DIV. ii PHYSIOLOGY 247 



and organs of similar function (p. 59) ; both diffusion and move- 

 ments in mass of the gases go on through these openings. 



- That the intercellular spaces were in direct communication with each other, and 

 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. 246). The leaf-stalk of an uninjured leaf 

 of Nymphaea is introduced into a glass cylinder which has been filled with and 

 inverted in water. The leaf- blade is under atmospheric pressure ; the pressure 

 on the cut end of the petiole is less than this by a fe.w centimetres of water. This 

 difference is, however, sufficient to maintain an active current of air from the cut 

 petiole. That this air enters by the stomata is shown by the stream ceasing when 

 the upper surface of the leaf is submerged and the stomata thus cut off from the air. 

 Intercellular air-spaces are extensively developed in water and marsh plants 

 (cf. p. 165), and may form two-thirds of their volume. The submerged 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 internal atmosphere is in turn replenished by slow diffusion with the 

 gases of the surrounding medium. As regards the rest of their gaseous interchange, 

 these plants are wholly dependent on processes of diffusion, since stomata, etc., are 

 wanting. Plants which possess these organs may also obtain gases by diosmosis 

 if the cuticle of their epidermis is permeable to gases. 



III. The Assimilation of the Food-Materials 



The plant grows and continues to form new organs ; for these 

 purposes it continually requires fresh supplies of food-materials. The 

 materials of the food become changed after their absorption, and the 

 substance of the plant is built up from them. They are said to have 

 been ASSIMILATED. By assimilation is understood the transformation 

 of a food-material into the substance of the plant. Those pro- 

 cesses of assimilation in which profound changes take place, e.g. the 

 change from inorganic to organic compounds, are especially interesting. 

 This is particularly the case when we are still unable to experimentally 

 bring about the reaction outside the organism. 



A. ASSIMILATION OF CARBON 

 1. Assimilation of Carbon Dioxide in Green Plants 



The assimilation of carbon dioxide by a green plant is a process of 

 the kind referred to in which organic substance containing carbon is 

 derived from carbon dioxide. In the assimilation of carbon dioxide, 

 soluble carbohydrates such as grape-sugar are formed in the chloro- 

 plast under the influence of sunlight. If we assume that the carbonic 



