ANNUAL RINGS. 139 



ring. That which is produced earliest (spring wood) has some- 

 what larger ducts and wood-cells than that which is formed later 

 (autumn wood) . The difference is not very striking when the 

 wood of a single 3'ear is examined, for the diminution in size 

 is gradual from within outwards ; but if the autumn wood of one 

 year is compared with the spring wood in the next ring, the dif- 

 ference is verj- marked. The cause of the difference in character 

 between the early and later wood formed during a single season 

 is supposed to be the greater pressure exerted bj- the tense bark 

 in autumn. The experimental evidence in favor of this view 

 will be presented in the chapter on " Growth." 



396. In climates where there is no marked arrest of vegetative 

 activity during the whole year, for instance, in that of the equa- 

 torial zone, the secondary wood seldom presents any clearly 

 defined annual rings. In the wood of warm, temperate zones, 

 however, well-marked annual rings are not uncommon. 



397. It has long been known that in temperate climates a tree 

 may exceptionally form a double ring in a single 3-ear. The 

 cause of this in cases which have been carefullj' examined ap- 

 pears to be : (1) a partial cessation of activity owing to injury, 

 followed by (2) a renewal of activity in the same season. Thus 

 an elm may be stripped of its leaves in early summer and suffer 

 a temporary check ; but the buds alread}- formed for another 

 year develop into full leaf in a short time, the assimilative activ- 

 ity is resumed, and two rings are formed as a result of this ces- 

 sation and renewal. Kny ^ has found this to be the case with 

 several trees which had been deprived of their foliage at the end 

 of June. Wilhelm has found by experiment that a tolerably 

 well-defined double ring was formed in Quercus sessiliflora, from 

 which he removed all the leaves on the 7th of June ; while in a 

 second case, where the foliage was removed later (July 10th), 

 the duplication of the ring was not apparent. 



398. From this statement it would appear that even in tem- 

 perate climates, where there is a prolonged period of complete 

 inactivity due to the cold, the number of rings shown in the 

 cross-section of a stem maj- not exactly coincide with the num- 

 ber of years through which the tree has lived. But, as matter 

 of fact, the lines of limitation in the intercalated rings are so 

 much less distinct than those on either side, that the two lesser 

 rings would be counted as one, and therefore be credited to the 

 growth of one 3'ear instead of two. 



1 Verhaiidl. d. botan. Vereins der Prov., Brandenburg, 1880. 



2 Child : Popular Science Monthly, December, 1883. 



