EXTERNAL CONFIGURATION OF PLANTS. 177 



tree must soon be stripped off or torn into shreds ; 

 a.nd even in still water they could hardly prove of any 

 use to the tree, because ordinary leaves are special- 

 ised, not for the purpose of absorbing water, but to 

 eliminate it. As soon, however, as a leaf is sub- 

 merged, evaporation must cease, and with its cessa- 

 tion the circulation of the sap within the stem and 

 branches comes to a standstill. Add to all this the 

 low specific gravity of ordinary wood, tending to 

 float the tree away, and it will be seen that an 

 oak growing on the sea-bottom is an impossibility. 

 A sea-weed in garden soil would be like a fish out 

 of water, and a tree under water is quite as much 

 *' out of its element." We see, then, that in land plants 

 the differentiation of the plant-body into leaf and 

 stem is a necessity arising out of the low density 

 of air compared with water. The axis of a land 

 plant must be massive and rigid ; flexibility is of 

 advantage to one growing in water. The leaves of 

 a terrestrial j)lant must necessarily be light in order 

 to float in the rarer medium. Terrestrial plants 

 have a further relationship with the soil from which 

 water and inorganic matter have to be taken up. 

 For this pvirpose an organ has been specialised in 

 all the higher plants — the root. The root also fulfils 

 the function of fixing the plant in one place; but 

 the mode of attachment differs strikingly from 

 that observed in marine plants. 



The foregoing illustrations may serve to i)ut the 

 reader's mind in the proper attitude for pursuing 

 the investigation before us. One other remark of 

 a general character may be made. 



The external forms of plants are of greater 

 physiological importance, and are, on the whole, less 

 varied than those of animals. This arises from the 

 circumstance that the food of plants is absorbed by 

 the external surface. Plants are uniformly destitute 

 of a stomach or alimentary canal. The body of one 

 of the higher animals may be regarded as consisting 

 of two concentric cylinders — the outer one repre- 



