Priestley and Ever shed. — Growth Studies. I. 235 
appear on the 14th day. It will be seen that the termination of the first S 
curve coincides with the appearance of secondary roots, and the termination 
of the second S curve with the appearance of tertiary roots (Figs, i, 2, 5, 6). 
The theoretical discussion of this phenomenon will be reserved for the 
second paper of this series. 
The second point that emerges is that this S curve, in so far as it applies 
to roots, is a restatement of the Sachs ‘ Grand Period ’ of Growth. 
5. The Sachs ‘ Grand Period ’ of Growth. 
Reference to any text-book of plant physiology (e.g. Jost ( 5 ), p. 288) 
will show that this generalization appears to be the complete expression 
of our knowledge of the quantitative phenomena involved in the growth 
of roots. It is based up till now entirely upon length , and therefore 
these data are only available for the time preceding the appearance of the 
lateral roots. Hence, the Sachs ‘Grand Period’ for the growth of roots 
covers the period represented in our experimental data by the completion 
of the first S curve. The 4 Grand Period ’ curves have a different shape, 
because they are usually plotted as rate curves (i.e. by plotting changes in 
length in the time intervals taken). But it is possible to convert the data 
given in Table I into a rough imitation of a Sachs ‘ Grand Period \ for the 
first fourteen days (before the secondary roots occur), by calculating the 
difference in weight between successive readings and reducing this to a rela- 
tive rate of increase of mass per day. This has been done and the results 
plotted in Fig. 6 . These data are not so suitable for this treatment as 
the more readily determined increments in length, but comparison of 
this curve with that of Fig. 1 (which was plotted from the same experi- 
mental data) shows, firstly, that the rising section of the ‘ Grand Period ’ 
curve represents the region where the increase in mass is in exponential 
relation to the time ; secondly, that the approximately horizontal region of 
the Sachs curve represents the straight portion of the curve in Fig. 1 ; and, 
thirdly, that the fall in rate at the end of the ‘ Grand Period ' represents 
that region of the weight curve whose shape we have attributed to the 
development of the secondary roots. 
It is clear, therefore, that, but for the inadequacy of previous methods 
of measurement, it would have been possible to demonstrate in the growth 
of roots (at any rate when produced upon cuttings) a series of ‘ Grand 
Period ’ curves. Each one of these represents the period between the origin 
of one crop of lateral roots and the appearance of a succeeding crop of 
rootlets arising from this first batch. 
The significance of this statement will be considered further in the next 
paper of this series. 
Presumably this type of curve, the S curve or ‘ Grand Period ’ curve, 
will occur until the root mass is in equilibrium with the mass of the leaf and 
