710 PART IV. — THE PHYSIOLOGY OF PLANTS. 



2. The Food of Plants. The constituents of the ash do not 

 form a merely accidental mixture ; it has been proved by experi- 

 ment that certain inorganic compounds are absolutely necessary 

 to the life of the plant. Those chemical elements which the 

 plant requires for its nutrition, and which must therefore be re- 

 garded as part of its food, are : — 



I. Non-metallic Elements : — Carbon, hydrogen, oxygen, nitro- 

 gen, sulphur, phosphorus, and perhaps chlorine. It must, 

 however, be clearly understood that all these elements 

 also exist in the plant to some extent as inorganic com- 

 pounds, carbonates, nitrates, phosphates, sulphates, of the 

 metals mentioned below. 

 II. Metallic Elements : — Potassium, calcium, magnesium, iron. 



Besides these we find in the ash of many plants — though they 

 cannot be regarded as essential to nutrition — the following ele- 

 ments : sodium, lithium, manganese, silicon, iodine, bromine, and 

 in rare cases, also aluminium, copper, zinc, cobalt, nickel, stron- 

 tium, and barium. Fluorine must also exist in vegetables, for it 

 is found in a perceptible quantity in the dentine of animals which 

 feed directly or indirectly on vegetables. 



The essential elements of the food will now be severally dis- 

 cussed. 



Carbon. Plants which possess chlorophyll obtain their carbon 

 mainly from the air (or, in the case of submerged plants, from the 

 water) in the form of carbon dioxide. The absorption of carbon 

 dioxide is, however, limited to those cells which actually contain 

 chlorophyll, and it can only go on even in those cells so long as 

 they are exposed to sufficiently intense light. 



Although plants possessing chlorophyll can and do use carbon dioxide as 

 carbonaceous food, yet there is reason to believe that they may supplement this 

 by absorbing more complf x carbon-compounds from the soil by their roots. 

 Thus, a remarkable case of symbiosis (see p. 273), has been observed in various 

 plants (especially trees belonging to Corylaceae and Fagaceae) : the absorbent 

 region of the roots is more or less closely covered by a felt-work of fungus- 

 mycelium, the whole structure being termed mycorhiza : itis sug gested, and the 

 suggestion is based principally on the fact that the development of the mycorhiza 

 is the more marked the richer the soil in decomposing organic matter (humus), 

 t hat the fun gus jaromotes the absorption of the organic matter by the roots oL 

 the t ree. In certain other cases {e.g. Drosera, Dionoea, Utricularia, etc.), green 

 plants are provided with a special mechanism, in the form of modified leaves, 

 for obtaining a supply of organic carbon-compounds. Such plants are said to be 



