246 Campbell . — Contributions to the 
all but the very youngest embryos, the dermatogen in the leaf 
is very plain. 
About the time that the first divisions in the ligule are 
formed, a few cells near the centre of the embryo cease to 
divide by transverse walls, but elongate in the direction of the 
longer axis of the embryo, and dividing longitudinally, form 
a bundle of narrow cells, the first trace of the fibro-vascular 
system of the young plant. This first group of cells lies so 
near the centre of the embryo that it is not possible to assign 
it certainly to either root or leaf; indeed in some cases it 
seems to belong to one quadrant, and again to the other. 
From it, in both directions, the development of the single 
axial bundle of both leaf and root proceeds, and by it they 
are both directly connected. A cross-section of these cells 
shows them to be of nearly equal diameter (Fig. 31). The 
section of the plerome- cylinder is somewhat elliptical, corre- 
sponding to the flattening of the leaf, and the appearance of 
the longitudinal sections differs as the section is made parallel 
to the flattened surface of the leaf, or at right-angles to it. In 
the former case, the diameter of the plerome is uniform, and 
it ends abruptly (Fig. 43). In the latter, it is narrower and 
tapers gradually to a point (Fig. 39). In both views its limits 
are clearly defined, as are those of dermatogen and periblem. 
The plerome-cylinder never extends entirely to the apex of 
the leaf, but is separated from it by several rows of cells. 
In the periblem, the divisions are mainly transverse, and the 
cells are therefore arranged in quite regular rows ; this is 
especially marked in horizontal sections (Fig. 43). 
Now begins a rapid growth in length, due partly to an 
increase in length of the cells at the tip of the leaf, and partly 
to the rapid transverse divisions and growth of the basal cells. 
The cells of the prothallium grow for a time, following the 
growth of the embryo, but soon the cells are ruptured and 
the young leaf protrudes, and having now developed abundant 
chlorophyll, appears to the naked eye as a bright green point 
attached to the spore. 
At first the leaf is composed of a compact tissue without 
