Life-History of Isoetes. 247 
intercellular spaces, but as it begins to elongate, the cells in- 
crease rapidly in size, and separating at the corners, numerous 
intercellular spaces result. There are usually two rows of 
very large ones (Fig. 49, i) that form broad air-channels 
extending the whole length of the leaf, but interrupted at in- 
tervals by layers of cells that form imperfect partitions across 
them. 
The primary xylem consists of small spiral and annular 
tracheids at the base of the leaf, and from these the formation 
of similar ones proceeds toward the tip. Their number is 
small, even in the full-grown leaf, and they are the only dif- 
ferentiated elements of the bundle, which for the rest is com- 
posed of elongated parenchyma-cells, differing in no essential 
particular from the original procambium-cells. 
The Root . — The first root arises from the posterior upper 
quadrant of the embryo. Ordinarily its axis of growth is in 
the same plane as that of the leaf, so that a vertical longi- 
tudinal division of the embryo through the centre bisects 
both root and leaf (Fig. 39). Not infrequently, however, the 
root diverges more or less from the median line, and when 
the leaf is nearly vertical, as sometimes happens, it makes 
almost a right-angle with it. In the root, as in the leaf, 
the primary tissues become early differentiated, but here the 
growth is due exclusively to the activity of cells near the 
apex. 
In the very young root (Fig. 27) the end is covered with a 
single layer of large cells, the dermatogen, continuous with 
that of the rest of the embryo. Beneath are two layers, con- 
centric with the cells of the dermatogen (Figs. 27, 28). Of 
these, the inner is the initial layer of the plerome, which soon 
becomes well defined and connected with the bundle of pro- 
cambium-cells in the centre of the embryo. The layer of 
cells below the primary epidermis is the initial meristem for 
all of the tissues of the root except the plerome, and the 
outermost layer of cells (primary epidermis) splits into two 
layers (Fig. 38, R), that take no further part in the growth of 
the root. 
T 
