220 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



wall. In Aneimia on the other hand the stomata are in the middle of an epidermal 

 cell (Fig. 169 s). This peculiar position, which occurs also in Polypodium Lingua, is 



FIG. 167. Pteris flabellata; development of the stomata. A very young, B nearly mature epidermal cells ; s in A 

 mother-cell of the guard-cells, v preparatory cell. 



due to the circumstance that the wall of the mother-cell of the stoma has the form of a 

 ring narrowing conically inwards, and set at right angles to the surface of the 



FIG. 168. Portion of a 

 leaf of Adiantitm (nat.size) 

 with the nerves spreading 

 fan-like from the base of the 

 leaf, and bifurcating re- 

 peatedly. 



FIG. 169. Aneimia fraxinifolia. Surface-view of a 

 stoma lying in the middle of an epidermal cell ; ss guard-cells 

 of the stoma, e epidermis, cl chlorophyll-corpuscles. 



epidermis, but touching no side wall. Occasionally we find here too an arrangement 

 such as is shown in Fig. 167. 



The fundamental tissue of the stem and of the leaf-stalk is in many species, such 

 as Polypodium atireum, P. vulgare, Aspidium Filix-mas, composed entirely of thin- 

 walled parenchyma ; in other kinds, as Gleichenia, species of Pteris and in the Tree- 

 ferns, certain portions of the fundamental tissue in the form of strings, ribbons or 



