42] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 177 



CHAPTER II. 

 SPECIAL PHYSIOLOGY OF THE NUTRITWE FUNCTIONS. 



42. Absorption. The food of plants is absorbed, generally 

 speaking, either from the soil or from the air. 



t Plants which do not possess chlorophyll (e.g. Fungi) usually 

 obtain the whole of their food from the soil ; whereas plants which 

 do possess chlorophyll absorb from the air one of the most im^ 

 portant constituents of their food, namely carbon dioxide, though 

 in exceptional cases it is obtained from other sources; for 

 instance, parasitic plants absorb their food from the hosts upon 

 which they live, and the " insectivorous " plants absorb a portion, 

 at least, of their food, from the insects which are caught by their 

 specially adapted leaves. Submerged aquatic plants absorb their 

 food entirely or mainly from the water in which they live. 



The food of plants is always absorbed in the fluid form ; either 

 as a liquid or as a gas. The liquid food, consisting of a watery 

 solution of various substances, is absorbed from the soil most 

 commonly by the roots, or, in the absence of roots, by other 

 members (shoots, leaves) which have become specially adapted 

 for the performance of this function; the gaseous food (C0 2 ) is 

 absorbed from the air by the green parts of plants, and, in 

 the more highly differentiated forms, more especially by the 

 leaves. It not infrequently happens that chemical elements 

 are found in plants which are known to be present in the soil 

 in the form of compounds which are insoluble in water. These 

 compounds are brought into solution by various means. For 

 instance, the soil usually contains carbon dioxide, which is 

 evolved from the decomposing animal or vegetable matter which 

 is commonly present, and from the absorbent organs them- 

 selves ; and it is well known that various substances, such as cal- 

 cium carbonate and certain silicates, which are insoluble in pure 

 water, are soluble in water charged with carbon dioxide. Again, 

 the sap which fills the vacuoles and saturates the walls of root- 

 hairs, has an acid reaction, due to the presence of organic acid, and 

 this acid sap will dissolve many substances which are insoluble in 

 pure water. The solvent effect of this acid sap is well demonstrated 

 by means of the familiar experiment with a slab of marble. If a 

 highly-polished slab of marble be fixed in the bottom of a flower- 



M.B. N 



