254 Campbell. — On the development 
root, but this is best done after the root has attained some 
length. Part of such a series is shown in Plate XV, P"igs. 
2-5, taken from an embryo of nine days. The younger 
segments, as will be seen on comparing them with those from 
a younger embryo (PL XIV, Figs. 20, 21), are much the same, 
but as the sections are made further from the tip of the root 
changes are observed which had not yet appeared in the 
younger embryo. The outer of the two original cells of each 
semi-segment divides by a tangential wall into two nearly 
equal cells, and these ultimately undergo further division by 
similar walls, the outer into two, the inner into three. 
About the time that these cells are formed (PI. XV, Fig. 3), 
intercellular spaces appear at the points where the inner and 
outer cells are in contact, and these appear larger and larger 
as the root increases in diameter. The two outermost layers, 
i.e. epidermis and hypoderma, undergo further radial divisions 
and form an uninterrupted double layer of cells, but the three 
original cells lying between the hypoderma and the plerome- 
cylinder divide subsequently only by horizontal walls and form 
single rows of cells separating the large intercellular spaces. 
The older sections (PI. XV, Figs. 4, 5) show a perfectly uniform 
radial structure. In the centre is a group of about nine cells, 
the young bundle, from which radiate at regular distances rows 
of three cells each. The lower cells of these rows are in con- 
tact and constitute the bundle-sheath, but the others are sepa- 
rated by the large intercellular spaces. Bounding the section 
are the two rows of cells forming epidermis and hypoderma. 
The cell-division in the cap-segments is illustrated in PI. 
XV, Fig. 2 #, b. In Fig. 2 b the central part shows the 
youngest cap-segment, the peripheral cells belonging to the 
next oldest segment. As seen here each segment-cell has 
divided into two nearly equal parts, and these, by walls at 
right angles to the first, also into approximately equal cells. 
These are next divided by tangential walls and the resulting 
marginal cells by radial walls, so that on section four central 
cells and a marginal circle of smaller ones are now visible 
(Fig. 2 a). These marginal cells later undergo further 
