4 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



visible pores (9). When it is thickened by the deposition of 

 sclerogen, passages are left in the latter communicating with 

 the sides of the tissue, and giving it the appearance of being 

 dotted or pitted. 



21. Cellular tissue is self-productive, one cell generating 

 others upon its surface. In Chara, Marchantia, &c. young 

 cells are said to be formed at the points of and in the spaces 

 between older cells ; in Confervse and in anthers new cells are 

 formed by the internal divisions of an older cell; while, 

 according to Schleiden, the most general mode of production is 

 from cytoblasts (10), generated in the mucus of vegetation (6). 



22. PITTED TISSUE (Boihrenchyma) is a modification of the 

 cellular, either consisting of ordinary cylindrical cells placed 

 end to end, opening into each other, and forming continuous 

 tubes ; or originally tubular 15 . Its sides are marked by pits, 

 resembling dots, produced in consequence of the sclerogen (13) 

 being unequally deposited over the inside of the cells. It is 

 common in wood, of which it forms what is vulgarly called 

 the porosity. Its office is to convey fluids with rapidity in 

 the direction of the woody tissue that surrounds it. Formerly 

 it was considered a form of vascular tissue, and called dotted 

 ducts, or msiform tissue. 



Pitted Tissue is articulated, when composed of short cylinders placed end to 

 end, or continuous when it was originally tubular. 



23. WOODY TISSUE (Pleurenchyma) consists of elongated 

 tubes tapering to each end, and, like the vesicles of cellular 

 tissue, imperforate to the eye. It may be considered a form of 

 the cellular tissue itself, to which it is frequently referred ; but 

 it is practically distinguished by its cylindrical form, great 

 length, extreme fineness, and toughness; the latter of which 

 properties is produced by the thickness of its sides. 



24. It is found in the wood, among the parenchyma of the 

 liber (124), and in the veins of the leaves, or other appendages 

 of the axis. 



25. Its functions are to give strength to the vegetable fabric, 

 and to serve as a medium for the passage of fluid from the 

 lower to the upper extremities. 



Common Pleurenchyma has its sides destitute of markings ; the glandular 14 is a 

 variety in which the sides of the tubes are furnished with circular disks ; the 

 latter occur chiefly in coniferous plants and such as have aromatic secretions. 



