PLANT FOOD 19 
The-results obtained with the plant seedlings were so 
striking as to suggest the possibility that a certain amount 
of organic matter in addition to mineral nutrients is 
essential for the maximum growth of plants. As in the 
usual water culture experiments with ordinary plant 
seedlings a fairly large quantity of organic matter is sup- 
plied to the young plant from the organic food reserve 
of the seed, it was necessary to experiment with a plant 
free from this objection. Lemna minor was eventually 
selected as being suitable. Its normal habitat is water, 
it multiplies rapidly by vegetative methods, and a reliable 
estimate of variations of growth in different culture 
solutions can be readily obtained by counting the plants 
at regular intervals of time. 
Preliminary experiments at King’s College during 
1915 showed that Lemna minor plants fail to grow for 
any length of time in a pure mineral culture solution, 
but if certain extracts from bacterised peat be added 
to the solution growth is normal and vigorous for an 
indefinite period. These experiments were repeated 
and extended last summer in the greenhouse laboratory 
of the Imperial College of Science and Technology, 
South Kensington. 
Fifty culture dishes, each containing 250 c.c. of the 
required solution, were prepared and arranged in five 
series of ten dishes each. The solutions employed 
were: Series I, Detmer’s standard culture solution ; 
Series II, Detmer’s solution together with a water 
extract of bacterised peat ; Series III, Detmer’s solution 
with a similar extract freed from humic acid ; Series IV, 
Detmer’s solution plus an alcoholic extract of bacterised 
peat; Series V, Detmer’s solution with the addition of 
