STEMS 691 



In young and middle-aged trees there often are such large fluctuations 

 in the width of the annual rings that the progressive phenomena out- 

 lined above are not easily recognized. Careful observation makes it ap- 

 pear that the growth rings are wider and the cell caliber greater when the 

 season is warm and dry than when it is cold and wet, thus appearing to 

 indicate-that strong transpiration facilitates the development of second- 

 ary as of primary wood. Stem elongation, on the other hand, is greatest 

 in wet seasons, being facilitated by weak transpiration (p. 736). 



Annual rings and climate. Annual rings are much more sharply 

 marked in periodic than in uniform climates, the greatest difference 

 being found where there is a well-defined winter and summer, or where 

 wet and dry seasons alternate. In uniform climates annual rings are 

 poorly marked or even absent, especially in the most pronounced ever- 

 greens (such as the conifer, Araucaria). The trees of eastern Java, which 

 has alternate wet and dry seasons, have much more prominent growth 

 rings than those of the uniform climate of western Java; even species 

 that are common throughout (as the teak, Tectona grandis) have uni- 

 form wood in the latter region and growth rings in the former. In the 

 teak the difference produced directly by climate is accentuated by the 

 fact that the tree is evergreen in western and deciduous in eastern Java. 

 That the deciduous habit favors ring formation is shown generally by 

 the presence of more prominent rings in deciduous than in evergreen 

 trees in the same climate. Probably the sudden cessation and renewal 

 of activity in deciduous trees as compared with the less interrupted 

 growth of evergreen trees sufficiently account for their more pronounced 

 ring development. The continued appearance of annual rings in certain 

 European trees transferred to uniform tropical climates shows that 

 hereditary as well as environmental factors may be of influence. 



Where the climate is detrimental to tree growth, as in alpine and arctic regions, 

 the annual rings frequently are eccentric rather than concentric, and are exceedingly 

 narrow, sometimes being discernible only upon microscopic examination; a 

 Juniperus stem has been reported as having a diameter of only thirty centimeters 

 and yet exhibiting four hundred rings. Some of the large polar kelps have a tissue 

 differentiation suggesting annual rings, there being alternating regions whose differ- 

 ences probably are due to seasonal variations. 



Rings other than annual. Sometimes two rings are developed in a single year, as 

 when a prolonged summer drought is followed by a pronounced wet period, or when 

 a tree puts forth new leaves after defoliation by insects or by storms, the new 

 foliage being accompanied by a second cylinder of "spring wood." Some tropica 1 

 trees shed their leaves two or three times a year, and in such cases the number 



