88 Brenchtey . — Effect of Concentration of the Nutrient Solution 
greatly the rate of growth of plants. Not only is the rate of growth affected, 
but the amount of growth is strictly limited by the quantity of available food 
when the nutrient solutions are dilute. Little work has yet been done with 
higher concentrations, but it is possible (see page 85) that toxic action due 
to over-nutrition from too great a supply of food-salts comes in to counter- 
balance or replace the increase of growth caused by increase of nutriment 
which occurs with lower strengths. 
Although the experiments fail to corroborate the idea that concentra- 
tion is unimportant within very wide limits, still they fully support other 
observations made by Stiles. In every case there is a drop in the dry 
weight of plants grown in any concentration according to the frequency with 
which the solutions are changed, the £ frequently changed ’ plants being 
heavier than the ‘ once changed ’, and the ‘ once changed ’ than the ‘ never 
changed \ With the normal strength it is probable that there is a sufficient 
supply of food material even when no renewal of solution takes place. 
In one case, barley was grown in such a normal solution for over eight 
weeks, and analyses made at the end of the experiment showed that 25 
per cent, of the initial nitrogen still remained in the solution, and as the 
nitrogen compounds are absorbed in greater quantities than other salts it is 
evident that an ample sufficiency remained, if the quantity of the salts were 
the only factor concerned. Thus it is probable that with this concentration 
the question of starvation does not arise, and that the steady decrease 
in weight is really associated with the change of balance of the nutrient 
salts, the plants being the better the closer the initial balance is maintained. 
With the lower concentrations, the drop in weight from ‘ frequently 
changed ’ plants to the others was much heavier. Since with the normal 
solution the decrease in weight due to the balance of the food-salts was 
so much less marked, it seems permissible to assume that the very heavy 
drop with the lower concentrations is due largely to quite another 
cause, that of varying degrees of starvation through lack of sufficient 
nutriment. When the solutions are changed as often as once in four days, 
twelve times altogether, the plant has access during its lifetime to a far 
greater' store of food material than when solutions are seldom or never 
changed. Consequently such plants suffer less from the starving effects due 
to the low concentrations of the food-salts in solution, but still the response 
corresponds strictly to the amount of food available at any one time. There- 
fore it seems evident that with the normal solution the change in the 
balance of food-salts has a hindering action upon the growth of barley, and 
that this hindrance is coupled with varying degrees of starvation as the con- 
centration decreases, being specially accentuated in those cases in which the 
solution is never changed. 
It has frequently been noted that the variation in the strength of 
the food solution not only affects the total dry weight of the plant, but 
