66 PLANT PHYSIOLOGY 



The water liberated in the cell by the condensation of 

 starch from sugar passes off in the ways to be discussed 

 later in connection with the transfer of water (see 

 p. 137). 



The food formed is used by the cell which forms it and by 

 the other cells of the plant, either immediately or later. 

 As we have seen, the food accumulates temporarily, hence 

 only small amounts are used immediately. What is removed 

 is soon disposed of in one or more ways; fii*st, it may be 

 used at once to build cell-walls, or to furnish energy by be- 

 ing respired ; second, it may be stored as reserve food in or- 

 gans other than those that produced it ; third, it may be still 

 further elaborated, serving as the basis for the construction 

 of nitrogenous food. This leads us from the relations of 

 carbon in the nutrition of plants to those of nitrogen. 



NITROGEN 



Although as essential a constituent element of the living 

 matter and therefore of the food of all organisms as car- 

 bon, nitrogen forms a much smaller percentage of the dry 

 weight of plants, and its distribution in the body is much 

 less uniform. Carbon is found in all the skeletal parts of 

 the body of a plant as well as in the living matter of the 

 cells. The cell-walls, containing no nitrogen, composed 

 mainly of a cellulose or of some derivation of a cellulose, 

 form the skeleton of the whole plant-body, of the dead as 

 well as the living parts. On the other hand, nitrogenous com- 

 pounds occur only in the living parts or as small remnants 

 in parts formerly living. They are found as reserve foods in 

 the cells and as the living constituents of the protoplasm. 

 The amounts and the distribution of nitrogen in the plant- 

 body are indicated by the following table : * 



In mushrooms 7.26% N. in the dry weight 



" lupin seeds 5.0% " " " " " 



" pea " 3.5% " " " " " 



" rye fruits 1.9% " " " " 



" green leaves 3-5 % " " " " 



11 potato tubers 0.3 % " " fresh condition. 



* Frank, A. B. Lehrbuch der Botanik, Bd. I., p. 563, 1892. 



