PLANT FOOD 23 
their growth; (2) That in these plants normal growth 
and multiplication cannot be sustained for any length 
of time in the absence of these organic growth-promoting 
substances ; (3) That these substances are essential 
for the effective utilisation and assimilation of the 
mineral nutrient supplied to the plants. 
These conclusions are contrary to the generally 
accepted botanical theory that green plants can build 
up complex protein compounds from mineral salts and 
mineral salts alone. Must we then consider these water 
plants as exceptions to this theory, or must we modify 
our present conceptions of plant nutrition ¢ Further ex- 
periments alone can decide this. The well-known fact 
that the seedlings of land plants can be grown to maturity 
in water culture solutions of mineral salts is not a fatal 
objection to the suggestion that all green plants may 
require traces of certain organic substances for their 
development, since it has been shown that such sub- 
stances are produced during the germination ef seeds, 
and the seedlings used in water culture experiments 
may already contain the necessary minimal quantities 
required for ordinary growth. 
It is impossible as yet to state definitely how these 
substances function. Some of them may be absorbed 
and utilised directly as plant nutrients, whilst others 
may have a similar effect to that of the accessory food 
substances or growth vitamines concerned in animal 
nutrition. Whatever may be the specific nature of 
these organic decomposition products, they all have 
the effect of promoting plant growth, and the term 
“ auximone ” (adé:wos, promoting growth) has been 
suggested as a general descriptive name for them. 
