2 43 Priestley and Pearsall — Growth S Hedies. II. 
It is clear that the phenomena so described above would admit of an 
alternative explanation. If some factor could be found that would inhibit 
the growth of the main root, then the development of the lateral roots 
might be ascribed to the accumulation of root-producing substances in the 
root. So far, however, we have found no adequate cause for the diminution 
in growth of the main root, and we are therefore inclined to assume that the 
growth of the main root is temporarily held up by the development of the 
lateral meristems. The alternative assumption certainly agrees with the 
case of the fibrous roots of Monocotyledons, where the strong and early 
development of lateral roots is apparently associated with the inhibition of 
the meristem of the primary root. 
Summary. 
1. Data, previously recorded for root growth, provide curves showing 
a series of successive inflexions of the S type so often obtained in growth 
experiments. 
2 . In a brief review of previous work, special attention is given to 
Robertson’s comparison of this type of curve with that of an autocatalytic 
chemical reaction. 
3. It is pointed out that analysis of the physiological conditions 
involved at different stages of growth is apparently a necessary preliminary 
in the interpretation of such growth-curves. 
4. Such an analysis is attempted for the growth of roots, after previous 
consideration of the data available for yeast. 
5. The early exponential portion of the curve is considered to represent 
the exponential increase in size of the apical meristem. 
6. The mass increase then becomes proportional to time, probably 
because the supply of root-forming material is delivered at uniform rate. 
7. The final decrease in growth is co-ordinated with the initiation of 
lateral meristems. 
8. It is shown that the hypotheses involved give an adequate ex- 
planation of the growth of roots at high temperatures (Leitch’s data), and 
permit certain assumptions as to the endogenous origin of lateral roots. 
References. 
1. Blackman, F. F. : Presidential Address to the Botanical Section. British Association Report, 
1908. 
2 . , and Matthaei, G. L. C. : A Quantitative Study of Carbon Dioxide Assimi- 
lation and Leaf Temperature in Natural Illumination. Proc. Roy. Soc., Ixxvi. 402-60, 1 905. 
3 . Blackman, V. H. : The Compound Interest Law and Plant Growth. Ann.., Bot xxxiii. 353— 
60, 1919. 
