[Chap. XXVIII TISSUES AND PROCESSES OF STEMS 



287 



Fig. 107. Photomicrograph of a small portion of a cross section of a sunflower 

 stem in which a continuous cambium has developed between the original bundles 

 of the younger stem as pictured in Fig. 97. 



Consequently the appearance of the inner portion of each annual ring 

 of xylem as seen in cross section is quite different from that of the outer 

 portion of the ring. Without these differences in spring wood and 

 summer wood the annual rings of growth would not be evident to 

 the eye. 



The width of the annual rings in particular species varies with the 

 age of the individual plant and with the fluctuations in environmental 

 factors from year to }ear. In certain localities the effects of fluctuations 

 in water supply and humidity are more prominent than those of all other 

 factors combined. Tree rings mav therefore be used, along with other 

 data, as an index of periods of drought and of abundant moisture. Such 

 studies have shown that there are rather definite and predictable cycles 

 of periods of drought. Comparison of annual rings of living trees with 

 those of wooden beams used in dwellings in the southeastern part of the 

 United States has made it possible to date quite accurately certain events 



