110 THE BIOLOGICAL PROBLEM OF TO-DAY 



these, they require the active energy of sunlight. 

 We can now see the chief requirements to which the 

 constitution and arrangement of the cells in a 

 multicellular plant must be adapted. Plant cells 

 may become clothed in a thick membrane, as that 

 would prove no hindrance to the passage of gases 

 and easily diffusible salts ; but they must be 

 arranged so as to present the greatest possible 

 surface to the surrounding media (i.e., to the soil 

 and the water, the air and the sunlight) whence is 

 drawn their supply of matter and force. The cells 

 must turn a broad face to the outside ; this they 

 do by becoming arranged in branching rows, or in 

 leaf-shaped flattened organs. That they may suck 

 up water and salts from the soil, the ceils are 

 arranged as a highly branched system of roots, 

 covered with delicate hairs, and penetrating the soil 

 in every direction. To inhale the carbonic acid 

 from the air, and to be subjected to the influence 

 of sunlight, the aerial part of the plant stretches 

 out its branches towards the light, and becomes 

 folded into the flat leaves, the structure of which 

 reveals a suitability for assimilation. Thus the 

 whole architecture of a plant is superficial and 

 visible; internal differentiation into organs and 

 tissues either is wanting, or, compared with animals, 

 is very scanty. It is only in the higher plants that 

 the internal fibro-vascuiar tissues appear; these 

 serve a double purpose : they act as channels along 

 which the sap passes, so bringing together the 

 different materials absorbed by roots and leaves ; and 

 they have the mechanical function of strengthening 



