CHAPTER X 



SECONDARY THICKENING 



IT was pointed out in the preceding chapter that the vascular 

 supply of the leaf is roughly proportional to its size (p. 113), 

 and in the same way the vascular system of the stem is correlated 

 with the area of leaf -surf ace which it bears. With the annual 

 increase of foliage exhibited by all woody perennials, a need for 

 additional conducting elements arises, and this want is supplied 

 through the activity of a rrieristem (the cambium, p. 79) situated 

 between the xylem and phloem of the bundles. Cambium is 

 found in this position in all Dicotyledons and Conifers (see p. 340), 

 but in Monocotyledons is present only in certain cases and as a 

 rudimentary vestige (e.g. in the leaf -sheaths of the Maize and in the 

 leaves of many other Grasses). The division of the cells of the 

 cambium leads to the formation of additional conducting ele- 

 ments, accompanied by a gradual increase in the size of the 

 stem, spoken of as secondary thickening. Enlargement does not, 

 however, always imply cell-division, since in Palms, where no 

 active cambium occurs, there is increase in girth mainly due to 

 growth of the cells already present. 



The cambium may be regarded as arising from an unaltered 

 remnant of the procambial strand (cf. p. 90), which has retained 

 its powers of division, but until it becomes active it is difficult 

 to recognise. The actual cambium is established by the appear- 

 ance of two parallel tangential walls in the persisting procambial 

 elements. There is thus cut out a single layer of radially flat- 

 tened cells (Fig. 57, C.) which, as seen in longitudinal section 

 (cf. Fig. 38, Ca., p. 83), have an elongated form and pointed ends. 

 Subsequent division of these cells takes place parallel to the two 

 tangential faces whereby files of segments are produced, both 

 on the outer and inner side, those adjacent to the phloem be- 



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