226 Priestley and Evershed. — Growth Studies. I. 
in form or size may be the expression of such subsidiary influences as 
intake and retention of water rather than of cumulative increase in living 
substance resulting from constructive metabolism. The aim of the work 
now presented was to obtain data for plant growth based upon mass 
measurements, with a view to seeing if the subsequent analysis of such 
data would be of value in throwing any light upon the inner .mechanism of 
growth. 
2. Experimental Material and Methods. 
Choice of Material. 
Need for quantitative data seems to have become apparent to many 
workers recently, and from Cambridge (Briggs, Kidd, and West (3)), the 
Imperial College (Gregory (4)), and Rothamsted (Brenchley (2)) valuable 
results have been published since our experiments began in the summer of 
1920 . Most of these figures refer either to the growth of the whole plant 
or to the growth of leaves. 
In choosing our experimental material it was necessary to bear in mind 
the fact that the leaf area represents the proportion of the plant engaged in 
the manufacture of food from raw materials. Figures giving an increase in 
leaf area would be expected to demonstrate the existence of an exponential 
law, because, as V. H. Blackman (1) has recently pointed out, increase in 
mass is directly connected with increase in photosynthetic area. Such an 
exponential relation between mass increase and time would depend upon 
a different train of circumstances from that connecting animal growth with 
time. 
In order, therefore, to find out if a similar relationship between growth 
and time existed in the case of plants, attention was restricted to roots , where 
increase in manufacturing surface does not directly follow upon increase in 
mass, and where conditions more approximate to those obtaining in experi- 
ments with animals. Seedlings were not used for the preliminary work, on 
account of the difficulty of securing a uniform start, as germination involves 
a number of factors, including the resistance offered by the seed-coat, its 
gradual decomposition by bacteria, the original rate of entry of water, &c. 
These obstacles were all removed by the decision to obtain data as to the 
production of roots upon cuttings. 
Experimental Method. 
In this work, first with Trade scantia Zehrina and later with tomatoes 
(Solanum Lycopersicum ), we endeavoured to obtain uniformity of conditions 
between the cuttings of each set of experiments. The cuttings, taken from 
plants grown in the same greenhouse, were of approximately equal weight, 
and were all started at the same time. At definite time intervals, the root 
production was measured in terms of wet weight, and then of dry weight. 
