34 Elementary Botany 



ROOT FUNCTIONS. 



1. The root is a great organ of support, tending to hold the 

 plant fixed in the soil or other situation in which it may be 

 growing. 



2. It is an organ for the absorption of food. 



This raises three questions, viz. the source of the food, the 

 nature of the food, and the manner of its introduction into the 

 plant. 



A. The source of the food. — The roots, as a rule, enter the 

 ground, and hence receive the plant-nourishment frotn that 

 source. In the case of some aerial plants the roots absorb the 

 food with the moisture present in the air, whilst in the case of 

 many parasites, as the Mistletoe (fig. 6i), the sucking roots 

 penetrate into the stem of the plant upon which it grows, thus 

 obtaining their nourishment second-hand. 



B. Nature of the food. — A chemical analysis of the sub- 

 stance of a plant shows it to be composed of the following 

 elements : — 



Carbon, C. Phosphorus, P. Potassium, K. 



Hydrogen, H. (Silicon, Si.) Calcium, Ca. 



Oxygen, O. (Chlorine, CI.) Magnesium, Mg. 



Nitrogen, N. Iron, Fe. 



Sulphur, S. (Sodium, Na.) 



The five placed in the first column are sometimes called 

 the organic elements, as they are required for the building up 

 of the protoplasm and cellulose. Those placed in brackets are 

 not so necessary or so universally met with as the rest. 



Since, then, the plant body is built up of these elements, 

 it is evident that they must enter into the composition of its 

 food. They are not, however, absorbed in their elementary 

 condition, being obtained from various compounds which con- 

 tain them. 



Carbon is obtained by green plants from carbon dioxide, 

 called also carbonic anhydride, and formerly known as carbonic 

 acid gas, CO 2, a gaseous compound of carbon and oxygen ; 

 perhaps partly by means of the roots ; but especially, as we 



