120 



THE RATE OF GROWTH 



[CH. 



a considerable period of time ; moreover, during the warm season, 

 the monthly values are regularly graded (approximately in a 

 sine-curve) with a clear maximum (in the southern hemisphere) 

 about the month of December. In the evergreen trees, on the 

 other hand, the amplitude of the periodic wave is very much 

 less ; there is a notable amount of growth all the year round, 

 and, while there is a marked diminution in rate during the coldest 

 months, there is a tendency towards equahty over a considerable 



Fig. 31. Periodic annual fluctuation in rate of growth of trees (in the 

 southern hemisphere). 



part of the warmer season. It is probable that some of the 

 species examined, and especially the pines, were definitely retarded 

 in growth, either by a temperature above their optimum, or by 

 deficiency of moisture, during the hottest period of the year; 

 with the result that the seasonal curve in our diagram has (as it 

 were) its region of maximum cut off. 



In the case of trees, the seasonal periodicity of growth is so 

 well marked that we are entitled to make use of the phenomenon 

 in a converse way, and to draw deductions as to variations in 



