73 



CELLULAR TISSUE. 



which is turned from the slit, the united ends, and the strip of the concave side 

 which borders on the slit is much less thickened. The last-named strip is seen, 

 in those cases where ridges of exit and entrance protrude far into the cavity, as a 

 channel on the inner surface of the wall, which appears in the transverse section of 

 the stoma like a broad pit. When the ridges of exit and entrance are very broad, 

 the middle of the convex side assumes a similar appearance (e.g. Ficus elastica, 

 Fig 1 8 C). The various modifications of this plan of structure, depending upon the 

 absolute and relative strength of the thickening and its varying protrusion inwards 

 and outwards, require no further description in detail; some of them are evident 

 from the above figures. Further, as regards genuine exceptions, which occur especially 

 in the Coniferse and Cycadese, we may refer to the special literature above-quoted 

 (comp. p. 35). Still the structure of the walls of the stomata of Equisetum, which is 

 anything but clearly explained in the writings of Duval-Jouve and Milde \ must not 

 be entirely passed over (Fig. z^f. The guard-cells themselves are here in no way 



FIG. 24,— Stem of Equisetum hiemale ; stoma with tile surrounding tissue (390). A view from tlie inner surface. 

 The pair of ffuard-ceiis is surrounded by the superposed edge of the pair of subsidiary cells. B transverse section 

 of the stem pa-ising in a median direction through a stoma, which lies in a depression of the surface. The narrow 

 entry of the slit is bordered by two flat guard-cells and the subsidiary cells which surround them ; each of the latter 

 has a curved pit, turned outwards. Further explanation in the text. The cells of the single-layered epidermis and 

 of the hypodermal schlerenchyma below it have numerous pits. C silica residue of a fragment of epidermis with a 

 stoma, after maceration in Schultze's mixture and subsequent ignition ; seen from outside. The curved figures are 

 the outlines of the prominences of the outer surface. 



remarkable except in their form, which is flattened obliquely outwards, and in the slight 

 difference in thickness between the ridges of exit and entrance and the other bands 

 of the membrane. But the subsidiary cells, which completely surround them (comp. 

 p. 43), have a thickening on the wall bordering on the guard-cell, in form of bands 

 protruding into the cell-cavity. These diverge in a radiating manner from the slit. 

 Hence the beautiful radiate-marking seen in surface view. The number, breadth, 

 and frequent branching of the radial bands vary according to species. In Milde's 

 Equiseta phaneropora (at least E. limosum) each band runs over the whole radiate 



• Milde, Monographia Equisetorum, Nov. Act. Acad. Leopold, torn. XXIV. pars II.- 

 Jouve, Histoire nat. des Equisetiim de France, Paris, 1865. 



' Sanio, Linnsa, Bd. 29, p. 389, Taf. Ill, — Strasburger, /. c. 



-Duval- 



