EP1DEKM1S OF THE LEAF. 



161 



452. The epidermis of the leaf is continuous with that of the 

 stem. Its principal features have been described in Chapter II., 

 and only the following need now be recalled. 1. It may be 

 simple, that is, composed of one layer of cells ; or multiple, - 

 of more than one. 2. Immediately below it may be found in 

 some cases one or more layers of 



cells known as the hypodermu. 

 3. The epidermal cells are in un- 

 broken contact with each other 

 except at (1) rifts, (2) water-pores, 

 (3) stomata. 4. Their surfaces 

 may exhibit nearly every form of 

 trichome. 



453. Glands secreting nectar 

 are found on different portions of 

 the leaves of various plants ; for 

 example, at the junction of the 

 petiole with the blade (Poplar), 

 at the base of the petiole (Cassia 

 occidentalis), on the lower side of 

 the midrib of the leaf (cotton- 

 plant) , or scattered over the lamina 

 (turban squash). Such glands are 

 particularly noticeable in insec- 

 tivorous plants, as Sarracenia and 



Nepenthes (see Part II.). On making a section of one of the 

 nectar-glands found on a young poplar leaf, the epidermis will 

 be seen to be transformed into a double layer of thin-walled, 

 elongated cells forming the secreting surface, which is charged, 

 together with the parenchyma lying below it, with a s3 T rup de- 

 rived from the transformation of starch. At times the secretion 

 from a gland is so abundant that drops of considerable size 

 collect upon the surface of the leaf, and if rapid evaporation 

 takes place, crystals of sugar are deposited at the gland. 1 



454. The leaves of submerged phrenogams, for example those 

 of Potamogeton and Myriophyllum, possess no true epidermis; 

 the parenchyma is therefore in direct contact with the surround- 



124 



1 Trelease : Nectar and its Uses, in Report on Cotton Insects (United States 

 Department of Agriculture, 1879), and Nectar-Glands of Populus, Botanical 

 Gazette, vol. vi. p. 284. 



FIG. 124. Transverse section through leaf of Camellia (Thea) viridis, showing: a 

 epidermis ; b, branched liber-cell ; d, oil-drop ; e, crystals. (Mirbel.) 



11 



