4 8 



SECONDARY INCREASE IN THICKNESS 



a 



former. This is due to the fact that when a cambium cell divides, 

 by the formation of a tangential wall, which it usually does, 

 the daughter cell facing the xylem much more frequently differ- 

 entiates into the permanent condition than the one facing the 

 phloem, the latter continuing as a cambium cell; but sometimes 

 the daughter cell facing the phloem grows to be one of the 

 phloem elements, while the one facing the xylem remains in the 

 cambium condition. 



The kinds of tissues which the cambium adds to the xylem 

 vary in different plants. In many Gyirmosperms wood paren- 

 chyma cells are formed, 

 but, except in a single 

 genus, neither tracheal 

 tubes nor wood fibers, 

 their place being 

 usurped by tracheids 

 which perform alike 

 the strengthening and 

 water-conducting func- 

 tions. In Angiosperms 

 are produced tracheal 

 tubes of the pitted type, 

 tracheids, and transi- 

 tional forms between 

 these two, xylem 

 parenchyma and wood 

 fibers, and transitional 

 forms between these 

 also (Fig. 22). On the 

 phloem side the cam- 

 bium adds sieve tubes, 

 and, varying with the kind of plant, companion cells or phloem 

 parenchyma, or both of these, and, in many instances bast fibers. 

 While, by its tangential divisions, the cambium is thus adding 

 to the radial diameters of the phloem and xylem, it is also, but 

 at a slower rate, increasing their tangential diameters by its 



FIG. 23. -Photomicrograph of cross section of stem 

 of Aristolochia sipho, where cambial activity is just 

 beginning, a, epidermis; b, collenchyma; c, thin-walled 

 parenchyma of the cortex, the innermost cell layer of 

 which is the starch sheath or endodermis; d, scleren- 

 chyma ring of the pericycle; e, thin-walled parenchyma 

 of the pericycle;/, primary medullary ray; g, phloem; h 

 xylem; i, interfascicular cambium; j, medulla or pith. 



X20. 



