Priestley and Pearsall — Growth Studies. II. 245 
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appeared at the same time on all plants in these cultures. The association 
of this appearance with the horizontal portion of the first S curve seems too 
close to be accidental, especially as the appearance of tertiary roots produces 
a similar effect in the second S curve. Tentatively, therefore, it may be 
supposed that the decreased growth-rate in region c of the growth-curve for 
roots is due to the diversion of food supply to the developing lateral roots. 
Support is given to this suggestion by the fact that the mass increase per 
unit of time is the same in the b regions of both the first and second S 
curves — indicating that the rate of food supply has not materially altered. 
The objection may then be raised that the mass increment would be 
the same whether the root-forming material were used at the apical meristem 
or in the incipient lateral apices. This by no means follows, however, for 
if the supply of food material is absorbed by the developing lateral 
meristems, it may well be that initially the greater part of it would be used 
in oxidation processes involving loss of weight. Such a ‘ lag ’ period is 
noticed by Briggs, Kidd, and West ( 4 ) in Kreusler’s data. It appears to be 
shown during the initial development of roots on cuttings, and is known to 
be a well-marked feature of the growth-curve when yeast in a resting stage 
is seeded into a nutrient solution ( 18 ). The pericycle cells from which 
lateral roots develop are not actually meristematic, and it seems probable 
that, as in yeast, a ‘ lag * stage may occur during their development into 
meristematic tissue. In addition, if meristematic activity is initiated above 
the main root apex, the food supply to the apex may temporarily cease and 
its rate of growth would then decrease, with a resultant decrease in the rate 
of membrane formation, &c., behind the apex. The flattening of the curve 
at the time of lateral root formation thus appears to be a very natural 
effect. While only further experiment can show to what extent the same 
curve is true for root development in general, we may now utilize the 
suggestion put forward to consider two further questions : 
(1) The rate of growth of roots at high temperatures, as recorded by 
Leitch (8). 
( 2 ) The causal factors in the development of endogenous secondary 
roots. 
The Rate of Growth of Roots at High Temperatures. 
Leitch (8) has recently given curves for the growth-rate of Pisum 
sativum at temperatures of 30° and 35 0 C., one of which is reproduced in 
Fig. 4. Leitch was quite unable to explain these curves, and only recorded 
them as being the results of repeated experimental confirmation. In the 
light of the analysis given above, however, it seems possible to put forward 
an interpretation of these curves. 
The rate of growth of roots from seeds must be assumed to depend on 
two processes: (1) the chemical reactions involved in the meristematic 
