7o 



SCIENCE PROGRESS 



internode-curve is especially encountered in species, genera 

 and families whose phyllotaxis varies from cyclic to acyclic. 



It is a familiar fact that various species of herbs differ in 

 their times of most active growth in length. Less generally 

 known and often subject to misconception are the facts con- 

 cerning the growth in length of trees. The statement may 

 at once be made that this is governed by an innate periodicity 

 and that the period of maximum growth in length coincides 

 neither with maximum temperature of soil or air, nor with the 

 maximum supply of humidity, nor with any combination of 

 these. From among the series of papers dealing with this 

 question those of Dr. D. Christison may be considered first, 

 not only because of their extensive nature but because Christison 

 simultaneously investigated in the same individual tree growth 

 both in thickness and in length of stem. He showed that the 

 periods of elongation and thickening do not synchronise either 

 in duration or in phase. His observations in 1888 and 1889 on 

 conifers were made on young specimens (ten to twenty years 

 old) of Abies lowiana and A. grandis, Pseudotsuga Douglasii, 

 Pinus Pinaster and Araucaria imbricata. Christison's results 

 as tabulated below were obtained by taking the mean of the 

 results obtained from all the coniferous specimens examined. 



Percentage of Annual Growth taking place in the 

 Bimensual Periods 



The results obtained in the two years agree in the main, and 

 show that nearly 80 per cent, of the height growth took place 

 in June and July, whereas growth in thickness is rather equably 

 distributed over the season when measured according to 

 bimensual periods. But the different species and even indi- 

 vidual trees of one species differ in their periodicity of growth 

 in length, and very different shares in growth are often taken 

 by the two months of the bimensual periods. I have therefore 

 roughly calculated the percentage share of elongation in the 

 different (approximate) months for two of the species as 

 measured by Christison. 



