SOME ASPECTS OF PERIODICITY IN PLANTS 67 



the phenology of a species varies within limits with site, latitude 

 and altitude. The causes of the inherent phenological differences 

 have been but little investigated ; in some cases at least, genetic 

 affinity and the original geographical source of the species appear 

 to be of importance. For example the relatively late seasonal 

 flowering of the Sweet chestnut and of the lime, when compared 

 with the majority of British trees, suggest quite correctly that 

 these belong to a more southern home and have their blood- 

 relations in warmer countries. But does the early flowering of 

 the Common ash imply genetic or geographical affinity to the 

 colder north? It certainly does not. The difference between 

 the two cases finds elucidation, I think, in the phenology of the 

 Fagales. In these the times of flowering seem at first to obey 

 rules opposed to such as might be anticipated. It is a general 

 rule that in one and the same species flowering is hastened in 

 the season by a warmer climate, so that Robinia Psendacacia in 

 southern Italy produces blossom before acquiring foliage, and 

 Daphne Mezercum on high mountains or far north postpones its 

 flowering until leaves are produced. Yet the Betulaceae, which 

 extend farther north and are more characteristic of cold-temperate 

 regions than the Fagaceae, generally flower earlier in the season 

 than do the oaks and chestnuts, many of whose species are sub- 

 tropical or even tropical. 



Here it would appear at first thought that the direction of 

 evolution has been opposed to that in which the climate acts 

 on the individual. Closer consideration, however, shows that 

 such is not the case. In the Fagaceae the inflorescences arise 

 and open on the current year's shoot; in the Betulaceae they 

 likewise usually arise on the current year's shoot, though 

 they do not open their flowers until the following vegetative 

 season. Thus in Castanca and Quercus, allied to subtropical 

 or tropical genera and species, and Fagus, whose distribution 

 is more temperate, the inflorescences open in the year of their 

 origin ; whereas in the Betulaceae, whose representatives extend 

 farther north into arctic regions, but scarcely reach the tropical 

 climate, the opening of the inflorescences is nearly always 

 postponed until the year following their inception. That this 

 explanation of the normal situation of the betulaceous inflores- 

 cences on the one-year-old twigs is the correct one is confirmed 

 by the behaviour of certain species of Alnits. The extreme 

 northern American species, A. sitchensis, produces the inflores- 



