224 J- H. Priestley and R. M. Tupper-Carey 
genous meristematic activity of the cortical region of the stem, as 
manifested for instance in periderm formation (Priestley and 
Woffenden (22)). 
In the root, then, the meristem directly in contact with the 
plerome is best nourished and as a consequence its rate of growth 
is more active than that of the superficial tissue, the outer layers 
with difficulty supplying enough cortical tissue to keep the plerome 
cylinder covered. Lundegardh ( 14 ) has already pointed out that the 
periblem appears to be constructed in layers, parallel to the surface, 
which have been carried past each other in a longitudinal direction 
by the greater extension of the plerome cylinder within. 
Schuepp( 24 ) has discussed the relative rates of growth of the 
different layers of the meristematic tissue at the stem growing point. 
As a result of measurement of the relative numbers of dividing cells 
he has shown that the superficial layers appear to be growing as 
rapidly as the deep-seated initials directly above the plerome. He 
points out that it must follow from this fact, that the outer layers 
of meristem will be unable to accommodate the products of their 
activity in one plane only and that in addition to increase in super¬ 
ficial area they must be able to increase in depth. Such an increase 
will lead either to the folding of the surface layer or to the super¬ 
position of one cell upon another, at right angles to the surface, 
which in its effect upon form amounts to the same thing. These 
folds or extensions, vertical to the direction of normal surface 
growth, would be the new leaf and stem initials which clothe the 
surface of the stem apex. 
The results recorded by Schuepp seem to us to be the natural 
consequence of the phenomena previously discussed and the exo¬ 
genous growth activity of the stem apex to follow from the readiness 
with which the nutrient solution is able to circulate freely through 
the tissue and supply equally both outer and inner layers of the 
meristem. So the rate of growth of both layers remains identical 
and from the volume relations thus established the lateral out¬ 
growths arise immediately behind the stem apex. The view is 
therefore put forward, though still requiring much experimental 
review, that the endogenous and exogenous habit of root and stem 
may be attributed to the conditions of food supply to the apical 
meristem. 
