358 RELAl^ION TO ENVIRONMENT. 



700. Lenticels. These are developed quite abundantly underneath 

 stomates on the twigs of birch, cherry, beech, elder, etc. The phellogen 

 underneath the stoma develops a cushion of cork which presses outward 

 in the form of an elevation at the summit of which is the stoma (fig. 416). 

 The lenticels can easily be seen. 



2. THE FIBBOVASCTTLAB SYSTEM. 



701. Fibrous tissue.* This consists of thick-walled cells, usually with- 

 out living contents which are elongated and taper at the ends so that the 

 cells, or fibers, overlap. It is common as one of the elements of the vas- 

 cular bundles, as wood fibers and bast fibers. 



702. Vascular tissue, or tracheary tissue. This consists of the vessels or 

 ducts, and tracheides, which are so characteristic of the vascular bundle 

 (see Chapter V) and forms a conducting tissue for the flow of water. The 

 vascular tissue contains spiral, annular, pitted, and scalariform vessels and 

 tracheides according to the marking on the walls (figs. 58, 59). These are 

 all without protoplasmic contents when mature. There are also thin- 

 walled living cells intermingled called wood parenchyma. In the conifers 

 (pines, etc.) the tracheary tissue is devoid of true vessels except a few spiral 

 vessels in the young stage, while it is characterized by tracheides with pecu- 

 liar markings. These marks on the tracheides are due to the "bordered" 

 pits appearing as two concentric rings one within the other. These can be 

 easily seen in a longitudinal section of wood of conifers. 



703. Sieve tissue. This consists of elongated tubular cells connected at 

 the ends, the cross walls being perforated at the ends. These are in the 

 phloem part of the bundle, and serve to conduct downwards the dissolved 

 substances elaborated in the leaves. 



704. Fascicular cambium. This is the living, cell -producing tissue in 

 the vascular bundle, which in the open bundle adds to the phloem on one 

 side and the xylem on the other. 



3. THE EPIDERMAL SYSTEM. 



705. To the epidermal system belong the epidermis and the various out- 

 growths of its cells in the form of hairs, or trichomes, as well as the guard 

 cells of the stomates, and probably some of the reproductive organs. 



706. The epidermis. The epidermis proper consists of a single layer of 

 external cells originating from the outer layer of parenchyma cells at 

 the growing apex of the stem or root. These cells undergo various 

 modifications of form. In many cases they lose their protoplasmic 

 contents. In many cases the outer wall becomes thickened, especially 



* Some fibers occur also very frequently in the Fundamental System, 

 forming bundle-sheaths, or strands of mechanical tissue in the cortex. 



