RHYTHM 455 



may be affected by external conditions, and by such internal 

 changes as the conversion of starch into sugar. Light acts 

 by activating the enzymes ; this occurs naturally when the 

 day lengthens in late spring ; it occurs experimentally with 

 continuous illumination in winter. That continuous growth 

 takes place in continuous illumination is due partly to the 

 demonstrated fact that the quality and intensity of the 

 electric light permitted only of feeble assimilation, and 

 partly to abundant salt supply from rich soil, and to weak 

 transpiration. The Lammas shoot is due to the coincidence 

 of maximum illumination, at the height of summer, with the 

 completion of leaf formation in the buds ; and it is significant 

 that only a few of the best illuminated buds take part. 



The hornbeam resembles the beech in its response to 

 continuous illumination. The oak and the ash, on the other 

 hand, cannot be forced in this way, but open their buds in 

 a suitable temperature, with favourable conditions of salt 

 supply. Continuous growth takes place in continuous 

 illumination, but activation is due to high temperature in 

 the dark. Thus the determining factor is not the same in 

 all trees. We have, however, a plausible explanation of 

 periodic development in its most extreme form, the cause 

 being the action of external factors in a peculiarly complex 

 fashion. Klebs lays great stress on the importance of the 

 relation of salt supply to assimilates, and thinks this may be 

 largely responsible for the periodic unfolding of leaves in 

 tropical trees. The supply of salts may accumulate till 

 sufficient is available for the unfolding of the buds and 

 growth, and then a rest period ensue till a further supply 

 is accumulated. It will be seen that this is a particular 

 application of the principle of limiting factors. It is of 

 great interest that changing conditions of illumination can 

 cause the beech to grow in the different modes characteristic 

 of the normal development of other trees ; the precise ratio 

 of supplies of assimilates and salts to the growing points is 

 clearly specific (see also Klebs, 1915). 



Klebs's interpretation is supported by analyses of twigs 

 of the apple and peach by Abbott (1923). She found that 



