86 
WHY PLANTS GROW, 
also absorbed by the leaves, either from drops of rain or dew, or from the vapor of 
water in the air. Air is largely absorbed by the leaves, and some also by the roots, 
either as dissolved in water, or else directly from the crevices and pores of the 
soil, which are tilled with air. 
264. Plants absorb their food by their sin face. Animals have an internal cavity, 
— a stomach , — - to hold their food ; and from the stomach it is taken into the 
system. Plants have nothing of this kind. They absorb their food by their sur- 
face,— by the skin, as it were ; and when very young and with the whole sur- 
face fresh and thin, by one part almost as much as another. But as they grow 
older and the skin hardens, they absorb mostly by their fresh rootlets and the tips of 
the roots, and by the leaves, — - the former spread out in the soil, the latter spread 
out in the air. For while the skin or bark of the older parts of the roots is hard- 
ening, new tips and rootlets are always forming in growing plants, with a fresh sur- 
face, which absorbs freely. And as to the leaves, they are renewed every year 
(even evergreens produce a new crop annually, and the old ones fall after a year 
or two) ; and the skin of every leaf, especially that of the under side, is riddled 
with thousands of holes or little mouths (called Breathing-pores ), which open into 
the chambers or winding passages of the pulp of the leaf, so that the air may cir- 
culate freely throughout the whole. 
265. Plants absorb their food all in the fluid form. They are unable to take 
in anything in a solid state. They imbibe or drink in all their food, in the form of 
water, with whatever the water has dissolved, and of air or vapor, by one or both 
of which their leaves and roots are surrounded. The reason they imbibe only fluid 
is this. The roots, leaves, and all the rest of the plant, under the microscope, are 
seen to be made up of millions of separate little cavities, each cut off from the 
surrounding ones by closed partitions of membrane. All that the plants take into 
their system has to pass through these partitions of membrane, — which fluid (air 
or moisture) alone can do. 
266. The common juices of plants are called Sap. What they take in from the 
soil and the air, not being digested or made into vegetable matter, is called Crude 
Sap. All that the roots imbibe has to be carried up to the leaves to be digested 
there. So while the roots are absorbing, the stem is 1 
267. Conveying the Crude Sap to the Leaves. There is no separate set of vessels, 
and no open tubes or pipes for the sap to rise through in an unbroken stream, in 
the way people generally suppose. The stem is made up, like the root, of cavities. 
