io6 PLANT PHYSIOLOGY 



ever, it is impossible to induce buds to open by means of external stimuli during 

 most of the summer, and removal of leaves which, earlier on, led to such unfold- 

 ing, is no longer effective. The buds remain in the resting condition, which 

 under natural conditions continues until the following spring. It is possible 

 as already remarked to stop it artificially from December onwards by raising 

 the temperature (' forcing ') ; but all attempts causing premature shooting by 

 raising the temperature between July and October lead to no result. In other 

 ways it is, however, possible, not indeed to suspend, but still to shorten this 

 resting period, as when the buds are treated with ether (JOHANNSEN, 1900). 



1. 43 P. 347, 1. 32, for which is fatal, an action . . . relation to seasons. 

 read which is fatal. Further, more recently HOWARD (1906) has succeeded in 

 shortening the rest period by the action of frost and by darkening. 



In many plants, however, the rest period remains for a definite time unaf- 

 fected after the application of such stimuli, a fact all the more remarkable since 

 the same bud which cannot be made to open out now was forced to develop 

 prematurely as we saw, e.g., by removal of the foliage-leaves. Some change 

 inside the bud must first take place which results in inhibition of growth and 

 which gradually fades away. As to the nature of this change, we have no 

 certain information ; we can only suppose that its seat is in the protoplasm. 

 As an indication of such a change in the protoplasm we may perhaps regard 

 the alteration of starch into sugar and other substances which takes place in 

 autumn ; it is at least worthy of note that one cannot successfully induce buds 

 to unfold before the starch has diminished to its minimum. 



At all events, this dissolution of starch in winter, followed later by its 

 reconstruction, shows us very clearly that ' rest ' in trees is only an apparent 

 rest, and there are other evidences which confirm this. First of all, respiration 

 is maintained in relatively higher degree in winter, or makes its appearance if 

 the temperature be sufficiently high (SiMON, 1906) ; further, growth phenomena 

 may be also induced during hibernation by wounding, e. g. cork and callus 

 formation, root growth and even bud growth (Josx, 1893 ; SIMON, 1906). 

 In contrast to one-year-old buds, buds several years old may often be made to 

 open in autumn ; they do not resist artificial forcing so well as the former. 

 Under quite natural conditions also buds and especially terminal buds may 

 commence growing in winter, as ASKENASY (1897) has shown in the cherry. 

 This growth is, however, embryonic, and we do not know how it is that at one 

 time it may be easily transformed into growth in length and at another time 

 not at all. If the incapacity for development of the chief bud appears, from 

 a purely physiological point of view, obscure, its biological significance is 

 evident enough : the tree, owing to the fixity of its resting period, is unaffected 

 by small climatic changes in winter, and thus saved from premature shooting. 



Reviewing what has been said, we see that the periodicity in leaf-formation 

 in trees is conditioned in the first instance by internal factors, but that it bears 

 a certain relation to seasonal periodicity in countries where plant growth is not 

 possible the whole year through. Periodic phenomena in plants may, however, 

 be made to accord with another distribution of seasons, though not equally 

 easily or successfully in the case of all plants. The transference of the shooting 

 period to another time of the year is most readily accomplished in cases where, 

 by employing ice, development may be delayed, as is done on a large scale with 

 elder or lily of the valley. When plants from a temperate climate are trans- 

 ferred to the tropics, however, where the conditions are continually favourable 

 for growth, the plants retain their periodicity, but it no longer shows any 

 relation to the environment. 



347, 1. 39 P. 348, 1. 35, for In other tropical climates . . . very well marked. 

 read Looking now at other periodic phenomena in plants, we come first to those 



