294 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
to the substance which it produces, but other growing tips are susceptible 
to it. 
The alternative assumption, that the substance is transported in the 
fluids of the plant and removed in some way from them in the cooled zone, 
does not serve any better than the first for the interpretation of certain 
experimental results. In fact, the hypothesis of inhibiting substances and 
their transportation, in whatever form we state it, does not account for the 
fact that within certain limits of temperature the effectiveness of a cooled 
zone of certain length in the stem of the bean seedling decreases with in- 
creasing distance from the buds to be isolated, even though the more distant 
zone may be more effective in inhibiting the growth of the chief tip. In other 
words, a cooled zone which has a marked inhibiting effect upon the movement 
of water and salts in the stem is not necessarily effective in isolating buds 
below, while a zone which has no appreciable inhibiting affect on regions 
above it may be effective in isolating buds below. 
Assumptions concerning the transportation of nutritive substances are 
not, I believe, any more satisfactory than the hypothesis of inhibiting sub- 
stances in aiding us to account for the facts of physiological correlation in 
the plant. As already noted, physiological isolation of buds below a 
cooled zone may be brought about without retarding the movement of 
water and salts to any marked degree. In the case of Bryophyllum, the 
leaf itself is able to produce starch, and in the bean seedling the reserves of 
the cotyledons are available for the buds in the axils of the cotyledons and 
those produced by the first pair of leaves are available for the buds of the 
second node. In the saxifrage runner the cooled zone may retard to some 
extent the passage of nutrition to the runner tip, but the results with the 
other species show clearly enough that this is not the primary factor in the 
physiological isolation of parts in plants. In the case of the Bryophyllum 
leaf with cooled zone about the petiole, the passage of water and salts to 
the leaf may be somewhat retarded, but the leaf contains plenty of starch. 
In the bean seedling the passage of water and salts to the buds to be isolated 
is not interfered with, since the cooled zone is above them, and here also 
plenty of carbohydrate is available. In both these cases, as well as in the 
saxifrage runner, physiological isolation and growth of buds occur. Again, 
if we cut off the stem of the bean seedling below the first foliage leaves and 
remove the cotyledons, the buds in the axils of the cotyledons, which are 
now the only buds on the plant, will develop, although in the absence of 
the more apical parts of the plants the movement of water and salts must 
be greatly decreased and the removal of leaves and cotyledons must have 
decreased the amount of carbohydrate available. In short, the result as 
regards development of the buds is essentially the same under experimental 
conditions which must determine very different internal conditions as 
regards the movement of nutritive substances and the amount available 
in a particular region. 
