THE EPIDERMAL TISSUE. 



105 



are applied to one of its side-walls (Fig. 84). Not unfrequently, especially with Ferns, 

 e.g. Asplenium biilbiferum, Pteris cretica, Cibotium Schiedei, &c., preparatory cells are 

 cut out in this manner from the epidermal cell before the formation of the mother- 

 cell, out of which the guard-cells are then 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 epidermal cell, the former is half, or more than half, en- 

 closed by the latter when looked at from above. In some Ferns and Sileneae the wall of 

 the mother-cell of the stoma is from the first so strongly curved that it touches the epi- 

 dermal cell only in a narrow band ; in Anemia 'villosa it touches it only at one spot, the 

 partition-wall seen from above appearing like a circle. In Anemia densa ^.nd fraxinifolia 

 the side-wall of the epidermal cell does not anywhere touch the wall of the mother-cell 

 of the stoma \ When first formed this cell has the form of a hollow cyhnder, or, 

 more exactly, of a truncated cone, the base and truncated end of which are portions 

 of the upper and lower wall of the epidermal cell ; out of the latter a cell is thus 

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



Fig. 87. — Superficial view of a stoma of Anetnia fraxini- 

 folia with the epidermal cell completely surrounding it ; e epi- 

 dermis, jj guard-cells ; cl chlorophyll-granules. 



Fig. 88.— Transverse section of a leaf of Piuw; 

 Pinaster (X 800) ; s guard-cells of the stoma; p 

 its cleft ; tj entrance ; I air-cavity ; c cuticularised 

 layers of the epidennis ; a middle lamella, i inner 

 thickening-layers of the cells beneath the epidermis ; 

 g parenchyma of the leaf containing chlorophyll. 



stoma; and thus arises the remarkable arrangement represented in Fig. 87, where, as 

 may be seen, the two guard-cells are entirely enclosed within a single epidermal 

 cell. Similar, but more complicated, is, according to Rauter, the structure in Nipho- 

 bolus Lingua. 



By further growth of the guard-cells and of the epidermal 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 depressed and apparently belong to a deeper layer of cells (Fig. 88) ; some- 

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



The stomata of Marchantieae may shortly be mentioned here in connexion with 

 what has already been said on Fig. 65, p. 78. After the formation of the air-cavities, 

 which are filled with outgrowths containing chlorophyll (Fig. 89, A, chl), one cell of the 



' Stiasburger, in Jahrb. fiir wiss. Bot. VII. p. 393 ; also Rauter, /. 



