EPIDERMIS. 



43 



■between the outer and inner wall, which touches no lateral wall, and which diminishes 

 conically inwards. In Aneimia (Fig. i6) it, or rather the stoma, is therefore sur- 

 rounded by an annular epidermal cell. Almost the same applies for Polypodium 

 lingua (see above, 3. c) ; the annular cell is in its turn as a rule surrounded by a horse- 

 shoe-Shaped neighbour, from which it was originally separated by a U-shaped wall. But 

 often (Rauter, Fig. 18) also this wall is not U-shaped, but annular, the stoma is thus sur- 

 rounded by two concentric. annular cells. In Aneimia Phyllitidis, and hirta, as in Poly- 

 podium lingua, it happens exceptionally, and in Aneimia villosa (according to Stras- 

 burger) it is the rule that the typically annular walls are U-shaped, and attached to a 

 lateral wall. Those of the mature parts are arranged accordingly. Further it occurs 

 not unfrequently that from one or from both ends of the stoma (and in the former case, 

 according to Strasburger, always the peripheral end) a membrane runs bridge-wise to the 

 nearest lateral wall (Fig. 16, c). In face of the many atteraplts to explain and interpret 

 this phenomenon it may be remarked, that from the first there is nothing more than the 

 appearance shows at once, that is a membranous band, arranged as described, growing 

 with the other membranes, and requiring an explanation of its appearance no more and 

 no less than any other membrane. 



The second case, which is to a certain extent peculiar, but which otherwise belongs 

 to the group (3. a), is the formation of the stoma of the Equiseta, It is here stated 

 according to Strasburger (h^.). The Initial cell, the first appearance of which was not 

 observed, is nearly cubical, the two flanks being parallel to the longitudinal axis of the 

 stem. Near to its own longitudinal axis, thus defined, there appear symmetrically, right 

 and left, two nearly radial longitudinal walls : both are concave on the sides facing one 

 another, and contiguous at their upper and lower ends. The initial cell is thus divided 



FIG. 17.— Development of the stoma of Hyacinthus orientalis. On the left the mother-cell, just divided ; ^, B successive 

 further stages of development ; on the right the formation of the slit is complete (/) ; the other letters as in Fig. lo, which 

 should be compared (800}. From Sachs' Textbook. 



into one central biconvex-, and two lateral plano-convex daughter-cells ; the two latter 

 lessening wedge-wise inwards, the central one outwards. The central cell is the mother 

 Cell of the stoma (it divides later by a longitudinal wall into the two guard-cells), the 

 two lateral ones are the subsidiary cells. The latter assume a form exactly similar to 

 the guard'cells, and over-arch them, so that they cover their whole outer surface, and 

 only leave a narrow space free above the true entrance of the stoma. Hence the form 

 of the double pair of guard-cells apparently covering one another. In the Equiseta 

 cryptopora of Milde the matter is further complicated by the depression of the stoma 

 with its subsidiary cells (comp. below. Fig. 23). 



To form the stoma, the mother-cell divides — after, rarely before the completion of 

 the last. division, which produces subsidiary cells^nto two halves, which are the guard- 

 cells ; and the slit appears thus : the division wall between the two splits in its central 

 part into two lamellae which gradually separate from one another (Fig. 17). This 

 separation .proceeds from the middle towards the ends, and from the entrance and exit 



