SECONDARY INCREASE IN THICKNESS 



or all of the leaves have been formed, and tracheal tubes com- 

 municating with these have been laid down in the stem, the 



cambium can devote itself 

 more exclusively to the pro- 

 duction of strengthening 

 tissues, the need for which 

 has been caused by the in- 

 creased size and weight of 

 the crown. The two differ- 

 ent regions of a ring of 

 annual growth are, there- 

 fore, seen to be an ana- 

 tomical expression of vary- 

 ing physiological needs. 



Sometimes it happens 

 that trees are stripped of 

 their leaves by insects, and 

 later become rehabilitated 

 by the growth of buds that 

 would under normal con- 

 ditions, have lain dormant 

 until the succeeding spring. 

 It is then found that the 

 new crop of leaves stimu- 

 lates the production of a 

 new ring of growth in 

 the stem, much as would 

 have happened the follow- 

 ing spring if things had 



FIG. 25. Diagram shewing the relation ct 



this year's leaves to the wood cf the current been left to their WOnted 



year. 



course. 



In those tropical regions where there is no pronounced dry 

 season and the leaves do not all fall off at once, but just a few 

 at a time with gradual renewal, there is no ring of growth formed ; 

 but where in the tropics a decided dry period provides plants 

 with too little water, the leaves drop off just as they do outside 



