150 Gibson . — Contributions towards a Knowledge of 
quently two only. In some species, e.g. vS. viticidosa , stomata 
with three or even four guard-cells may be found, similar to 
those described and figured by Haberlandt 1 as occurring in 
the sporogonium of Mosses, and by Farmer 2 in Iris. The 
dermatogenic layer immediately over the midrib undergoes 
division so as to form a band of variable breadth of almost 
square cells, at first (i.e. near the base of the young leaf) quite 
indistinguishable from each other. Presently some of these 
cells, after division has ceased, become rounded and contain 
denser protoplasm ; these are the mother-cells of the stomata. 
The dermatogenic cell therefore becomes the mother-cell of 
the stoma directly, and undergoes division by formation of a 
vertical wall in the long axis of the leaf. In some cases this 
wall is oblique. The median wall splits, and a central ellip- 
tical pore is formed, as in the normal mode of formation of 
stomata. In the older leaves of many species the walls both 
of the guard-cells and of the surrounding epidermal cells 
become greatly thickened, as, for example, in , 5 . Martensii ^ 
S.prodncta, S . concinna, S. viticulosa , &c. The stomata, which 
are in some species formed directly on the margin, do not appear 
to differ in any respect from those of other parts of the leaf. 
3. The Epidermal Cells (Figs. 6, 11, 12, 13, 16, &c.). 
Reference has already been made under the individual 
species to the great variety in length and general form of the 
epidermal cells. Generally speaking, one may distinguish 
three types of cell in the superficial layer of the leaf: (1) 
conical cells, having their somewhat square or more commonly 
polygonal bases turned outwards, and their truncated ends in 
continuity with the mesophyll (Fig. 22) ; (2) elongated, square- 
ended, or tapering plates, with more or less sinuous lateral 
walls (Figs. 6, 11); (3) sclerotic, warty, and elongated fibres, 
which occur along the margin in many species (Fig. 4), or 
scattered irregularly amongst the epidermal cells of either 
of the two preceding types (Figs. 11, 16, 19). Even on the 
1 Beitrage zur Anatomie und Physiologie der Laubmoose. Pringsh. Jahrb. 
XVII. 3. 1886. 
2 Ann. Bot., No. xiii, p. 174. 
