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 
