60 PLANT LIFE 



swiftly flowing streams, the roots may ex- 

 hibit new and remarkable developments that 

 especially fit their possessors to occupy such 

 stations. 



Let us inquire somewhat more closely as 

 to what are the special qualities, both of 

 general behaviour and anatomical structure, 

 which render a terrestrial life possible for 

 plants. If we select a concrete example of 

 a land plant, such as an oak tree, we observe 

 that there is a large branching top, covered 

 with leaves for part of the year. Below, this 

 crown passes into the trunk, and the latter 

 again ends in the branched root system under- 

 ground. The leaves are, of course, the fac- 

 tories in which the operation of food-making 

 is going on so long as they are exposed to the 

 light. The roots are absorbing water from 

 the soil, and such salts as are dissolved in it, 

 whilst the trunk forms an intermediate con- 

 ducting region through which exchange be- 

 tween the substances in the root and the rest 

 of the tree can take place. The circulation 

 of materials in a plant is not really like the 

 circulation of the blood in animals, although 

 an analogy largely a false one is often 

 drawn between them, for there is no con- 

 tinuous circulating system in the oak tree 

 at all comparable with the arteries and veins 

 of the animal body. Nevertheless there is 

 a process of exchange, though arranged on 

 different lines, and serving quite different 

 ends. In order to grasp this clearly, it will 



