it is that which admits,
regulates, or restrains the visible motions of the leaf; while the system
of circulation can only be studied through the microscope. But the ribbed
leaf bears itself to the wind, as the webbed foot of a bird does to the
{45} water, and needs the same kind, though not the same strength, of
support; and its ribs always are partly therefore constituted of strong
woody substance, which is knit out of the tissue; and you can extricate
this skeleton framework, and keep it, after the leaf-tissue is dissolved.
So I shall henceforward speak simply of the leaf and its ribs,--only
specifying the additional veined structure on necessary occasions.

10. I have just said that the ribs--and might have said, farther, the stalk
that sustains them--are knit out of the _tissue_ of the leaf. But what is
the leaf tissue itself knit out of? One would think that was nearly the
first thing to be discovered, or at least to be thought of, concerning
plants,--namely, how and of what they are made. We say they 'grow.' But you
know that they can't grow out of nothing;--this solid wood and rich tracery
must be made out of some previously existing substance. What is the
substance?--and how is it woven into leaves.--twisted into wood?

11. Consider how fast this is done, in spring. You walk in February over a
slippery field, where, through hoar-frost and mud, you perhaps hardly see
the small green blades of trampled turf. In twelve weeks you wade through
the same field up to your knees in fresh grass; and in a week or two more,
you mow two or three solid haystacks off it. In winter you walk by your
currant-bush, or your vine. They are shrivelled sticks--like bits of black
tea in the canister. You pass again in May, and {46} the currant-bush looks
like a young sycamore tree; and the vine is a bower: and meanwhile the
forests, all over this side of the round world, have grown their foot or
two in height, with new leaves--so much deeper, so much denser than they
were. Where has it all come from? Cut off the fresh shoots from a single
branch of any tree in May. Weigh them; and then consider that so much
weight has been added to every such living branch, everywhere, this side
the equator, within the last two months. What is all that made of?

12. Well, this much the botanists really know, and tell us,--It is made
chiefly of the breath of animals: that is to say, of the substance which,
during the past year, animals have breathed in