126 BOTANICAL GAZETTE | AUGUST 
both plants and animals. The toxic effect of a substance is 
evidently due to one of two things. In plant tissue the effect 
upon the furgescence of the cell must play an important part,’ 
since when the turgor-pressure is suddenly and decidedly 
decreased, the growth is either retarded or inhibited. An inhi- 
bition or a retardation of growth must then be regarded as 4 
symptom of poisoning. In comparison with turgescence the 
direct effect upon the protoplasm is by far the more important, 
since in simple turgor experiments the retardation of growth Is 
due to the irritation of the protoplasm as well as to the turgor 
change. Now, since the irritability of the protoplasm of dif 
ferent plants differs widely, we may reasonably expect quite ; 
wide range in the amount of different compounds which various 
plants will withstand. 
Why and how certain substances have a toxic effect, and 
certain others a nutritive value, is not known. For example, tt 
is at present impossible to say why it is that potassium sulfate 
has a nutritive value while copper sulfate produces a toxic effect. 
Those substances which are poisonous to plants are generally 
such substances as are not accessible to plants in their normal : 
habitats, at least to any extent, while those substances which ate 
generally present in the soil have no injurious effect, oF at least 
not in the same degree of concentration at which we find sae : 
in the soil. If the poisonous action of various substances ie 4 
ies matter of adaptation and adjustment, as seems . Z a 
highly probable, then we might expect that by adding gradually : 
more and more copper sulfate to the soil in which a plamt ” — 
rowing it would come in the course of time to adapt itself oe 
a a large amount of this salt, which is normally extremely 
Porsonous to the majority of plants. So far as known nt a 
pcb of exactly this nature have been carried out, 4" 
experimental test of this would be highly interesting: ae 
2. METHOD; 
In the experiments performed, three different pe 
used: Pisum sativum, Zea Mais, and Cucurbita Pepe. 
7 Annals of Botany g : 385, 1895. . 
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