SOME ASPECTS OF PERIODICITY IN PLANTS 75 



Mean Diurnal Increments of Growth in Girth and Height 

 during the named successive periods 



In reference to the mysterious temporary stoppage of growth 

 in thickness Dr. Christison concludes that " the retardation in 

 girth-increase in conifers is synchronous with rapid growth 

 of the new top and therefore may be due to the incapacity on 

 the part of the tree to carry on the two growths in full measure 

 at the same time." But an inspection of the figures given in 

 the table above shows that though they harmonise well with 

 Christison's hypothesis in the case of Abies, this is not true 

 in the case of Pinus : yet in both these species the second 

 maximum of rate of growth in thickness occurring in late summer 

 synchronises with great reduction or stoppage in the rate of 

 growth in height. The cause of the halt in thickening then is 

 unknown. Possibly it is due to temporary shortage of water or 

 of food-material that are being diverted to growth in length of 

 the stem and leaves and to transpiration ; available statistics 

 give no support to this suggestion, in particular von Hohnel's 

 figures concerning the monthly rate of transpiration in conifers 

 betray no signs that transpiration is a contributory cause. 

 We do not know whether this growth in thickness is checked 

 simultaneously throughout the tree or if it is synchronous 

 with a sudden increase in rate of thickening of other parts of the 

 tree (the root, for instance). Possibly it may be a genuinely 

 innate pause, such as we might anticipate could characterise 

 a species whose ancestral home lay in a region where there 

 was a rest in the middle of the hot season associated with 

 drought, and where the climate so far approximated to that 

 now prevailing in the regions characterised by sclerophyllous 

 vegetation. From this point of view the investigation of living 

 sclerophyllous types would be of interest. This revived activity 

 of the cambium late in the season of growth is by no means 



