THE PLANT AS A WORKING MACHINE 13 
and let it remain for a few minutes, and then remove it and 
examine it by sectioning, definitely stained regions would be 
seen, other regions being unstained, thus showing not only 
that the liquid passed upward through the leafstalk, but that 
it passed through certain tissues of the stalk. If a leafstalk 
of celery is carefully broken and one part pulled slowly away 
from the other, there are seen fibers, or threads, which are 
quite like those shown in the cornstalk (fig. 7). These threads 
are known as fibrovascular bundles, which means simply “ col- 
lections of thread-like tubes.” It is through these fibrovas- 
cular bundles that water and substances in solution in water 
pass from the soil through roots, through stems, and into 
leaves. Through them also plant foods may pass from leaves 
downward through the plant. Indeed, there are certain parts 
of each bundle through which water passes upward, and other 
parts through which the organized plant foods are carried. 
The fibrovascular bundles, therefore, are the chief transporta- 
tion lines of the plant. 
11. Leaves. Most leaves are expanded so that they expose 
much more surface than would stems of equal weight. In 
some cases (fig. 3) the entire leaf is expanded, while in others 
there is a leafstalk, or petiole, and the expanded portion, the 
blade. The leaf blade may be single (simple) or sub-divided 
(compound). From the plant stem the fibrovascular bundles 
extend into the leaf, where they are known as the veins of the 
leaf. They terminate in the leaf, sometimes in its tip and 
sometimes in the margin as well as in the tip. Water from 
the soil may therefore pass through the fibrovascular bundles 
of the roots, the stem, and the leaves, into the interior of the 
leaf. From the leaf some of this water is evaporated into 
the air. 
Part of the water in the leaf, instead of being evaporated 
into the air, is used in the construction of plant food by means 
of a process which is of very great importance to all living 
things. Carbon dioxide from the air enters the leaf through 
its surface. The leaf is green because of the presence of a 
