CHAPTER 111 



SECONDARY INCREASE IN THICKNESS 



Dicotyledons and Gymnosperms 



If we follow the history of any particular region of stem 

 or root we find that its growth in thickness up to the time when 

 its growth in length ceases is due to the enlargement of the cells 

 that arise from the division of the cells of the primordial and 

 primary meristems. In other words, increase in thickness is at 

 first due to the enlargement of the cells of the epidermis, primary 

 cortex, pericycle, phloem, xylem, medullary ray, pith. Such 

 increase, known as primary increase in thickness, soon ceases, 

 and subsequent growth in thickness is due to the differentiation 

 of additional tissues following the production of new cells by 

 the division of the cambium, or cork cambium in the bark. 



Growth of the Vascular Bundles. — It will be remembered 

 from the preceding chapter that the cambium ring is composed 

 of two parts: the fascicular cambium consisting of procambium 

 cells lying between the phloem and xylem which remain prac- 

 tically unchanged in form and retain their power of division, 

 and the interfascicular cambium which is formed by the tan- 

 gential division of primary medullary ray cells that lie in a 

 line connecting the fascicular cambium of contiguous bundles. 

 The cambium cells begin active cell division immediately follow- 

 ing the differentiation of the primary tissues told about in the 

 last chapter, and by the differentiation of these new cells the 

 fascicular cambium adds tissues to the xylem toward the inside, 

 and to the phloem toward the outside, and the interfascicular 

 cambium makes additions in like manner to the primary medul- 

 lary rays. 



It is found on comparing the rate of growth of phloem and 

 xylem that the latter increases much more rapidly than the 



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