52 The Ottawa Naturalist. [June- July 



and new leaves in the spring. In the tropics various factors 

 mav be effective at various times, which may also be said of 

 the warmer desert regions, where rain may induce the produc- 

 tion of foliage at anv time of the rear. '{Foiiquieria, Cannon, 

 22). 



The loss of foliage in areas of marked seasonal change is a 

 response to environmental stimuli, found in conditions which 

 are usually and on the whole unfavourable for growth or for 

 the physiological processes which take place in the leaf. If it 

 happens, as in exceptional years it will, that such or analogous 

 unfavourable conditions intervene at unusual times, general 

 defoliation will ensue just as promptly and completely as at 

 the usual time. Only last year (1913) in Nebraska, an almost 

 unprecedented period of high temperatures and meagre rainfall, 

 together with low relative humidity, caused, in addition to a 

 far-reaching prejudice to crops, a marked shortening of the usual 

 vegetative period. Herbaceous plants hastened to fruitage, and 

 "early leaf maturity and leaf fall was common among native 

 and exotic forest trees." During the late summer, after the 

 drought had been broken, refoliation occurred, but the new 

 leaves were small (Pool, R. J., 23). Klebs (24), cites a similar 

 occurrence in Germany during the summer of 1910, caused by 

 dryness in July and August, followed by refoHation, and speaks 

 of the case as'^a natural experiment on a large scale to support 

 his contention that the periodicity of trees expressed in leaf-fall 

 is a response to external conditions, and not, as Volkens (2 5) 

 has argued, especially in regard of tropical plants, a periodic 

 phenomenon independent of the external environment, and 

 dependent on inherited and inherent causes. The basis for this 

 view was Volkens' failure to observe any relation between the 

 march of climate and defoliation, as, e.g., in Ficus fulva. Klebs 

 insists, however, that the time of defoliation may be shifted by 

 disturbance in surrounding conditions, and cites, among others, 

 the fact that tropical plants could be made to shed their leaves 

 in the verv short time merely by a reduction of light. 



However the attack on this problem may turn out, it is 



worth while to indicate that a conclusion, such as Volkens 



arrives at, is a sort of mental anaesthetic, which, like the vitalistic 



theory, lulls the mind* and inhibits vigorous and critical attack. 



As Klebs very rightly puts it, every life-process depends in some 



degree upon the external world, and it is only by experimental 



methods that we can hope to come at a right analysis of this 



complex relation. 



(To be continued) 



