362 
AMERICAN JOURNAL OF BOTANY 
IVol. io, 
drawing solutes from, or secreting them into, the vessels canying the 
water, might also influence their movement. 
The fact that solutes have actually been found in the water-conducting 
vessels and that they have been obtained from cut and bleeding stems has 
been offered as proof that they are carried with the water. As just stated, 
however, unless it is known that the water moves by mass flow through 
open tubes and not by diffusion or through occasional membranes, or 
unless it is known that neighboring cells do not remove solutes from a 
passing stream, perhaps transferring them and reintroducing them at a 
lower level, this can not be considered as conclusive evidence. Further¬ 
more, the method of flow or bleeding from a cut stem may be different 
from that in a normal uninjured stem in the same sense that a movement 
of water through a series of cells with membranes enclosing solutions may 
be very different from the flow that would occur if some of the membranes 
were cut open. 
The fact that, when cut stems are placed in solutions of various dyes, 
the solutes can be found at considerable distances in the xylem tissues 
has been considered as proof that solutes must be carried with the water. 
Normally, however, the water-conducting system may be considered as 
a closed system with no actual openings. The objections offered to the 
evidence from cut and bleeding stems should, therefore, be considered in 
this type of experiment also. Experiments involving the injection of 
dyes or other solutes through incisions of one type or another might be 
invalid for the same reason. 
A few experiments have been reported in which the movements of dyes 
or of salts, such as those of lithium or iron, have been studied in rooted and 
uninjured plants, and some of these indicate a movement, or in some cases 
an accumulation, in the xylem; but, because of the toxicity of the materials 
used or of their tendency to be taken up by thick cell walls, or, as with lithium, 
the ease with which it penetrates membranes and the lack of quantitative 
determinations, these experiments have not yet given clear indications as 
to the method of movement or as to the tissues concerned in the movement 
of nutrient salts. 
Since nitrogen is so commonly deficient in soils, and since the absorp¬ 
tion of nitrogen can be relatively easily determined both quantitatively 
by analysis and qualitatively by the color of the plant and by its texture 
and growth responses, experiments were planned to determine the effects 
of ringing on the upward movement of nitrogen in stems. In a number of 
cases the ash contents of the leaves were also determined. 
In most of the ringing experiments, twigs or branches were selected 
in pairs so that the ringed and the normal, unringed branches w^ere simi¬ 
lar in size and position. The precaution was also taken to select a branch 
for ringing such that there were always branches below the ringed one, so 
that the roots and trunk below would be well supplied with food and their 
