TISSUE SYSTEMS 



829 



so great in quantity that a coating of waxy particles is found to 

 be deposited on the exterior. 



The cells of the epidermis in the lower plants and in some 

 aquatic members of the Phanerogams contain chloroplastids, 

 but as a general rule these are absent from them. The epidermis 

 of aquatic plants does not become cuticularised. 



Though usually only one cell thick, there are many cases 

 where the epidermis consists of several layers. In the leaves of 

 certain plants, such as Ficus {fig. 642), there are two or three 

 layers. In the root-cap (fig. 763) we have several layers ; in 

 various aerial roots, such as tliose of epiphytic Orchids, there 

 is a special epidermis consisting of many layers of cells which 

 have curiotisly pitted walls and no ceU-contents. 



Fig. 705. 



Fig.. 706. 



Fig. 705. Simious epidermis with stomata, from the garden Balsam. 



Fig. 706. Vertical section of the epidermis of ffotta carnosa treated with 

 caustic potash, a. The detached cuticle, if. The partially cuticularised 

 layers of the outer walls of the epidennal cells. After You Mohl. 



The cells of the epidermis are, with certain exceptions, in close 

 contact with each other, forming a continuous covering to the 

 plant without any intercellular spaces. This continuous coating 

 is, however, interrupted at the apex of shoots whose growing 

 point is marked by the presence of an apical cell {fig. 707), 

 the latter always extending to the surface. In some shoots, as 

 in the thaUus of Polvetia, one of the Pucaceae, the growing point 

 is depressed, in eonseqvience of tlie growth of the lateral segments 

 of the apical cell {fig. 708). 



The epidermis of the shoot in all the higher sporophytes 

 possesses a number of apertures, produced by the splitting of 

 the common wall of two contiguous cells, known as guard-cells, 

 immediately over a conspicuous intercellular space of the sub- 

 jacent region. These apertures, known as stomata, may be 



