+ 
228 BOTANICAL GAZETTE [APRIL 
( fig. 5). Whether or not the dividing cylinder is more than 
one cell-layer in thickness I could not determine. 
The cortex is very bulky and consists throughout of iso- 
diametric parenchymatous cells abundantly filled with starch 
together with a little oil. The cells undergo no divisions, but 
as they are forced outward by the activity of the cambium 
they increase very greatly in size (figs. 6 and 7). In this way 
by the growth of the cells in all directions the cortex expands 
upwards and downwards as well as outwards, and as a result 
carries up the older leaves to a height considerably above the 
stem apex. This is illustrated in fig. 68, in which the unbroken 
lines represent the form which the cortex would take if its cells 
underwent no enlargement as they are pushed out from the 
cambium, and the dotted lines the form and dimensions which it 
actually assumes. 
The stem apex lies at the base of the conical depression 
formed in the manner just explained. In small plants it is dis- 
tinguishable in longitudinal sections as a slight elevation (fig. 3); 
in older plants it is merely a flattened area between the bases of 
the young leaves (jig. 2). 
The method of growth of the apical meristem was first cor- 
rectly described by Hegelmaier (2). Hofmeister (1) had erro- 
neously ascribed it to the segmentation of an apical cell, having 
been led to that conclusion probably by too exclusive study of 
young plants. There is neither an apical cell nor such a group 
of initials as might result. from the division of a rectangular apical 
cell like that of the Marattiacee. Only two or three layers of cells 
show their meristematic nature by their contents. The superficial 
layer appears to divide only in an anticlinal direction except when 
young leaves are about to be formed; but this layer, as Hegelmaier 
showed, can on no account be regarded as a dermatogen. 
Although all the species of Isoetes are perennial, only 4 
small portion of the plant persists from year to year. The roots, 
the leaves, and the bulky cortex are shed or decay annually, 
and are as often renewed from the stem apex and the meristem- 
atic zone which surrounds the small central permanent cylinder. 
