SOME ASPECTS OF PERIODICITY IN PLANTS 73 



of conifers at repeated intervals during the period of growth. 

 The macroscopic method was adopted by Sir R. Christison, 

 and subsequently by Dr. D. Christison, who observed the rate 

 of increase by means of a tape measure (made of steel in the 

 case of the latter's observations). The macroscopic method 

 measures the increments due to a complex of phenomena — 

 namely, the algebraic sum of (i) additions to the bark and 

 wood caused by the phellogen and cambium ; (2) expansion 

 and contraction due to increased or decreased supplies of water 

 and heat ; (3) decrease due to peeling of dead bark ; (4) change 

 due to cracking of the bark ; (5) expansion and contraction of 

 the steel tape due to change of temperature. Selection of 

 appropriate trees eliminated the difficulties concerning the 

 dead bark. It was evident that contraction with loss of water 

 or fall of temperature could take place, as sometimes a slightly 

 decreased girth was registered by the tape after the lapse of 

 a week. Yet, that Dr. Christison's results may be accepted 

 as trustworthy is demonstrated by the consistent character of 

 his results and their general agreement with those of others 

 who used the microscopical method. 



The date of commencement of activity of the cambium in 

 one and the same species varies with the part of the tree in one 

 individual and in different individuals according to site and 

 climate. In central Germany it begins to be active usually in 

 the second half of April or the first half of May. At Edinburgh 

 at five feet from the ground in the main trunk Christison found 

 that in 1890 the dates at which the different species commenced 

 and ceased to thicken varied from April 6 to May 17 and 

 from August 23 to October 1 1 respectively. Strasburger 

 states that the production of phloem is prolonged later in the 

 season than that of xylem : this would account for the fact that 

 compared with the results obtained by continental observers, 

 who fixed their attention on the production of the wood, 

 Christison's statistics show that the process of thickening is 

 prolonged later into autumn. 



The exact time at which the several parts of the annual ring 

 are produced has been investigated by several botanists, who 

 have demonstrated, inter alia, that the term " autumn wood " 

 conveys a false impression. Mer, for instance, found that in 

 the oak the transition from "spring" to "autumn" wood is 

 effected about the middle of June and that in conifers the 



