292 Blackman. — Optima and Limiting Factors . 
Further consideration of this question would, however, involve the 
quotation of definite examples, and must be postponed. 
In the case of growth, besides the more subtle factors, there are 
two fairly obvious factors which must, each in its turn, and under different 
conditions, play the part of a limiting factor. These are the temperature 
and the water supply. To variations in either of these growth is very 
susceptible, and indeed a vigorous transpiration in dry air has been 
observed by Frank Darwin 1 to cause actual temporary loss of weight in 
a growing Cucurbita fruit, water being drawn off to supply the leaves. 
In a quite recent paper by R. H. Lock 2 we meet with what I take to 
be a most interesting example of the action of limiting factors in growth. 
The author made series of measurements of the daily, and sometimes 
of the hourly, rate of growth of Bamboos growing in the open at 
Peradeniya, and at the same time noted the temperature and the relative 
humidity of the air, and the rainfall. The conclusion at which he arrives 
is that the daily rate of growth is almost entirely a question of the water 
supply, and that the hourly growth during the day follows very closely 
the curve of humidity of the air. It is striking that temperature does not 
appear to come into the causation of the fluctuations at all, and indeed 
in the fifteen conclusions given temperature is not mentioned. Now 
during the months of June, July, and August, when the measurements were 
taken, the temperature was always high, and it is recorded that the extreme 
range of temperature for these months was between 19 0 C. and 30° C. Here, 
then, temperature is favourable to growth, and would presumably permit 
more growth than actually takes place, so the amount of water attainable 
is the factor which limits the growth of the bamboo stems. Hence the 
close correlation between observed growth and water supply. 
Now suppose the temperature had been much lower and the water 
supply not diminished in proportion. Then the temperature would have 
kept the growth-rate down and would have been the limiting factor, and 
we should have expected the daily growth to correspond to the fluctuations 
of temperature, and not to those of water supply, which is now in excess. 
I think it abundantly justifies this way of looking at the phenomena 
when we find that Shibata 3 in 1900, measuring the daily growth-rate of 
Bamboos growing in the open in Japan, obtained numbers which showed 
the closest agreement with the fluctuations of temperature and very little 
relation with the humidity. The temperatures in Japan fluctuated between 
1 jl*6° C. and 20-7° C. during the measurements, so that the whole range is 
quite below the Peradeniya range of temperature and the case entirely fits 
our hypothesis. 
Shibata’s measurements are analysed by Lock in his paper, but 
1 Annals of Botany, vol. vii. 2 Annals of R. Botanic Garden, Peradeniya, vol. ii, August, 1904. 
3 Journal Coll. Sci., Tokyo, 1900; cf. Lock, 1. c., p. 215. 
