104 
MORPHOLOGY OF TISSUES. 
the stoma then appears, when perfect, as if it had been formed according to the 
Hyacinthus type, in which each guard-cell has been again divided into an upper and 
a lower cell. But, according to Strasburger, this is not the case; the two pairs of 
guard-cells lie originally in one plane, and, strictly speaking, it is only the middle cell, 
— which is divided by a vertical wall, and the splitting of which forms the cleft, — 
that is to be considered as the mother-cell of the stoma; the two oblique divisions 
by which the two lateral cells are formed that afterwards lie uppermost must be 
regarded merely as a preparation for the formation of the mother-cell. Preparatory 
divisions of this kind occur in many Dicotyledons ; one. of the young epidermal cells 
becomes the primary mother-cell of the stoma, and is divided successively in dif- 
ferent directions by walls at right angles to the surface; finally (Fig. 85) we have 
a cell surrounded by several cells formed in this manner, which afterwards forms the 
two guard-cells (as in Crassulaceas, Begoniaceae, Gruciferae, Violacege, Asperifolieae, 
Solanaceae, Papilionaceae). In other plants, on the contrary, especially Monocotyledons, 
after the formation of the mother-cell of the stoma which results from the division 
Fig. 86. — Development of the stomata in a leaf of Cotninelyna ca-lcstis; A very young stomata ; B nearly mature ; 
s s\n A and B the mother-cells of the stoma in C ; s s va. C the guard-cells; A and B show the formation of the 
neighbouring cells. 
of a young epidermal cell, divisions also take place in the adjoining epidermal cells, so 
that the stoma is surrounded by a pair or by two decussate pairs or by some other 
arrangement of neighbouring cells (Fig. 86) ; as in Aloe socotrina, Gramineae, Juncaceae, 
Cyperaceas, Alismaceae, Marantaceae, Proteaceae, Coniferae, Pothos crassinewia, Ficus 
elastica, Tradescantia %ebrina. The origin of the mother-cell of the stoma in Planta- 
gineae, CEnothereae, Sileneae, Centradenia, and miiny Ferns deserves special study in 
reference to the mode of cell-division. In these cases the mother-celP is cut out on 
one side from the young but already tolerably large epidermal cell by a wall bent in 
a U-shape, the convexity of which faces the cavity of the epidermal cell, while the ends 
^ Strasburger calls them ' special mother-cells.' I think it, however, better entirely to abandon 
this expression, the more so as its first introduction in the formation of pollen depended on an 
obsolete view of the formation of the cell-wall (compare our description, pp. 33, 34). 
