WATER-GLANDS IN THE LEAF OF SAXIFRAGA CRUSTATA, 41] 
of subsequent growth. It never seems to grow as large as 
the adjoining epidermal cells, and may always be recognised 
in the fully developed condition as interfering with the ar- 
rangement of the four or five epidermal cells which would 
otherwise surround the stoma in a symmetrical manner 
(figs. 9 and 10). 
Gland hairs.—At the same time that the mother-cells of 
the water-pores are making themselves apparent by their 
increased size, outgrowths have arisen on the surfaces of 
numerous dermatogen cells. These are the rudiments of 
the small knob-like hairs which are borne in the mature 
leaf on the sides of the depression, which, as has already 
been mentioned, marks the position of the gland. They are 
especially numerous on the outer side of the lobe, and when 
fully developed are almost perfectly spherical, with very 
thick and very highly refractive cell walls (fig. 5). Cell- 
division though not cell-growth of the water-gland tissue 
practically ceases with the division of the mother-cell of the 
water-pore; but in the leaf, cell-division still goes on, 
ceasing only with the formation of stomata. As a conse- 
quence of this, depressions are produced over the glands, 
the sides of which are fringed by the delicate highly refrac- 
tive hairs. 
Structure of the mature gland.—By far the larger portion 
of the gland-tissue is made up of polygonal cells, slightly 
longer than broad, closely fitting one to the other, with no 
intercellular spaces. The cell-walls are thin and the cells 
themselves are much smaller than those of the surrounding 
ground tissue. The protoplasm is very granular. At first 
it fills the cells entirely, with the nucleus in the centre. 
When mature a central vacuole makes its appearance, and 
the protoplasm then forms a thick layer, which closely 
invests the cell wall; the nucleus is either imbedded in the 
protoplasm, or it is suspended in the vacuole by strands of 
protoplasm, its position being no longer central but ec- 
centric (see fig. 7). The whole gland is roughly pear- 
shaped, broad towards the surface of the leaf, tapering 
inwards towards the fibro-vascular bundle (figs. 5 and 6). 
It is invested by a sheath of cells continuous with the 
endodermis of the fibro-vascular bundle, except where it is 
in contact with the epidermis. The endodermis cells are 
long and cylindrical with moderately thick walls. They 
contain numerous chlorophyll granules. External to this 
is the mesophyll of the leaf. Free communication between 
the gland tissue and the external mesophyll is afforded by 
VOL. XXI,—NEW SER. EE 
