Priestley and Pearsall . — Growth Studies. II. 
247 
Causal Factors in the Development of Endogenous 
Secondary Roots. 
While the problem of the development of lateral roots cannot yet be 
fully discussed, it must be considered in the light of evidence hitherto 
unpublished. 
The evidence available supports the view that the supply of sap and of 
food material for growth is restricted within the endodermal cylinder, which 
is closed at the apex by the meristematic tissues of the root tip. Within 
this endodermal cylinder all the cells are vacuolated and differentiated at 
a fairly early stage, with the exception of the endodermis and the pericycle, 
which remain relatively embryonic for a much longer time. The fact that 
they occupy the most favourable position in relation to the balance of 
supply of food from the conducting elements and of oxygen through the 
intercellular spaces of the cortex may possibly be the reason for this. 
The endodermal cylinder is thus lined along the inside with a relatively 
embryonic tissue, and owing to the normal action of osmosis a considerable 
hydrostatic sap-pressure also accumulates inside it (Priestley (10)). 
Some factor then induces the development of meristematic activity on 
the part of some of the pericycle cells, usually those opposite the xylem. 
The exact position of the new activity seems to depend either on the degree 
of maturation of the pericycle cells or upon the slow accumulation of some- 
thing diffusing downwards from the shoot, since the lateral roots never 
approach the root apex closely, but always develop acropetally. 
It has been observed that the formation of phellogen below a wound is 
preceded by a blocking of the exposed surface, a factor presumably leading 
to increased hydrostatic pressure in the cells beneath the surface, and it has 
been pointed out also that cork formation is confined to places receiving a 
free supply of nutrient sap. In addition, the phellogen in roots normally 
develops inside the endodermis. Both these groups of observations support 
the suggestion that the supply of nutrient sap and the development of 
hydrostatic pressure within the endodermis are important factors in initiating 
the development of the latent activity of the pericycle cells, and in explaining 
the endogenous origin of lateral roots. 
The initiation of this meristematic activity disturbs the gradient of 
substances diffusing to the main root apex, and as a consequence the 
growth of the main apex is lessened or stopped, as recorded in our ex- 
periments by a fall in the growth-rate. The new lateral meristems in their 
turn leave behind them vacuolated and differentiated tissues, and the 
previous growth-rate is thus restored, though apparently not increased, 
indicating that the total mass increase per unit of time is not dependent 
upon the amount of meristematic tissue, but upon some other limiting 
factor, probably in this case the supply of food material. 
T 
