466 



VENTILATING SYSTEM 



Fig. 181. 



Occluded stoma from the upper side of the leaf of 

 Pilea elegans. (T.S.) 



a decided tendency to thicken their outer tangential walls. As a rule, 

 one intrusive cell in each chamber develops its cellulose pad immedi- 

 ately below the stoma, which thus becomes completely occluded 

 (Fig. 181). AVhen plants of Trachscantia viridis are grown in the 

 relatively dry atmosphere of an ordinary room, the epidermal elements 



adjoining the stomata put forth 

 vesicular processes which become 

 closely apposed to the guard-cells 

 and thus obstruct the pore. 220 

 Another remarkable mode of sto- 

 matic occlusion has been observed 

 by the author in Dischidia ben- 

 galensis, an epiphyte which is very 

 common in Java. During dry 

 weather the hypostomatic air- 

 chambers of this plant become 

 completely filled with a resinous 

 substance, which is highly refractive 

 and readily soluble in alcohol ; this secretion appears to be derived 

 from the subsidiary cells, which put forth vesicular processes towards 

 the chamber; it generally extends outwards as far as the pore passage. 

 The thickness of the outer epidermal walls varies in different parts 

 of the same plant ; in the same way, the above-mentioned arrange- 

 ments for the " protection " of the stomata may attain various degrees 

 of development in different organs. As the accompanying drawing 

 (Fig. 182) shows, even the two surfaces of the same leaf may differ in 

 this respect. In Populus pyramidalis, most of the stomata of the adaxial 

 leaf-surface are provided with a moderately deep, slightly funnel-shaped 

 external air-chamber (Fig. 182 a); on the lower side of the leaf 

 the stomata are scarcely depressed at all (Fig. 182 a). Similarly, 

 in Plantago major, the front cavities of the adaxial stomata are on an 

 average considerably wider than those of the abaxial stomata (cf. 

 Figs. 182, b and Bj). In both the cases mentioned, the protective 

 features of the stomata are more pronounced on the side of the leaf 

 that is more exposed to the influence of factors such as light and 

 warmth which tend to increase the activity of transpiration. 



So far, attention has been mainly directed to those structural 

 features of stomata, which serve to restrict transpiration, and which 

 hence are characteristic of xerophytes. Some account must now be 

 given of the peculiarities of the stomata of plants which thrive in a 

 humid climate, or in moist localities. The leaves of certain floating 

 water-plants have been examined by the author from this point of 

 view.- 2 ' Here the stomata often diverge widely from the ordinary 



