410 WALTER GARDINER. 
on in the primary ground-tissue, for those cells immediately 
next the gland have become differentiated in a regular and 
definite manner so as to form a sheath of one or at most two 
layers of cells, entirely surrounding the gland with the ex- 
ception of that portion of it which is covered by epidermis. 
This sheath is continuous with the endodermis of the bundles 
of the leaf. 
Development of the water-pore.—One, two, or rarely three, 
of the cells of the dermatogen, where it is in immediate re- 
lation to the outer end of the gland, increase in size. Each 
is distinguished by a large and distinct nucleus, and each is 
the mother-cell of a water-pore (fig. 4). 
The nucleus of this cell divides and a wall is formed, 
dividing the cell into two parts. The two cells thus pro- 
duced separate from one another, leaving a hole or opening 
between them. This is the water-pore or water-stoma. It 
thus consists of two guard cells, which when fully developed 
contain chlorophyll. There is a difference in the time of 
formation of a water-pore as contrasted with that of the 
ordinary stomata. ‘Thus the water-pores are fully developed 
before any trace can be detected of the ordinary stomata. 
This difference in point of time is exhibited in a still more 
striking manner by Crassula coccinea, in which the water- 
pores may be seen completely formed, when the cells of the 
epidermis are just beginning to divide to form stomata in the 
peculiar and complicated manner so characteristic of the 
Crassulace. And not only does the water-pore differ from 
the stoma in its time of development, but also in size, shape, 
and mode of development (figs. 8 and 9). It is slightly 
larger than the stoma. Its contour is rounded as compared 
with the elliptical shape of the ordinary stomata, the breadth 
being greater in the former than in the latter case. The 
actual pore or opening of the stoma is, however, larger than 
the water-pore. 
The water-pore isthus produced by the simple division into 
two equal halves of a cell of the dermatogen, which has 
become larger than its neighbours. This is well shown by 
fig. 4, which is a surface view. But the ordinary stoma is 
formed in a more complicated manner. One of the cells of 
the dermatogen divides. Of the two cells thus produced 
one becomes the mother-cell of the stoma. It increases in 
size, and after previous division of the nucleus, a wall makes 
its appearance in nearly every case at right angles to the 
plane of the first division, and thus forms the two guard- 
cells of the stoma. The second cell of the two formed in 
the first instance becomes somewhat displaced in the course 
