ABSORPTION AND MOVEMENT OF WATER 155 



the whole plant is due to the high gas-pressure in the inter- 

 cellular spaces as well as to the turgescence and form of its 

 component cells. How much of the size, and to a certain 

 extent of the form also, of plants depends upon maintaining 

 the air-spaces in expanded condition can only be roughly 

 guessed from these figures : * 



| to of the volume of the leaves of most land plants 

 is air-space. 



71% of the volume of Pistia texensis (a floating plant) is 

 air-space. 



3.5% of the volume of Begonia hydrocotylifolia (succulent) 

 is air-space. 



53% of the volume of the leaf of Polypodium setigerum} 

 is air-space. 



TRANSLOCATION OF FOODS 



There remains for us to consider in this chapter the trans- 

 location of foods. Through the xylem elements, especially 

 through the ducts and tracheids, aqueous solutions of cer- 

 tain food-materials are transferred from the absorbing root- 

 hairs to the elaborating chlorophyll-containing parenchyma- 

 cells of the leaves. To these same chlorophyll-containing 

 cells the other food-material, carbon-dioxide, makes its way 

 through the stomata and the intercellular spaces. In these 

 cells water and carbon-dioxide are consumed in elaborating 

 a carbohydrate, a food which accumulates hi the same or in 

 chemically closely related form in the cells which manufac- 

 ture it. From these cells the food must be removed, unless 

 it is also to be stored there, to parts needing it at once or 

 , in which it can be kept in reserve for future use. 



In order to secure the transfer of the non-nitrogenous 

 food manufactured hi the green tissues during the hours of 

 daylight, it is usually necessary to change the chemical con- 

 stitution or composition of the food. If starch or oil is 

 the form in which the carbohydrate elaborated by the 



* Pfeffer W. Pflanzenphysiologie. I., p. 164. Engl. transl., I., p. 182. 

 Various papers there referred to. in wMch other data may be found. 



I Estimated from Stahl's figure in Botan. Zeitung. plate IV., fig. 7, 1894. 

 This is a land plant from one of the rainiest regions in the tropics. 



