454 THE BIOLOGY OF FLOWERING PLANTS 



a continuous succession of leaves seen. Generally a race 

 or generation of leaves unfolds itself quickly, and a more or 

 less prolonged period of rest follows. One, two, three, or 

 more such races may be seen on a branch at the same time. 

 The fall of the oldest race frequently occurs just before 

 or just after the unfolding of the youngest ; or the old 

 leaves may fall irregularly. Sometimes leaves are to be 

 seen unfolding on one branch, mature on another, while 

 a third is bare. In many trees an occasional " general 

 cleaning " takes place, and the tree stands for a time nearly 

 bare. The general effect of the tropical rain forest is of 

 perennial verdure, but the details of leaf-fall and renewal 

 present a picture of extraordinary complexity, from which 

 it comes out clearly that rhythmical development, apparently 

 unrelated to external conditions, is the rule. 



That this rhythm is inherited in the structure of the 

 protoplasm does not follow. Klebs (1914, 191 7) has offered 

 an alternative explanation which we may consider in relation 

 to the case he has most fully investigated, that of the beech. 

 Like most of our shrubs and trees the beech cannot be forced 

 by placing it in a hothouse in winter, but Klebs found that, 

 by subjecting it to continuous illumination by electric light, 

 the buds could be made to open at any time, though more 

 easily in September or February (after 10 days) than in 

 November (after 38 days). He further found that in con- 

 tinuous illumination the beech could be prevented from 

 forming buds, and forced to continuous growth during at 

 least four months ; by suitable changes it could be made 

 to form buds which at once opened, or to form resting buds. 



He considers that bud formation, initiated by the 

 suppression of the leaf and the conversion of the stipules 

 into scales, is due to the monopolisation of the salt supply 

 by the cambium and by the vigorously growing and 

 transpiring leaves in early summer, combined with a supply 

 of abundant carbohydrates to the growing-point. The 

 resting condition of the bud is determined by inactivation 

 of en2ymes through the presence of excess of carbohydrates. 

 This inactivation is most extreme in November, when it 



