THE VEGETATIVE ORGANS OF PLANTS. 283 
The final result of the filling up of the cells of the heart- 
wood is to make this part of the stem almost or quite im- 
passable to sap, so that the interior wood may be removed 
by decay without disturbing the vigor of the tree. 
Passage of Sap through the Stem,—The stem, besides 
supporting the foliage, flowers, and fruit, has also a most 
important office in admitting the passage upward to these 
organs, of the water and mineral matters which enter the 
plant by the roots. Similarly, it allows the downward 
transfer to the roots, of substances gathered by the foliage 
from the atmosphere. To this and other topics connected 
with the ascent and descent of the sap we shall hereafter 
recur. ° 
The stem constitutes the chief part by weight of many 
plants, especially of forest trees, and serves the most im- 
portant uses in agriculture, as well as in a thousand other 
industries. 
§ 3, 
LEAVES. 
These most important organs issue from the stem, are 
at first folded curiously together in the bud, and after- 
wards expand so as to present a great amount of surface 
to the air and light. 
The leaf consists of a thin membrane of cell-tissue, ar- 
ranged upon a skeleton or net-work of fibers and ducts. 
It is directly. connected with, and apparently proceeds 
from, the cambial-layer of the stem, of which it may, ac- 
cordingly, be considered an expansion. 
In certain plants, as the cactus (prickly pear), there 
scarcely exist any leaves, or, if any occur, they do not, 
differ, except in external form, from the stems. Many of 
these plants, above ground are in form, all stem, while in 
structure and function, they are all leaf 
