Permeability 17 
In a similar way the unhealthy development or the death of a 
plant may be taken as evidence of the penetration into it of a poison¬ 
ous substance if this is present in the external medium in addition to 
all the necessary nutrients. The test is equally legitimate if the pre¬ 
sence of a dispensable substance in the external medium should have 
a stimulating effect on the plant and bring about a development above 
the normal. Thus the penetration of copper salts into roots of cereals 
and other plants has been made abundantly clear from the work of 
Phillips (1821) and many later writers, particularly Otto (1893), Coupin 
(1898), Kahlenberg and True (1896), Copeland and Kahlenberg 
(1900), True and Gies (1903) and Miss Brenchley (1910) among others. 
In a similar way the absorption of compounds of zinc, arsenic, man¬ 
ganese and boron has been shown, for a review of the work on which 
the reader is referred to the paper and book of Miss Brenchley 
(1914 a, b) dealing with the effect of these substances on plant growth. 
In Miss Brenchley’s book will be found abundant references to the 
literature of the subject. Even compounds of the alkali metals 
(Coupin, 1901) have been shown to penetrate the roots of cereals by 
their harmful effect on vegetative development. The penetration of 
cyanides has been indicated in the same way (Brenchley, 1917). 
Since this method can only afford a qualitative test of permeability, 
and has done so only incidentally, it is not worth while to deal with 
it in any greater detail here. It must, however, be pointed out, that 
the toxicity of a substance may indeed be a clear indication that it 
reaches the protoplasm, but it is no indication that the substance 
has the power of passing through the protoplasm before the pro¬ 
perties of the latter, in respect of permeability, have been com¬ 
pletely changed by the action of the substance. The test of toxicity 
is perhaps the least useful of all the tests of permeability. 
10. Plasmolytic Methods 
Concentration required to produce plasmolysis. It has already been 
pointed out in Chapter IX that a solution of a substance incapable 
of penetrating into the cell which just does (or just does not) produce 
plasmolysis, is, on the simple osmotic view of the cell, isotonic with 
the cell sap at the moment when plasmolysis is just about to com¬ 
mence. It must be pointed out, however, that if the sustance pene¬ 
trates the cell sap but is immediately withdrawn from solution to 
form an insoluble compound, plasmolysis may still occur if the quan¬ 
tity of salt withdrawn from the external solution is insufficient to 
alter materially the concentration of the external solution. It is not 
Phyt. XXII. I. 2 
