cross section of an internode.
_E_, of a node of a somewhat older leaf. _F_, _G_, young sexual organs
seen in optical section. _o_, ooegonium. _An._ antheridium. _H_,
superficial view. _G_, _I_, group of filaments containing
spermatozoids. _J_, a small portion of one of these more magnified,
showing a spermatozoid in each cell. _K_, free spermatozoids. _L_, a
piece of a leaf with ripe ooegonium (_o_), and antheridium (_An._).
_B_, _H_, x 150. _J_, _K_, x 300. _I_, x 50. _L_, x 25.]

By making a series of longitudinal sections with a sharp razor
through the top of the plant, and magnifying sufficiently, it is
found to end in a single, nearly hemispherical cell (Fig. 23, _B_,
_S_). This from its position is called the "apical cell," and from
it are derived all the tissues of the plant. Segments are cut off
from its base, and these divide again into two by a wall parallel to
the first. Of the two cells thus formed one undergoes no further
division and forms the central cell of an internode (_y_); the other
divides repeatedly, forming a node or joint (_x_).

As the arrangement of these cells is essentially the same in the
leaves and stem, we will examine it in the former, as by cutting
several cross-sections of the whole bunch of young leaves near the
top of the plant, we shall pretty certainly get some sections
through a joint. The arrangement is shown in Figure 23, _E_.

As the stem grows, a covering is formed over the large internodal
cell (_y_) by the growth of cells from the nodes. These grow both
from above and below, meeting in the middle of the internode and
completely hiding the long axial cell. A section across the
internode shows the large axial cell (_y_) surrounded by the
regularly arranged cells of the covering or cortex (Fig. 23, _D_).

All the cells contain a layer of protoplasm next the wall with
numerous oval chloroplasts. If the cells are uninjured, they often
show a very marked movement of the protoplasm. These movements are
best seen, however, in forms like _Nitella_, where the long
internodal cells are not covered with a cortex. In _Chara_ they are
most evident in the root hairs that fasten the plant to the ground.

The growth of the leaves is almost identical with that of the stem,
but the apical growth is limited, and the apical cell becomes
finally very long and pointed (Fig. 23, _C_). In some species the
chloroplasts are reddish in t