SECT. II PHYSIOLOGY 213 



brought about in maintaining a sufficient supply of water, not only arrests tran- 

 spiration, but also prevents the entrance of CO.2 into the plant. 



In s]iite of their enormous number, the total area of the stomatal apertures is 

 only about 1 })er cent or little more of the whole surface area, for each individual 

 opening is very small (about O'OOOl sq. mm. in Helianthus). Brown and Escombe 

 have, however, shown that the minuteness of the openings, combined with the 

 peculiar distribution of the stomata, results in a greatly accelerated rate of 

 diffusion. Diffusion through a number of fine apertures is much greater than 

 through a single aperture of the same total area. When the small openings are 

 placed about ten times their diameter apart, the diffusion is nearly as rapid as when 

 no separating wall is present. The distribution of the stomata in the epidermis 

 very nearly meets these requirements. For example, a square metre of the surface 

 of a Catalpa leaf absorbs abovit two-thirds the amount of carbonic acid gas taken 

 up in an equal time by the same area of potash solution freely exposed to the 

 air (2»). 



The Movement of Gases from cell to cell, or from intercellular spaces to the 

 cells and their movement within the intercellular spaces, takes place according to 

 the principles discussed on p. 188. 



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. Considered 

 generally this process of assimilation might take place without chemical 

 change of the substance that has been absorbed. As a rule, howevei', 

 a chemical change is involved. Those processes 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. This process is of such great im- 

 portance that it is frequently spoken of without any qualification as 

 " Assimilation." This cannot be justified, since the assimilation of 

 carbon dioxide is only one of a number of processes of assimilation 

 which, taken together, are chai'acteristic of the vegetable organism. 



In the assimilation of carbon dioxide, soluble carbohydrates such 

 as grape-sugar are formed in the chloroplast under the influence of 



