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 

 is 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 



