22 EXPLOITATION OF PLANTS 
substances, it was suggested that the beneficial effects 
of the added organic matter might be due to a neutralisa- 
tion of the toxicity of the distilled water used. Accord- 
ingly a parallel series of experiments, using glass distilled 
or ‘ conductivity’ water instead of ordinary distilled 
water, was carried out. The figures obtained showed 
that whilst the toxic substances in the ordinary distilled 
water had a certain injurious effect upon the growth 
of the plants, the use of pure non-toxic water will not 
suffice for continued normal and healthy growth. 
The necessity for the presence of certain organic 
substances in a mineral culture solution in order to 
ensure healthy growth is not peculiar to Lemna minor. 
Experiments with Lemna major, Salvinia natans, Azolla 
filiculoides and Limnobium stoloniferum, growing in both 
Detmer’s and Knop’s culture solution, are equally 
sensitive to the presence or absence of organic matter. 
As the organic substances employed in the above 
experiments were evidently decomposition products 
of vépetable matter (peat),.experiments were made to 
test to what extent similar growth-promoting substances 
are present in other sources of decomposing vegetable 
matter used for cultural purposes. Extracts of stable 
manure, leaf-mould, and even a well-manured garden 
soil were found to supply the beneficial growth-promot- 
ing substances, although they are present in relatively 
much smaller proportions than in bacterised peat. 
The results thus obtained appear to justify the follow- 
ing conclusions: (1) That bacterised peat contains 
certain organic substances which, when supplied even 
in small quantities to water plants growing in a com- 
plete culture solution, have a remarkable effect upon 
