Apical Meristems in the Roots of Monocotyledons. \2*j 
divide transversely once or twice and then cuts off one or two 
egments by periclinal walls. These segments may, by transverse 
divisions, form layers whose cells do not divide tangentially, or 
each layer may give rise to two or more layers, the outer layer 
usually consisting of smaller cells than the inner. Thus it is 
obvious that the endodermis cannot be differentiated at the apex 
as it is the innermost layer and the last to be cut off. All the cells 
of the periblem are quadrangular in both longitudinal and transverse 
sections. In transverse sections they shew a remarkably regular 
arrangement in concentric circles. The exodermis with its broad 
cells is clearly seen, widening in one direction only, that is, along 
the radius. Transverse sections also shew that as the cells towards 
the exterior grow longer, they divide in a radial direction ; in such 
sections the cells of the endodermis appear to be five-sided. The 
cells of the endodermal layer form a ring immediately outside the 
ring formed by the cells of the pericycle; but an individual cell 
of the endodermis does not lie immediately outside an individual 
cell of the pericycle, but the apex of its pentagon fits in between 
two cells of the pericycle. This is clearly seen in the illustration 
of Alisma (Fig. 3). 
V .—The Root-cap. 
The root-cap arises from the single layer of cells known as the 
calyptrogen, which covers the apex of the root. Each cell of this 
single layer may divide anticlinally, thus increasing the number of 
cells in this single layer. All the cells except those furthest from 
the apex may also divide periclinally. As the result of this, the 
original single layer becomes double, except in regions furthest 
from the root apex where it remains as a single layer. The next 
periclinal walls are formed in the innermost row of the double¬ 
layered root-cap These walls are formed in all the cells save those 
furthest from the root apex. Thus at the apex the root is covered 
by a three-layered cap, a short distance from the apex this becomes 
two-layered and further away still the root-cap consists of a single 
layer only. The subsequent divisions in the cells of the root-cap 
take place on a similar plan, that is, periclinal walls are continually 
being formed in the median cells of the innermost layer of the 
root-cap, so that after several layers have been thus formed very 
few cells are concerned in the process. Although this is the ground 
plan periclinal walls may also be formed at the same time in other 
outer layers of the root-cap ; no walls, save anticlinal ones, are, 
however, formed except in the innermost layers. Anticlinal divisions 
