14 Campbell. — Observations on the Development 
type. When the base of the apical cell is triangular (Fig. 40), 
the segments are cut off from the base, and these take part in 
the formation of the plerome-cylinder. The root-cap arises 
in part from the outer segments of the apical cell, but is 
formed also in part from the outer cells of the newly formed 
segments. In later roots a single apical cell is not always 
certainly to be seen, and it is probable that as the roots 
increase in size one or more of the segments of the apical 
cell assume the function of initials and thus a group of 
initial cells is formed that replaces the original single apical 
cell. 
The vascular cylinder of the root is usually tetrarch. 
At four points in the periphery small spiral or annular 
tracheids arise, and from them the formation proceeds as 
usual toward the center of the bundle. The phloem is made 
up of nearly uniform cells with moderately thick colourless 
walls. The bundle-sheath is much less definite than is usual 
in ferns. 
The foot is much less prominent than is usual, and its limits 
are not at all clearly defined. In fact all of the superficial 
cells of the central region of the embryo become enlarged and 
appear to act as absorbent cells. 
As the embryo grows, the surrounding prothallial cells 
divide rapidly, so that the young plant is completely enclosed 
within a sort of calyptra for an unusually long time. Owing 
to the position of the cotyledon and stem, which are turned 
toward the upper surface of the prothallium and grow ver- 
tically, a very conspicuous elevation is formed upon its upper 
side, and finally the cotyledon bursts through this and appears 
upon the upper surface of the prothallium. The root grows 
downward, its axis almost coinciding with that of the 
cotyledon ; but it does not break through the prothallium 
until the cotyledon has nearly reached its full development. 
This fact is especially significant, as, in the leptosporangiate 
ferns, the root is usually the first organ of the young plant to 
break through, and is fastened in the ground before the 
cotyledon is visible from the outside. Of the other ferns, 
