t. From this it naturally results that the growth and
spread of the leaves must largely depend upon the supply of carbon, as
the growth and fatness of sheep depends upon the supply of pasturage.
Under most circumstances, to be sure, there is carbon enough and to
spare lying about loose for every one of them; but conditions do now
and again occur where we can clearly see the importance of the carbon
supply. Water, for example, contains practically much less carbonic
acid than atmospheric air, especially when the water is stagnant, and
therefore not supplied fresh to the plant from moment to moment. As a
consequence, almost all water-plants have submerged leaves very narrow
and waving, while floating plants, like the water-lilies, have them
large and round, owing to the absence of competition from other kinds
about, which enables them to spread freely in every direction from the
central stalk. Moreover, these leaves, lolling on the water as they do,
have their mouths on the upper instead of the under surface. But the
most remarkable fact of all is that many water plants have two entirely
different types of leaves, one submerged and hair-like, the other
floating and broad or circular. Our own English water-crowfoot, for
example, has the leaves that spring from its stem, below the surface,
divided into endless long waving filaments, which look about in the
water for the stray particles of carbon; but the moment it reaches the
top of its native pond the foliage expands at once into broad lily-like
lobes, that recline on the water like oriental beauties, and absorb
carbon from the air to their heart's content, The one type may be
likened to gills, that similarly catch the dissolved oxygen diffused in
water; the other type may be likened to lungs, that drink in the free
and open air of heaven.

Equally important to the plant, however, with the supply of carbonic
acid, is the supply of sunshine by whose aid to digest it. The carbon
alone is no good to the tree if it can't get something which will
separate it from the oxygen, locked in close embrace with it. That
thing is sunshine. There is nothing, therefore, for which herbs, trees,
and shrubs compete more eagerly than for their fair share of solar
energy. In their anxiety for this they jostle one another down most
mercilessly, in the native condition, grasses struggling up with their
hollow stems above the prone low herbs, shrubs overtopping the grasses
in turn, and trees once