PERIODICITY IN DEVELOPMENT. I 



345 



mining factors which lead to this behaviour in individual branches are unknown, 

 and the conclusion that it is due to a periodicity in internal causes can scarcely 

 be avoided. 



There are many observations, however, which scarcely support the view that 

 internal factors play an essential part in this periodicity, they rather go to prove 

 that the periodicity in the plant still stands in a definite relation to the change 

 in the seasons of the year. If, as is clear from what has been said, this is often 

 incorrect as regards the commencement of the resting period, it is so for the 

 close of this period, for the opening of the buds takes place as a general rule in 

 spring, when the temperature is rising. Previous to the institution of scientific 

 experiment on the subject gardening experience had shown that the close of the 

 resting period, in these latitudes at all events, was dependent in many plants only 

 on such a rise in temperature, and that it is possible artificially to induce leaves 

 and flowers to appear in the middle of winter if what may be termed an ' artifi- 

 cial premature spring ' be provided. This precocious development, however, has 

 limits ; in the case of the elder it is possible to bring about an opening of the 

 principal buds and subsequently of the flowers by raising the temperature shortly 

 before the beginning of November ; in the summer months preceding, these buds 

 cannot be made to develop although the organs concerned have been laid down 

 in the bud. On the other hand, the development of these buds (but without 

 flowers) may be induced, as already noted, in early summer, immediately after 

 the first shoot has formed, by removal of the foliage leaves. Between these two 

 dates there is a period of rest, i.e. from July to October, when raising the tempera- 

 ture has no effect. This forms the special period of rest which depends on 

 internal factors only but which may be easily lengthened by external factors, 

 but is with difficulty shortened. According to Askenasy's (1877) researches 

 a complete rest, where growth is at an absolute standstill, does not take place in 

 the buds. In the flower-buds of the cherry a continuous but feeble embryonic 

 growth goes on from summer till the next spring. Syringa also behaves 

 doubtless in a similar way. We do not know why this embryonal growth at 

 definite times, viz. from July to October, is not followed by growth in length. 

 Certain observations which we have yet to speak of will provide us with 

 a starting-point for further research. 



Johannsen's (1900) ingenious method of shortening the resting period by 

 etherization is especially worthy of notice. Plants which are near the beginning 

 or completion of their resting period may be made to send out shoots by exposure 

 for two periods of twenty-four hours each to ether vapour. It is possible that the 

 action of the ether is that of an anaesthetic, i. e. that certain functions which 

 tend to retard growth in the plant are inhibited by the ether. It is, however, 

 more likely that we have not to deal with a specific action of the ether itself but 

 that other poisons may produce a similar effect. What we have to think of is 

 the stimulatory action which we have seen poisons to possess when below that 

 degree of concentration which is fatal, an action which affects the incitation of 

 metabolic activity and increases respiration more especially. We have every 

 right to assume that active metabolic processes go on in the plant during the 

 resting period. A. Fischer's (1890) researches, based on the valuable observa- 

 tions of Russow (1882), have added much to our knowledge of these processes. 

 According to them the reserve substances in trees undergo very extensive 

 changes in the course of the winter. In autumn the parenchyma is filled with 

 starch ; in October this starch begins to undergo dissolution, fats and. in part, 

 also glucoses taking its place. In some trees this transformation takes place in 

 the rind only, the starch in the wood remaining unaltered, whilst in others {Tilia, 

 Betula, Pinits) all the starch undergoes transformation, and hence no starch can 

 be found in these trees from November to the end of February. Starch is re- 

 formed jn March and shortly before the development of the young shoots it is 



