ANNUAL RINGS 681 



the function of the various elements of the woody cylinder, the answer 

 to this question would appear to be in the affirmative. The transpiring 

 surface exposed by the crown of a tree increases in size from year to 

 year. When, therefore, vegetative activity is resumed in spring-time, 

 one of the first requirements that has to be met is the demand for 

 additional water channels. This want is supplied by the inclusion of 

 numerous vessels in the wood produced during spring and early 

 summer. Hence, when the transpiratory activity of the crown reaches 

 its height in July and August, the additional water channels are there 

 ready for use. The water-conducting arrangements being thus perfected 

 so far as the current season is concerned, the tree can then concentrate 

 its attention upon increasing the mechanical strength of the trunk ; 

 strands of wood-fibres therefore appear, while the thick-walled character 

 and radial compression of the autumnal elements increases the amount 

 of mechanically effective tissue in the annual ring. 



The foregoing anatomico-physiological explanation of the formation 

 of annual rings was first put forward by the author in the original 

 [German] edition of the present work. Subsequently similar opinions 

 were advanced by Hartig and Strasburger. On the basis of a large 

 number of measurements performed upon complete transverse sections 

 of the trunk of Pinus sylvestris, F. Schwarz comes to the significant 

 conclusion that " the distribution and quantitative development of 

 autumn wood do actually conform very closely to the prevailing- 

 mechanical requirements." One-sided incidence of mechanical stresses 

 not only leads to an increase in the total growth in thickness on the 

 compressed side, but also raises the ratio of autumn to spring wood in 

 the individual annual rings. 



Further evidence in support of the view, that the characteristic 

 differentiation of spring and autumn wood is determined by the general 

 ecological relations of the plant, is furnished by the occasional duplica- 

 tion of annual rings, a phenomenon known to Unger, and more recently 

 studied by Kny, Wilhelm and Jost. 3tiG This duplication, consisting in 

 the formation of two more or less sharply defined zones of secondary 

 wood in a single vegetative season, occurs, as a rule, when the young 

 foliage of the tree is destroyed by frost, the ravages of insects or some 

 other injurious agency, but is replaced during the same season, owing 

 to the unfolding of buds which would ordinarily have lain dormant 

 until the following year. As Jost has recently once more pointed out, 

 the phenomenon of duplication clearly demonstrates, that a correlation 

 exists between the production of leaves or the unfolding of buds on 

 the one hand, and the formation of annual rings on the other, and that 

 this correlation involves a possibility of self-regulation in respect of the 

 aforesaid processes. 



