24 



EPIDERMIS. 



extended over all the parts of plants exposed to the air, except the 

 stigma. On the extremities of newly-formed roots, the integument 

 consists of loose cells, which are considered either as being the ordinary 

 cellular tissue of the plant, or as being an imperfectly-formed epider- 

 mis, which has received the name of Epiblema (tvl, upon, and phy/ax, 

 wound, as being the tissue which first covers wounds). This latter 

 kind of tissue occupies the place of the epidermis, in the parts of plants 

 which are always under water. On the aerial roots of Orchidaceous 

 plants, there is an epidermal layer consisting of spiral cells (fig. 23), 

 containing air. 



48. The epidermis is usually formed by a layer or layers of com- 

 pressed cells, which assume a more 

 or less flattened tabular shape, 

 and have their walls bounded by 

 straight or by flexuous lines. Fig. 

 69 e e, represents an epidermis 

 formed of regular hexagonal cells ; 

 fig. 72, one composed of irregular 

 hexagons ; while in fig. 71, the 

 boundaries of the cells, e, are flexu- 

 ous and wavy. The cells of the epi- 

 dermis are so intimately united 

 together, as to leave no intercel- 

 lular spaces (fig. 74 e e). 



49. The epidermis is sometimes 

 thin and soft, at other times dense 

 and hard. In the former case it 

 may be easily detached from the 

 subjacent cells ; in the latter, the cells become thickened by deposits, 

 and sometimes the layers are so produced as to leave uncovered spots, 



fig. 71. Epidermis, from lower surface of the leaf of Madder (Rubia tinctorum). e, Cull of the 

 Epidermis. s, Stoma. 



Fig. 72. Epidermal layer, from upper surface of a leaf of Ranunculus aquatilts when growing 

 out of water, e e, Epidermal cells, ssss, Stomata. 



Fig. 73. Vertical section of lower epidermis of the leaf of Rochea fakata. e , Double epider- 

 mal layer, with very large external cells, small internal ones pierced by a stoma, s, which com- 

 municates with a lacuna, I. p, Parenchyma of the leaf. 



