MOVEMENTS OF NUTRIENT MATTERS, 8343 
are formed in a plant are produced in the leaves, 
and must necessarily find their way down to nourish 
the stem and roots. The facts just mentioned demon- 
strate, indeed, that they do go down in the bark. We 
have, however, no proof that there is a downward 
Jlom of sap. Such a flow is not indicated by a single 
fact, for, as we have before seen, the only current of water 
in the uninjured plant is the upward one which results 
from root-action and evaporation, and that is variable and 
mainly independent of the distribution of nutritive matters. 
Closer investigation has shown that the most abundant 
downward movement of the nutrient matters generated 
in the leaves proceeds in the thin-walled sieve-gglls of the 
cambium, which, in exogens, is young tissue common to 
the outer wood and the inner bark—which, in fact, unites 
bark and wood. The tissues of the leaves communicate 
directly with, and are a continuation of, the cambium, and 
hence matters formed by the leaves must move most rapid- 
ly in the cambium. If they pass with greatest freedom 
through the sieve-cells, the fact is simply demonstration 
that the latter communicate most directly with those parts 
of the leaf in which the matters they conduct are organized. 
In endogenous plants and in some exogens (Piper me- 
dium, Amaranthus sanguineus) the vascular bundles con- 
taining sieve-cells pass into the pith and are not confined to 
the exterior of the stem. Girdling such plants does not give 
the result above described. With them, roots are formed 
chiefly or entirely at the base of the cutting, (Hanstein,) 
and not above the girdled place. - 
In all cases, without exception, the matters organized in 
the leaves, though most readily and abitidantly moving 
downwards in the vascular tissues, are not confined to 
them exclusively. When a ring of bark is removed from 
a tree, the new cell-tissues, as well as the vascular, are in- 
terrupted. Notwithstanding, matters are. transmitted 
downwards, through the older wood. When but a narrow 
