AND WHAT THEY DO. 
87 
or cells divided off by whole partitions ; and to rise an inch the sap generally has 
to pass through several hundred such partitions. When there is much wood, the sap 
rises mostly through that. Now the fibres and the vessels of the wood are tubes, 
most of them several times longer than wide ; but their ends do not open into 
each other ; a closed partition divides each cavity from the next, which the sap has 
to get through some way or other. How it gets through so readily, we do not 
altogether know ; but there is no doubt about the fact. 
268. Carried into the leaves, and distributed through their broad surface, the 
crude sap is exposed to the light and air. A large part of it is water ; and each 
drop of this serves to bring up a minute portion of earthy matter, which it dissolved 
out of the soil. Most of the water, no longer wanted, is evaporated from the leaves 
by the warmth of the sun, and exhaled ; that is, it escapes in vapor into the air, 
mostly through the breathing-pores (264). What remains, the plant is at the same 
time 
269. Digesting or Assimilating. Assimilating is the proper word. To assimilate 
is to make similar, or to turn into its own substance. This is just what plants do 
in their leaves. They change into vegetable matter that which was mineral matter 
(air, earth, or water) before. This they do only in the leaves, or other green parts, 
and in these only when they are exposed to the light of day, that is, to the influ- 
ence of the sun. We see, therefore, why plants are so dependent on the light. 
They cannot grow without it, except so far as they are fed by vegetable matter 
prepared beforehand; — as the seedling is fed at the beginning, by vegetable matter 
of the parent plant stored up in the seed ( Chap. II. Sect. II.) ; and potato-shoots, 
by that provided in the tuber or potato (74, 75), &c. This enables them to begin 
their growth in the dark. But the inheritance only serves to set up the young 
plants; when they have exhausted it, they have to work for themselves, to take 
in air and water, and a little earth, and assimilate it, — i. e. make vegetable matter 
of it, — in their leaves or other green parts, with the help of sunshine. This they 
do throughout the whole growing season. 
270. The new-made vegetable matter is dissolved in the water or the sap in the 
leaf, and forms a thin mucilage. This is prepared or Elaborated Sap , fit to be 
used in growth ; for it contains the same material as that which the plant itself is 
built of. It is to the plant just what the prepared clay is to the earthen vessel, or 
to the bricks of which the house is built. It has only to be conveyed where it is 
wanted and used for growing. 
