5° 



SECONDARY INCREASE IN THICKNESS 



continues, as a rule, to the middle or end of August, but the 

 growth of the phloem continues after this even until frost. 



The kinds and 

 relative amounts of 

 the tissues in second- 

 ary xylem and 

 phloem vary a great 

 deal in different 

 families, genera, and 



species: 



and this fact 



is often very useful 

 to the anatomist and 

 pharmacognosist in 

 characterizing and 

 identifying materials. 

 Thus, the xylem 

 parenchyma may 

 vary from abun- 

 dance to entire ab- 

 sence; tracheids may 

 prevail or be lack- 

 ing; tracheal tubes 

 may vary greatly in 

 size and number, or 

 in their contact with 

 one another or com- 

 plete isolation; wood 

 fibers may be present 

 or absent, numerous 

 or infrequent. The 

 character of the 

 phloem may vary in like degree; bast fibers may or may not 

 occur; and so with the companion cells and phloem parenchyma. 

 One of the most wonderful things about plants is that the 

 daughter cells of the cambium may become such various things. 

 How is it that a daughter cell facing outward is directed to 



Fig. 24. — Portion of cross section of four-year-old 

 stem of Aristolochia sipho, as shown by the rings of growth 

 in the wood. The letters are the same as in Fig. 23, but 

 new tissues have been added by the activity of the cam- 

 bium; and a cork cambium has arisen from the outermost 

 collenchyma cells and given rise to cork. The new tissues 

 are: /, cork cambium; k, cork; g, secondary phloem from the 

 cambium, and just outside this is older crushed phloem; 

 n, secondary xylem produced by the cambium; m, second- 

 ary medullary ray made by the cambium (notice that this 

 does not extend to the pith). Half of the pith is shown. 

 Notice how it has been crushed almost out of existence. 

 Compare Figs. 23 and 24, tissue for tissue, to find out what 

 changes the primary tissues undergo with age, and to 

 what extent new tissues are added. Photomicrograph 

 X20. 



