50 



ORGANOGRAPHY. 



hardened on their surface ; while others again regard the cuticle 

 as the permanent original outer wall of the parent cells of the 

 epidermis, which has become chemically altered by the action 

 of the air. It appears to be of the same nature as the Inter- 

 cellular substance which will be described hereafter. A Iiomo- 

 geneous membranous layer resembling, if not actually identical 

 with cuticle, is found upon the surface of plants living under 

 water ; and upon that of the Algse, Lichens, and Fungi, which 

 have no true epidermis. 



c. Stomata or Stomates. — These are orifices situated between 

 some of the epidermal cells, opening into the intercellular cavities 

 beneath, so as to allow a free communication between the internal 

 tissues and the external air {figs. 106, s and 107, s); hence they 

 are also commonly called breathing jpores. These orifices are 

 surrounded by cells of a diiFerent form from those of the epi- 

 dermis ; and they also usually contain some chlorophyll granules. 

 There are generally but two cells surrounding the orifice, which 



Fig. 104. 



Fig. 105. 



Fig. 104. Epidermis of the Lily, with stomata. Fig. 105. Vertical section 



of a portion of the frond of Marchantia polymorpha. g, g. Stoma divided 

 perpendicularly, h, h. Rings of cells forming its walls. Alter Carpenter. 



are commonly of a more or less semilimar form {figs. 92 and 

 104), so that the whole has some faint resemblance to the lips 

 and mouth of an animal, and hence the name Stomata applied to 

 /such orifices {eroixa, a mouth). The bordering cells of the 

 (orifice have been called ' stomatal cells,' or ' pore cells,' or ' guard- 

 cells,' and have the power of opening or closing the orifice which 

 they guard according to circumstances, as will be explained when 

 treating of the functions of stomata. Instead of two stomatal 

 cells, we sometimes, although but rarely, find four, or even more ; 

 thus, in some of the Liverworts {Hepaticaceai), the stomata are 

 rounded apertures between the epidermal cells, surrounded by 

 three or more tiers of stomatal cells, each tier being itself com- 

 posed of four or five cells, the whole forming a kind of funnel 

 or chimney {fig. 105). 



Upon making a vertical section through the stomata, we 

 usually find that the stomatal cells are placed nearly or quite 



