FOOD-MATERIALS AND THEIR ABSORPTION 203 



example, they are not able to utilise all nitrogenous compounds 

 as sources of nitrogen, nor are they able to obtain their necessary 

 carbon from all kinds of carbon-compounds. 



A compound to be of service as a food-material capable ot 

 supplying a particular element for the nutrition of a plant, must 

 (i) be soluble and able to diffuse through the cell-wall and proto- 

 plasm of the cells, and (ii) must also possess a certain chemical 

 structure. 



The carbon dioxide gas present in the air is the chief source 

 from which the carbon is obtained ; the absorption and subse- 

 quent use of the gas is discussed in the succeeding chapter. 



The food-materials furnishing the rest of the elements needed 

 by plants are obtained from the soil by osmosis through the 

 root-hairs. Before they can enter the latter they must be in 

 solution, since no solid particle however small is able to 

 pass through the closed cell-membranes of the absorbent 

 hairs. 



Moreover it is only from weak solutions of food-materials that 

 plants can absorb what they need ; plants grown by the 

 method of water-culture make the most satisfactory progress 

 when the total amount of solids dissolved in the water does not 

 exceed from '2 to '5 per cent, or 2 to 5 parts in 1000 of water. 

 Solutions containing 2 or 2^ per cent, of dissolved substances 

 act injuriously upon the protoplasm of the plant, and prevent 

 growth : hence the importance of avoiding readily soluble 

 manures in excess. 



The water of the soil from which plants obtain all they need 

 usually contains not more than 'oi to "03 per cent, of solid 

 matter dissolved in it. 



Carbon dioxide gas is produced within the soil in the processes 

 of putrefaction and decay of the manures present, and is excreted 

 to a slight extent in the respiration process carried on by the 

 protoplasm of the root-hairs. This gas indirectly assists plants 

 to absorb useful food-materials, for some of the latter which are 



