PHYSIOLOGY 



245 



also with the outer atmosphere was rendered highly probable from anatomical 

 investigation, and has been positively demonstrated by physiological experiment. 

 It is, in fact, possible to show that air forced by moderate pressure into the inter- 

 cellular passages makes its escape through the stomata and lenticels ; and con- 

 versely, air which could enter only through the stomata and lenticels can be drawn 

 out of the intercellular passages. The method of conducting this experiment can 

 be seen from the adjoining 

 figure (Fig. 212). 



Intercellular air-spaces are 

 extensively developed in water 

 and marsh plants, and occupy 

 the greater part of the floating 

 portions of the plant. The sub- 

 merged portions of water plants 

 unprovided with stomata thus 

 secure a special internal atmo- 

 sphere of their own, with which 

 their cells maintain an active 

 interchange of gases. This in- 

 ternal atmosphere is in turn 

 replenished by slow diffusion 

 with the gases of the surround- 

 ing medium. In marsh-plants, 

 which stand partly in the air, 

 the large intercellular spaces 

 form connecting canals through 

 which the atmospheric oxygen 

 without being completely used 

 up can reach the organs growing 

 deep in the swampy soil and 

 cut off from other supplies of 

 oxygen. In some cases the 

 need of a supply of oxygen 

 to such roots is met by special- 

 ised roots (pneumatophores) 

 which project vertically from 



FIG. 212. Experiment to show the direct communication 

 of the external atmosphere with the internal tissues of 

 plants. The ^lass tube R, and the leaf }', are fitted air- 

 tight in the bottle ; upon withdrawal of the air in the 

 bottle by suction on the tube B, the external air pene- 

 trates the intercellular spaces of the leaf, through the 

 stomata, and escapes in the form of small air-bubbles 

 from the cut surface of the leaf-i>etiole. (From DKTMER'S 

 Pliysiol. I'rakt.) 



the muddy soil (Fig. 213). 



Phosphorescence. Under 

 the same conditions as the 

 respiratory process a limited 

 number of plants, particularly 

 Fungi and Bacteria, emit a 



phosphorescent light. This phosphorescence at once disappears in an atmosphere 

 devoid of oxygen, only to reappear on the admission of free oxygen. On this 

 account the phosphorescent Bacteria, according to BEYERINCK, afford a delicate 

 test for the activity of assimilation. All the circumstances which facilitate 

 respiration intensify phosphorescence ; the converse of this is also true. According 

 to the results of investigations concerning the phosphorescence of animals, from 

 which that of plants does not probably differ in principle, 'the phosphorescence 

 is not directly dependent upon the respiratory processes. 



The best-known phosphorescent plants are certain forms of Bacteria which 



