196 Walter Stiles and Ingvar Jorgensen . 
H ere the recorded difference in dry weights is considerable 
(46 per cent.)* The probable errors are however large owing to 
the small number of plants and individual differences between the 
plants. Nevertheless the difference between the dry weights of the 
aerated and non-aerated cultures is between two and three times 
as great as the sum of the probable errors, and it seems legitimate 
to regard this difference as having a real significance. 
Concluding Remarks. 
In the foregoing pages we have emphasized the complexity of 
problems of water cultures, and have endeavoured to illustrate it 
especially in relation to aeration. Thus we have obtained similar 
results to those of Hall, Brenchley and Underwood in relation to 
the increase in growth of barley resulting from aeration. On the 
the other hand with buckwheat we have obtained a result similar 
to that of Free who found no benefit result from aeration. 
It does not follow however that the same result would be 
obtained under all conditions of experiment. It is very likely that 
there are conditions in which aeration would produce no effect on 
the rate of growth of barley, or which would bring about an 
increase in the rate of growth of buckwheat. 
We have also shown how different conditions bring about 
different increases in the rate of growth as a result of aeration 
^contrast Series 3 with Series 1 and 2). 
There is at present no explanation why under the conditions of 
our experiments aeration should increase the rate of growth of 
some species and not of others. We do not know whether it is to 
be correlated with oxygen supply, removal of carbon dioxide, 
removal of diffusion gradients, or change in solubility conditions 
or other causes. There seems no doubt however that water culture 
conditions react in a different manner on different species. 
It must be emphasized that a great deal of work must be done 
on the physical chemistry of water culture solutions before work 
can be done with necessary definiteness. Particularly investigation 
must be extended on dynamical principles as is clearly indicated 
from the investigations of Pantanelli (6). 
Further it is obviously desirable to work towards getting some 
more explicit principle which will take into account the co-opera¬ 
tion of the various activities of the plant. It seems clear that at 
present neither the law of the minimum nor the principle of limiting 
factors as employed by Blackman in regard to the sub-aerial part 
