ioo Plant Growth-Rhythms. [Sess. 



ture, and heat ; the latter conditions are not uniform in 

 nature, but are continually varying, so that even were the 

 organism adapted for continuous growth, and the supply of 

 energy were continuous, the variation in the necessary con- 

 ditions acting on the sensitive organism would tend to produce 

 variation in the growth. If growth is intermittent under 

 uniform conditions, the inference is that the machine is 

 adapted for intermittent growth, and the movement is an 

 autonomic one ; if the growth varies exactly as the conditions, 

 we conclude that the movement is induced. 



Observation and experiment show that all growth move- 

 ments are rhythmic, but whether this is due to the nature of 

 the organism or to the influence of external conditions, or a 

 combination of the two, is not in every case quite clear. 



1. Annual Periodicity. — In countries which have a seasonal 

 alternation — for example, a wet and dry season in the course 

 of the year — the vegetation exhibits alternating phases of 

 activity and rest. Commencing in spring, growth accelerates 

 to a maximum in summer, slows off gradually during autumn, 

 and ceases in winter. This runs parallel with variations in 

 the light, temperature, and general climatic conditions ; the 

 natural tendency is to associate the two as cause and effect, 

 and to regard the annual periodicity as an induced phenomenon. 

 This does not exhaust the subject ; experiment has supplied 

 other facts which throw additional light on the question. 



When stored in the dark, even at suitable growth tempera- 

 ture, tubers will remain in a dormant condition until a given 

 period has elapsed, when they will sprout even if the tempera- 

 ture be lowered. Deciduous trees retain their annual period- 

 icity when cultivated under uniform conditions in a warm 

 and well-illuminated greenhouse. Beech and oak transported 

 to Madeira, where the temperature is favourable for growth 

 throughout the year, continue to foliate and defoliate as 

 usual. On the other hand, in South Java, under a temperate 

 and uniformly moist climate, the oak and the apple after 

 several years lose their periodicity and grow like evergreens ; 

 but while the organism as a whole has lost its rhythmic 

 growth, there is still a rhythmic growth in the individual 

 members, although these rhythms do not synchronise ; thus 

 on the same tree shoots may be found simultaneously showing 



