95 
Rate of Growth of Plants in Water Culture. 
different ions at different rates which would result after a time in an altera¬ 
tion of the relative proportions of the different substances in the solution. 
The necessity for a definite balance between the substances in a nutrient 
solution has been emphasized by many workers recently. The effect of 
this selective absorption would be extremely difficult to foretell, as it would 
probably produce different results in solutions of different concentrations. 
In the strongest solutions, however, the toxic properties of the substance in 
excess would probably be most marked, while in the weakest solution 
a starvation effect owing to exhaustion of some particular salt or ion might 
result In any case it would appear to be essential in many water-culture 
experiments to renew the culture solutions at frequent intervals, and possibly 
to use culture jars or bottles of large capacity. 
One point which may be worth mentioning is that of the ratio of the 
dry weight of the shoot to that of the root. It would appear that with 
decreasing concentration of the solution the growth of the shoot is affected 
much more than that of the root, a fact which is also indicated by Hall, 
Brenchley, and Underwood’s figures. 1 When the culture-solutions are 
not changed frequently, the growth of the shoots is again affected more 
than that of the roots. 
It seems necessary to lay emphasis on the extreme variability of plants 
growing in water-cultures, particularly as regards their dry weight. The 
numbers given by Brenchley, already referred to in this paper, and some 
given by Hall, Brenchley, and Underwood, 2 make it quite clear that in order 
to obtain definite results by the water-culture method it is essential to 
use a fairly large number of plants, and to weigh the dry matter of each plant 
separately and calculate the probable error. 
Only by this means can an indication be obtained as to whether any 
difference is significant. By reference to the figures in this paper it will be 
seen that working with sets of ten plants under the same conditions does 
not allow of the measurement of moderately small differences even when 
a pure line of seed is used. Hall, Brenchley, and Underwood’s cultures 
were only grown in duplicate, and this may account for the differences 
between their results and those of other observers. 
What bearing the results of experiments with water-cultures can have 
on the question of the soil solution it is difficult, and would indeed be 
premature, to say. To argue from a comparatively simple medium, such 
as a nutrient solution of mineral salts, to a complex structure like the soil, 
is indeed a risky thing to do in the present state of our knowledge. It 
seems, however, safe to say that the present experiments, like those of most 
other observers, support Cameron’s contention that the soil solution, dilute 
as he supposes it to be, is yet quite concentrated enough to support 
vegetation. In this connexion it is interesting to compare the quantities of 
1 Hall, Brenchley, and Underwood: 1 . c., pp. 191, 193. 2 1 . c., pp. 191, 195. 
