244 Priestley and Pea? sail. — Growth Studies. II. 
The Growth of Roots. 
The growth of roots from cuttings, as shown in the first paper of this 
series, may be expressed by the curve given in Fig. 3. This curve consists 
essentially of two successive S curves. 
The exponential region of the initial curve appears to represent the 
period during which a meristem is growing in amount, and in which each 
meristematic cell divides to give two meristematic cells in a given time 
interval, as in the a region of Fig. 2. Actually the data may show the 
position of the change later than it really occurs, since what we measure is 
not merely the mass of the growing meristem but probably to a greater 
extent the subsequent mass increase due to the differentiation of the cells. 
This would involve vacuolation (not measured in dry weight determinations), 
the deposition of thicker walls and membranes, and the inclusion of various 
accumulation products. There may also be an initial ‘ lag * period due to 
the transformation of resting cells in the shoots into meristematic cells, as in 
the case of yeast resting cells. 
The meristem soon ceases to increase exponentially, and the mass 
increase becomes proportional to time. We have here to seek another 
factor governing the rate of the reaction. Loeb ( 9 ) has shown that the rate 
of regeneration of roots is directly proportional to the mass of the shoot 
upon which they are produced. The same influence can be traced in the 
results of the Tradescantia experiments (Priestley and Evershed ; see ante , 
pp. 231-2). This suggests that the total mass of meristem produced on 
the root system will be determined by the supply of root-forming material 
from the shoot. Under such conditions a constant mass of shoot would 
maintain the activity of a constant mass of meristem, and the increase in 
mass of tissue would become proportional to time. In the b region of the 
curve it may therefore be supposed that an equilibrium has been attained 
between the secretion of root-forming materials by the shoot and their 
utilization in the root apex. The diffusion gradient of these materials will 
be approximately constant, and hence in this part of the curve the rate of 
food supply is assumed to be the factor limiting the rate of growth. 
As in Fig. 2, the same number of cells remain actively meristematic in 
succeeding intervals of time, other cells passing into the resting stage, which 
in this case means that they proceed to vacuolate and extend in size behind 
the growing-point. 
Since the shoot on which the roots are borne is gradually increasing in 
size, there should be a slight increase in growth-rate in the later parts of 
the curve. There is actually, however, a rapid retardation of growth j 
(region c), which suggests a falling off in the supply of food to the apical 
meristem. In the cases examined, this retardation is associated with great 
regularity with the development of a crop of secondary roots, which 
