MOVEMENTS OF GROWTH. 401 



1 Literature : Sachs, in Pringsheim's Jahrbiicher, Bd. 2 ; Fr. Haberlandt, in 

 Wissenschl.-praktische JJntersuchungen auf dem Gebiete des Pftanzenbaues ; and 

 Detmer, Vergleichende Physiologic des Keimungs processes der Satnen, 1880. 



163. The Annual Period. 



In many plants the general course of development is inter- 

 rupted by periods of rest to which their organs are subjected at 

 particular times. Most of our native trees and shrubs shed their 

 leaves in autumn, and the buds previously formed pass the winter 

 in a state of rest, not unfolding till the following spring. Here, 

 undoubtedly, we have to do with a phenomenon originally induced 

 by the alternation of seasons, but which, as it now presents itself, 

 by no means exhibits a direct dependence on external factors. 

 When, viz., the same external conditions constantly act in definite 

 rotation on an individual plant, or on generations of individuals, 

 the plants thereby acquire, in consequence of after-effects, specific 

 peculiarities which may even become hereditary, and these pecu- 

 liarities then frequently appear so firmly fixed that they very 

 largely determine the entire behaviour of the plant. This must 

 especially be remembered when we attempt to explain why the 

 winter buds of our trees and shrubs do not always open at once 

 when, under suitable conditions, exposed in winter to higher 

 temperature. The resting period of buds is, it is true, primarily 

 induced by the alternation of the seasons, but ultimately it has 

 become in the manner indicated a specific peculiarity of the plant, 

 which cannot be at once laid aside. 



If at the end of October twigs of Prunus avium, not too small, 

 are placed with their lower ends in water, and kept in the hot- 

 house, their buds do not burst in spite of the favourable conditions 

 presented, but gradually die ; Prunus twigs, on the other hand, 

 brought into the hothouse in the middle of December, flower after 

 about four weeks. If we do not cut the twigs till the middle of 

 January, they flower still more rapidly. Similar results are ob- 

 tained with branches of other trees, e.g. Tilia. Forsythia twigs 

 also sprout very rapidly. I placed some twigs in water at the be- 

 ginning of December, and they unfolded their leaves and flowers 

 in three weeks. 



Moreover the degree of energy with which the winter buds of 

 different kinds of plants develop when shoots cut from them are 

 placed in water and brought into a warm room varies very much. 



P.P D D 



