AND WHAT THEY DO. 89 
circulation of the two kinds of sap; and no crude sap exists separately in any 
part of the plant. Even in the root, where it enters, this mingles at once with 
some elaborated sap already there, and as it rises through the stem they mix more 
and more. But whatever is not assimilated must be, before the plant can use it; 
that which has been assimilated, can be used wherever it may be. 
274. The elaborated sap, like the crude, is contained in the cavities, cells, or 
short closed tubes which make up the fabric of the plant, and circulates or passes 
frem one to another through the partitions. How it passes through, and what at- 
tracts it where it is wanted,-we do not very well know. And what we do know 
could not be well explained to the young beginner, for whom this book is written. 
The same may be said about 
275. Growth. Growth is the increase of a living thing in size and substance. 
In plants it is done by the increase in the number of the cells (or cavities divided 
off by partitions) which make up the fabric, and by the increase of each in size to 
a certain extent. That is, growth is the building-up of the plant, or of additions to 
it, out of vegetable matter.. And this vegetable matter was made in the leaves 
(either of the same plant or of its parent), out of: mineral matter drawn from the 
earth and the air,— was mostly made of air and water. For the earthy part 
which is left behind when we burn a plant (and so turn all the vegetable matter 
back into air and vapor of water again) forms only a very small part of its bulk. 
276. If the pupil would learn more particularly how growth takes place, and 
how plants change mineral into vegetable matter, they must study three or four les- 
sons of the Lessons in Botany already referred to. But our short and simple 
account of the plant in action, i. e. vegetating, is sufficient for answering the main 
question, viz. : — : 
277. What Plant do. Vegetation consists essentially of two things, namely, as- 
similation and growth. In assimilation plants are changing mineral matter — air, 
water, and a little earth — into vegetable matter; and in growth this vegetable 
matter is wrought into all manner of beautiful and useful forms. This is the work 
which the vast variety and infinite number of plants over all the earth are busily 
engaged in. It is their peculiar work ; for only plants can live upon (or assimilate) 
mineral matter; they only have the power of changing air, water, and earth into 
organic matter. 
278. What is the effect of this action of plants, especially upon the air we 
breathe? And what becomes of all the vast amount of vegetable matter which 
