140 PART II. ANATOMY AND HISTOLOGY. [ 35 



The Tissues developed from the Cambium. In stems and roots 

 in which the growth in thickness is normal, the cambium gives 

 rise to secondary wood, secondary bast, and secondary conjunctive 

 tissue (medullary rays). 



The structure of the secondary wood differs essentially from 

 that of the primary wood only in that it includes no_ spjirad or 

 annular vessels resembling those of the protoxylem (see p. 126). It 

 always includes tracheal tissue ; nearly always wood-parenchyma 

 (see p. 91); frequently sclerenchyma: the cell-walls of all these 

 forms of tissue are usually more or less completely lignified. 



The secondary tracheal tissue may consist either solely of 

 tracheae (e.g. Platanus, Fraxinus excelsior and Ornus, Citrus, 

 Viscum, Hydrangea) ; or solely of tracheids (e.g. Coniferse, Drimys 

 Winter I) ; or, as is generally the case, of both tracheae and 

 tracheids. The cell-walls of the tracheal tissue are, as a rule, 

 marked with bordered pits; but occasionally, especially in soft 

 wood, the walls are reticulately thickened. 



The secondary icood-parenchyma consists of oblong cells, which 

 are generally so arranged that their long axes are parallel to that 

 of the member of which they form part : they occur in short 

 longitudinal strands, consisting commonly of a single row of cells 

 (Fig. Ill (7), but sometimes, in the middle only, of more than one 

 row. They are true cells, containing protoplasm and a nucleus, 

 and other substances, such as starch (especially in perennial steins 

 and roots in winter), tannin, etc. Their walls are generally 

 lignified, but usually not very much thickened, and have .circular 

 or elliptical simple pits. In many soft fleshy stems ._ and_jLQOts 

 (e.g. Potato, Radish, Turnip, Beetroot), where this tissue is the 

 principal product of the activity of the cambium, the cell- walls are 

 not lignified. 



The secondary sclerenchyma consists of elongated prqsenchy- 

 matous cells, with more or less thickened lignified Avails marked 

 Avith narrow oblique bordered pits (Fig. 72, p. 93; Fig. Ill 

 A, B). Two forms of this tissue are distinguishable: woody 

 fibres destitute of protoplasmic contents, Avhich are connected 

 by transitional forms with the tracheids (see p. 92) : fibrous 

 cells, with protoplasmic cell-contents, which are allied to the 

 wood-parenchyma ; in fact, one fibrous cell corresponds to a row 

 of wood-parenchyma cells ; the walls of the fibrous cells sometimes 

 remain thin, as in Viscum and some other plants, Avhere they 

 replace the Avood-parenchyma both structurally and functionally. 



