THE TISSUES OF Till. STEM 



63 



of annual rings, each of which represents the increment of growth 

 of one year. Consequently it is possible by counting them to estimate 

 the age of the trunk (Fig. 40). The reason why these rings are recog- 

 nisable is that the wood formed in the spring has larger and more 

 numerous vessels and on the average thinner walls than that formed 

 as the season advances. The physiological advantage of this may be 



m.r. 



auWs 



Fig. 41. 

 The tissues in the small outlined square in Fig. 40, magnified 200. au.W = autumn 

 wood of fourth season. sp.W = spring wood of fifth season. c6 = cambmm. phl.= 

 phloem. *u.r. = medullary ray. V, V = vessels of wood. S = sieve-tubes. 



that the arrangement ensures an increase in the water-conducting 

 tissues in spring, called for by the new leaves; but in the later 

 part of the season, when nutrition is high, more material is spent in 

 thickening the walls, thus adding to the mechanical strength. The 

 result of continued development of a woody trunk in the manner 

 described is to give a column constructed after the plan shown in 

 Fig. 42, where the relation of the annual rings and medullary rays 

 is clearly seen. 



