474 



VENTILATING SYSTEM 



As regards the position of stomata with reference to the surface of 

 the leaf [or the organ] on which the) T are borne, it has already been 

 stated that these structures usually stand at the same level as the 

 ordinary epidermal cells, in the case of non-xerophilous plants. The 

 significance of the " sunken " stomata which occur in xerophytes and 

 in all plants that cannot afford to transpire actively, has likewise been 



discussed at some length (p. 461). At this 

 stage, therefore, it is only necessary to 

 consider the third possible arrangement, 

 namely, that in which the stomata are 

 raised above the general level of the epi- 

 dermis. Great variety prevails with regard 

 to the degree of elevation, which is some- 

 times scarcely visible, while in other 

 instances {e.g. on the peduncle of Cucurlita 

 Pejw, Fig. 188) it is so pronounced that 

 each stoma comes to occupy the apex of a 

 conical or. cylindrical papilla, which is 

 traversed by a passage corresponding to the 

 internal air-chamber. The ecological im- 

 portance of such raised stomata is not 

 quite clear ; from the fact that they not 

 infrequently occur in plants which inhabit 

 most shady spots (e.g. many ferns), it might be thought that the 

 exposed situation represents a device for accelerating transpiration. 

 Leaves which are readily wetted are also often furnished with raised 

 stomata ; here the elevation may possibly serve to provide protection 

 against prolonged capillary occlusion of the pores by water. On the 

 other hand, elevated stomata are also to be found on some very hairy 

 leaves, in which case they are situated in common external air-chambers 

 or furrows ; in this latter instance no plausible explanation of the fact 

 is forthcoming. 



8. Loss or change of function on the part of stomata. 



In many plants, the stomata lose their power of adjustment more or 

 less completely, or at any rate become incapable of closing tightly after 

 a certain age. This physiological degeneration of the stomata takes 

 place at a comparatively early age in floating and other aquatic plants, 

 and also in a number of shade-loving hygrophytes, doubtless because 

 members of these ecological classes never require much protection 

 against excessive transpiration. It should, however, be explicitly stated, 

 that neither Leitgeb nor the author himself have met with a single 

 instance in which the stomata are devoid of the power of adjustment 



Fig. 188. 



Raised stoma from the fruit-stalk of 

 Cucurbita Pepo. 



