Priestley and Ever shed. — Growth Studies. I. 233 
4. Interpretation of Experimental Results. 
If the curves obtained in all these series of quantitative observations 
on root growth are compared, it will be seen that they are of one general 
type, which we venture to describe as a progressive series of S curves. The 
S curve is of frequent occurrence, when quantitative data of the progress of 
growth are recorded, both in the case of animals (Robertson (8)) and of plants 
(Gregory (4), Schuepp (9), &c.). 
Fig. 4. 1 Dry weight ratio ’ of roots on cuttings of ‘ Sunrise ’ tomato (see Table 4 a). The 
arrow indicates the time of appearance of secondary roots. 
A full discussion of this curve will appear in the second paper of this 
series, so we shall confine ourselves now to two observations which are 
directly relevant to this special case of the appearance of the S curve in the 
growth of roots. At the beginning, the cuttings were all rootless, therefore 
the curve starts at the origin, and indicates an increasing rate of growth 
which is connected exponentially with time. Subsequently the rate of 
S 2 
