of leaf- 

 surface amounts to between 100-300. Much higher figures are 

 recorded for special cases ; thus the lower leaf -surface of Olea europca 

 bears 675 stomata per sq. mm. (according to Weiss) and that of Brassica 

 Rapa 716 (according to linger).- 32 The number of the stomata like 

 their detailed structure is correlated with the transpiratory activity 

 of the plant, and hence with the nature of its habitat ; but the relation 

 of structure to environment in this respect is far more complicated 

 than might at first sight be supposed: for, as a matter of fact, the 

 structure of the stomata varies greatly in different plants. The 

 differences in the diameter of the pore passage are very considerable, 

 the position of the guard-cells with reference to the general level of 



