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 : * 



I 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. 



3.55J of the volume of Begonia bydrocotylifolia (succulent) 

 is air-space. 



53^ of the volume of the leaf of Polypodium setig-erumf 

 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 in 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 in 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 wbich other data may be found. 



f Estimated from Stahl's figure in Botan. Zeitung, plate IV., flg. 7, 1894. 

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



