432 ZONES AND REGIONS [Pt. Ill, Sect. II 



deserts, not only the difference in the temperature but also that in the 

 moisture comes into play. 



The following considerations relate chiefly to the cold belt of the north 

 temperate zone, as at present precise physiological observations on the 

 warm or sub-tropical belts are lacking ; at the same time, in the sub-tropical 

 belts we have to deal with the same phenomena as occur further north, 

 though in a less pronounced form. 



i. METABOLISM AND INTERCHANGE OF ENERGY IN 

 MESOTHERMIC PLANTS AT DIFFERENT SEASONS. 



i. PERIODICITY IN THE GEAN-TREE. 



A description will not be given here of periodic phenomena easily visible 

 without detailed investigation, such as foliation and defoliation, the develop- 

 ment of the flowers and the maturing of the fruit, as these are generally 

 known. But quite recently we have begun to gain a better insight into 

 these processes, by means of physiological experiments and of the 

 microscope, and thus to prepare the way for their explanation. It is to 

 be hoped that a careful consideration here of these investigations may 

 stimulate similar observations and experiments in other climates. 



The periodic phenomena in a cold temperate climate appear on the 

 whole to run the same course throughout the entire vegetable kingdom, 

 except in annuals and a few plants that are constantly in blossom. Yet 

 in details manifold differences reveal themselves, so that it seems advisable 

 to direct our attention first of all to a definite example. The gean-tree, 

 Prunus avium, appears to be eminently suitable, as it has been thoroughly 

 investigated by several authors, specially by Askenasy and A. Fischer, 

 and at any rate noticed by others ; it can moreover serve as a type of 

 the majority of our broad-leaved trees. 



Roughly considered, the season of activity and the season of rest of the 

 gean-tree in South and Central Germany include each about six months — 

 the active period from about the middle of April to the middle of October, 

 the resting period during the remainder of the year. 



During the period of rest, the branches are leafless and bear only scaly 

 buds, which, as will be shown hereafter, are nearly always actually growing. 

 This growth is however usually extremely slow, and in default of exact 

 measurements is first noticeable towards the end of the period of rest 

 as swelling. 



The most significant moments during the active period are, as regards 

 the reproductive functions, the flowering time in April or May, and the 

 ripening of the fruit in June or July. The vegetative phenomena 1 that 



1 Askenasy, op. cit. 



