358 RELATION TO ENVIRONMENT. 



700. Lenticels. — These are developed quite abundantly underneath 

 stomates on Ihc 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 (iig. 416). 

 The lenticels can easily be seen. 



2. THE FIBROVASCULAE SYSTEM. 



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

 out living contents which arc 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 V.)undles, as w'ood fibers and bast filjcrs. 



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

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

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

 vascular tissue contains spiral, annular, piitted, and scalariform ^■esscls and 

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

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

 walled living cells intermingled called wood parenchyma. In the conifers 

 (pines, etc.) the tracheary tissue is de\'oid 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 ctther. 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 arc in the 

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

 substances elal)C)rated in the U-aves. 



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 e]iidennis and the various out- 

 growths of its cells in the form of hairs, or In'riinmcs, 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 la^'cr of 

 external cells originating from the outer la\"cr of parcnch\'ma cells at 

 the grooving apex of the stem or root. These cells undergo ^*arious 

 modifications of form. In many cases they lose their jirotoplasmic 

 cfmtents. In luany cases the outer Awall becomes Ihickcneil, especially 



* Some fibers occur also very frei|uently in the Fundamental System, 

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



