283 
Ipomoea paniculata . 
wall spreads centrifugally through the gland, and then the 
central cells undergo a further tangential division, a second 
cell being cut off from the upper one which borders the lumen. 
This cell usually has a large vacuole and a clear appearance, 
while the upper one contains very granular protoplasm and 
a large nucleus and nucleolus about the middle of the cell. 
Finally, two or three of the surface-cells in the very centre 
of the gland undergo radial divisions (perpendicular to the 
surface), so that we now have four or five elongated cells 
(s. e.) y supported on one basal polygonal cell pas 1 ), and 
below this another cell pas 2 ), which has thin walls, — all these 
being derived from one original epidermal cell. A single 
section through a young gland would show all these stages, 
since the development does not take place simultaneously 
throughout. (See Fig. 13.) 
The two lower cells derived from the epidermis remain 
clear and contain vacuoles ; the lateral walls of the uppermost 
one pas 1 ), adjoining the true secretory cells, become thickened 
and cuticularized, and the cell itself becomes one of the 
polygonal layers already mentioned (see Fig. 18). 
In the next oldest leaf on the shoot, whose blade was 
11 mm. in length, the multiplication of the surface-cells of 
the gland by vertical divisions had continued, and now we 
have as many as ten or twelve secretory cells supported on 
one basal polygonal cell (Fig. 14). These secretory cells are 
full of protoplasm, which, however, is not quite so granular in 
appearance as before, and the large nuclei are usually in the 
upper third of the cell. 
The next oldest leaf (blade 15 mm. long) had a gland 
whose secretory epithelium exhibited the same general struc- 
ture as the previous one (see Fig. 15). The secretory cells 
have become longer and narrower, and are crowded closely 
together ; and it is not easy to distinguish the various groups 
of cells from one another. The protoplasm is not so granular, 
and small vacuoles are beginning to appear. 
When the secretory epithelium has become fully developed 
it presents the appearance shown in Fig. 16. This drawing 
x 
