CHAP, i.] PLANT ARCHITECTURE. 27 



the latter are, as stated above, permanently fixed to one 

 place by the root. This marked difference depends on 

 the nature of the food. Plant food consists of carbonic 

 dioxide obtained from the atmosphere along with certain 

 soluble substances present in the ground that are taken 

 in by the roots in solution in water; now the composition 

 of the atmosphere is practically the same everywhere, 

 and the substances derived from the ground are very 

 widely diffused, hence it may be stated in broad terms 

 that wherever a plant grows, it is certain to be surrounded 

 by its food, consequently the power of locomotion is 

 unnecessary, the only fluctuating factor in determining 

 the presence or absence of plant life being water, and 

 in proportion to the relative scarcity of the latter we 

 observe a corresponding scarcity of plant life, which is 

 altogether absent from the dryest parts of the earth's 

 surface, not on account of the absence of plant food, but 

 the absence of water for dissolving the food which can- 

 not be absorbed by the plant in the solid form. All 

 plant food is absorbed in the gaseous or liquid condi- 

 tion, never as a solid. Animals, on the other hand, feed 

 on organic food that is never so universally diffused as 

 the inorganic food of plants, consequently the power of 

 moving from place to place for the purpose of obtaining 

 the requisite amount of food. 



(2) Leaves. The green, flattened portions of a plant 

 known as leaves perform several functions, the most 

 important and universal being the following: Nutrition, 

 in the sense of taking in food ; carbonic dioxide, as pre- 

 viously stated, being absorbed from the atmosphere by 

 the green parts of plants. Transpiration, or the escape 

 of water into the atmosphere in the form of vapour 



