Apical Meristems of the Roots of Monocotyledons. 125 
Van Tieghem (2) considers that all roots whether adventitious 
or not, whether of Monocotyledons or of Dicotyledons with very 
have three sets of initials, giving rise to (i.) the plerome, (ii.) the 
periblem, (iii.) the many-layered epidermis acting as a root-cap. The 
plants of which he gives figures, and which I have also examined, 
are Alisma, Butomus, Vallisneria and Stratiotes. 
According to my investigations the number of the initials for 
the different layers is usually as follows :—for the plerome one 
initial, for the periblem and the dermatogen a single common initial 
with occasionally an undivided segment on either side, though 
sometimes two initials appear to be present. 
III .—The Plerome Cylinder. 
The plerome cylinder is surrounded by the pericycle. This 
layer is well-marked in all the plants examined and appears to be 
differentiated very near the apex, and is apparently often continued 
over the apex of the cylinder by the initial cell. The first cells cut 
off from the initial of the plerome on either side become the first 
pericyclic cells and remain undivided as a single layer, or rather 
cylinder, of cells surrounding the remaining pleromic tissues. 
Sometimes, as is seen in Limnocharis (Fig. 18), the pericycle is not 
differentiated at the very apex, but the cells near the initial split 
behind to form part of the inner tissues of the cylinder. In this 
case the side segments of the initial cell form the whole of the 
pleromic tissues except the central single well-marked axial row 
which is formed by the posterior segments of the initial. When 
these segments form a strongly-marked central row they do not 
divide further, but the pleromic cylinder is completed, either by 
divisions of the primary lateral segments of the initial, into peri¬ 
cycle and inner tissue, or by the division of cells cut off at the same 
time as the pericycle (which then remains undivided) from the side of 
the initial. If the axial row be not so strongly marked it may cut 
off lateral segments to complete the cylinder, but it does not divide 
to form rows of cells itself. The root remains, in transverse section 
radially symmetrical, with a single central cell. The segments, in 
whatsoever manner they may be formed, divide as they grow older, 
this forming double rows, many of which divide again. These cells 
divide, however, in no predetermined order. 
All the cells a short distance behind the apex are quadrilateral 
in longitudinal section. In transverse section they are seen to be 
arranged regularly in concentric rings around one central cell. 
The cells appear in transverse section to be five or six-sided, and 
