Metals and Plants. 
471 
acid among the poisons, and Peiigot (21) agrees with him. 
Paul and Kroenig do not find it extremely toxic. Traces of boron 
have been found in the ash of numerous plants. 
It is likely that iodine alone among the substances we have 
tested injures plants directly, acting in the elementary state as 
the elementary halogens have been found before to attack 
plants more violently than do their acids. But iodine is so dis¬ 
tinct from the metals in all respects that its behavior throws 
no doubt on our views as to the mode of their action. 
With the exception, because of their extreme resistance to 
corrosion, of the first four or five metals in our table, we find 
that just those metals poison plants when present in water 
whose salts are already known to be toxic; which is the final 
evidence in favor of our original view, that in our experiments — 
and presumably in Nageli’s too—it is still strictly the salts act¬ 
ing in their ordinary, characteristic ways, which kill the plants. 
It is a well recognized fact in animal physiology that the 
phenomena of stimulation and of poisoning are very intimately 
related, and the applicability of the same principle in plant 
toxicology has been constantly forced upon us. The same thing 
has been observed to apply to certain fungi by Raulin (24), 
Pfeifer (22. Foot note, p. 233), and Richards (25) for manganese, 
zinc, iron, cobalt and so on; and a statement of Risse in Sach’s 
Experimental Plant-Physiology indicates that zinc sometimes 
exerts a similar influence on phanergams. Frank and Krueger 
(5) find that by proper application of copper a potato plant can 
be stimulated to the production of sturdier leaves and more 
chlorophyll, and to more active transpiration and assimilation, 
and longer life. In our own experiments we have seen copper and 
cobalt certainly, and boron, lead, and tungsten probably, exert 
a stimulating influence in individual instances. And we have 
found that, except when the control plants were selected as 
especially thrifty, those that grew in the presence of gold and 
platinum, which can have entered into solution only in the 
most infinitisimal amounts, were uniformly of more vigorous 
growth. The subject of chemical stimulants is a most inviting 
one for further study. 
University of Wisconsin , Madison, Wis., December, 1899. 
