Influence of Aeration of Nutrient Solution. 183 
appear between individuals of the same species on account of their 
different ancestry. In some recent work recognition of this fact 
has led to the use of pure lines of seed where inherited differences 
are reduced to a minimum. The interaction of hereditary factors 
and environmental factors has, however, never been dealt with in 
water cultures—that is, no attempt has been made to determine 
whether different strains of the same species will react differently to 
various environments. 
Related to this question is that of the variability of different 
species as to the conditions which suit them best, and in general, 
their different modes of reaction to different environmental 
conditions. It has been far too common to draw conclusions as 
to the behaviour of plants in general from the behaviour of one or 
at most a few species, when there is no evidence or reasonable 
expectation that all species should behave in the same way. 
2. Germination conditions. The germination of seeds for 
experimental work is a more serious matter than is generally 
assumed. The effect of preliminary soaking has been known for 
many years to have a great effect on the time required for 
germination, and on the vigour of the seedling produced. After 
effects may be caused by the germination conditions which are 
attributed to the later environment, and so completely false 
conclusions may be drawn. The effect of temperature on germin¬ 
ation, the time of preliminary soaking in water, the medium in 
which germination takes place, as well as the different effects of 
these on different species, all require consideration. 
3. The technique of the bottle and cover. The use of glass jars 
or wide mouthed bottles covered with paper or black paint to 
exclude light and growth of algae is universal in water culture 
experiments, but there is no uniformity as to the size of the vessel. 
Thus in recent work of some American investigators ( e.g . 9) jars of 
250 c.c. were used to carry 6 plants, thus giving 42 c.c. of solution 
per plant. At the other extreme we have Wortmann (11) who 
recommends the use of vessels of 26,500 c.c. capacity for a 
single plant. Most investigators have used vessels inter¬ 
mediate in capacity between these two extremes. It is to be 
expected that the size of the vessel employed has influenced results 
with water cultures, principally owing to the different rates of 
change in the composition of the culture solution to which we shall 
refer below, but also possibly owing to other causes. 
The cover of the vessel has generally taken the form of a cork 
