395] E. E. Free 197 
but wilting begins immediately and the leaf usually falls 
long before it is entirely yellow. The first effect of chromium 
is a brown discoloration in the vascular bundles of the petioles 
and veins. This is followed by a change of the leaf color 
to a dark green, and the early fall of the leaves. The regu- 
larity and specificity of these changes is attested by many 
repeated observations on different leaves and different plant 
individuals. Similar specificities were observed with the 
plants other than Pelargonium. It seems probable, therefore, 
that the recognition of a poisonous agent by the specific symp- 
toms of its action is as possible with these plants as with 
animal organisms. 
Certain features of the localization of injury in the plants 
is suggestive of relations to transpiration. For instance, with 
boron and lithium on Pelargonium the limitation of injury to 
the edges of the leaves implies its occurrence only where the 
final evaporation of the water of the transpiration stream 
localizes the poison in a concentration sufficient to be toxic. A 
similar conclusion follows from the fact that injury occurs 
first, and sometimes only, on leaves of moderate age, that is 
on those which are in their period of most vigorous transpira- 
tion. Younger leaves and older leaves on the same plant are 
usually uninjured. Similarly, when a Pelargonium plant 
ig poisoned but not killed, by either boron, lithium, mercury 
or iodine, new leaves produced thereafter do not show injury 
while they are young, but develop it after from two to six 
weeks of growth. The same observation was made with 
boron and iodine on Chrysanthemum. Further confirmation 
is the failure of Bryophyllum, which has a very low transpir- 
ing power, to show injury with any poison except boron. 
Even in this case the injury developed eleven weeks later than 
it did on Pelargonium and all the other plants. All of this 
evidence suggests that, in the concentrations used, the poisons 
were carried into the plant incidentally by the transpiration 
stream and produced injury only when and where the evap- 
oration of the transpired water increased the concentration of 
the poison in a local tissue area. The symptoms observed 
