88 MORPHOLOGY OF TISSUES. 



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 perpendicular 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 Phanerogams ; one of 

 the young epidermis -cells becomes the primary mother -cell of the stoma, and is 

 divided successively in different directions by walls, which stand at right angles to 

 the surface ; finally we have a cell surrounded by several cells formed in this manner, 

 which afterwards forms the two guard-cells (as in Crassulacese, Begoniaceae, Cruciferse, 

 Violarieae, Asperifolieae, Solanacese, Papilionaceae). In other plants, on the contrary, 

 after the formation of the mother-cell of the stoma, which results from the division of a 

 young epidermis-cell, divisions also take place in the adjoining epidermis-cells, so that 

 the stoma is surrounded by a pair or by two decussate pairs, or by some other arrange- 

 ment of epidermis-cells, which stand in relation to the stoma according to their origin 

 and development; (as in Aloe socotrina, Gramineae, Juncaceae, Cyperacese, Alismaceae, 

 Marantaceae, Proteaceae, Pothos crassiner'via, Ficus elastica, Goniferae, Tradescantia %ehrina). 

 The origin of the mother-cell of the stoma in Plantaginaceae, Oenotherese, Silenese, Cen- 

 tradenia, and many Ferns is of special interest in the mode of their cell-division. In 

 these cases the mother-cells^ are so developed that from the young but already tolerably 

 large epidermis-cell, a small piece is cut out on one side by a wall bent in a U-shape, the 

 convexity of which faces the centre of the epidermis-cell, while its margins are applied to 

 one of its side-walls. Not unfrequently, especially with Ferns {e.g. Asplenium bulbiferum, 

 Pteris cretica, Cibotium Schiedei, &c.), preparatory cells are cut out in this manner from, 

 the epidermis-cell before the period of the formation of the stoma-cell, out of which 

 moreover the guard-cells are formed by simple longitudinal division. 



In consequence of the U-shape of the division -wall which separates the mother- 

 cell of the stoma from the epidermis-cell, the former is half, or more than half, en- 

 closed by the latter, when the epidermis is looked at from above. In some Ferns (and 

 Silene?e) the wall of the mother-cell of the stoma is from the very commencement so 

 strongly curved that it touches one side of the upper epidermis-cell only in one narrow 

 band ; in Aneimia 'villosa it touches it only at one point, the curved partition-wall as 

 seen from above appearing annular. In Aneimia densa and A. fraxinifolia the side- 

 wall of the upper epidermis-cell does not anywhere touch the wall of the mother-cell 

 of the stoma '^. At its commencement this cell has the form of a hollow cylinder, or, 

 more exactly, of a truncated cone, the bases of which are portions of the upper and 

 lower wall of the upper epidermis-cell ; out of the latter a cell is thus cut out like a 

 piece out of a cork by a corkborer; the piece thus cut out is the mother-cell of the 

 stoma, and thus arises the remarkable arrangement represented in Fig. 76, where, as 

 may be seen, the two guard-cells are enclosed by a single annular epidermis-cell. 

 Similar, but more complicated, are, according to Rauter, the arrangements in Nipho- 

 bolus Lingua. 



By further growth of the guard-cells and of the epidermis-cells which surround 

 them, different relative positions of the former to the surface may be brought about ; 

 the guard-cells may, when mature, lie in one plane with those of the epidermis, or may 

 be deeply pressed down and apparently belong to a deeper layer of cells ; sometimes 

 they are on the contrary elevated above the surface of the epidermis. 



^ Strasburger calls them ' special mother-cells.' I think it, however, better entirely to abandon 

 this expression, the more so as its first introductitin in the formation of pollen depended on an 

 obsolete view of the formation of the cell-wall (compare our description, pp. 32, 33). 



2 Strasburger, Jahrb. fiir wiss. Bot. VII, p. 393. 



