04 



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 j 

 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 Crassulaceae, Begoniaceae, Gruciferae, Violaceae, 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 Commelyna coeUsits; A very young stoniata ; B nearly mature ; 

 ss'va. A and B the mother-cells of the stoma in C; s s xa C the gfuard-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, Gramineaj, Juncaceae, 

 Cyperaceae, Alismaceae, Marantaceae, Proteaceae, Coniferae, Pothos crasshier'via, F'tcus 

 elastica, Iradescantia %ebrina. The origin of the mother-cell of the stoma in Planta- 

 gineae, CEnothereae, Sileneae, Centradenia, and many 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). 



