272 BOTANY part i 



interrupted by resting periods ; in the course of the year several periods 

 of expansion of shoots may occur. Periodic activities are also seen 

 in our native trees, the growth of the shoots of which generally com- 

 mences in the spring while the leaves are shed in autumn. In this 

 case at first sight we are inclined to recognise a direct result of the cold 

 winter season. That this is not the case is shown by further considera- 

 tion and some observations. The rise of temperature in spring is 

 doubtless the cause of the commencement of growth, but the cause of 

 the preceding cessation of growth does not lie in the lower tempera- 

 ture. The unfolding of the leaves of many trees ceases completely 

 in May or June. Further, our trees, when transferred to a tropical 

 climate, frequently exhibit a periodicity similar to the native plants 

 of the new locality. It must thus be assumed that they possess a 

 i:)eriodicity depending on internal causes, and that they become adapted 

 to the seasonal changes in countries where these occur ; with us the 

 resting period is the winter while in other countries it occurs in the dry 

 period. This does not hold for all plants, however. In our climate 

 there are some herbs, such as Senecio vulgaris, which continue to grow 

 throughout the whole year if the external conditions permit, and in 

 the tropics plants which grow continuously also occur. 



Tlie Oak, Beech, Apple, and Pear retain their resting period in the sub-tropieal 

 climate of Madeira, while under uniformly favourable conditions in the mountain 

 regions of Java the periodicity may be disturbed in particular individuals. This 

 even occurs in the several branches of the same tree, which may then bear leafy 

 and leafless boughs at the same time (Oaks, Magnolias, Fruit and Almond trees, 

 together with some endemic species). Other trees again gradually accustom them- 

 selves to the new conditions, as the Peach, for instance, which in Ceylon has become 

 an evergreen tree. The Peach is stated to produce flowers and fruit throughout 

 the entire year ; while the Cherry, like many other trees of tjie temperate zone, 

 ceases altogether to bear flowers in tropical climates. 



The resting period may be more or less shortened by a jiremature provision of 

 favourable conditions for growth ; previous exposure to frost, dryness, darkness, 

 to ether-vapour, or to a suitable rise of temperature may favour this by providing 

 preliminary steps. The awakening from the resting stage does not succeed equally 

 well at every period. The best time naturally is shortly before the normal resump- 

 tion of vital activity, but a period shortly after the resting condition has been 

 assumed is nearly as good. In the intervening period of complete rest attempts to 

 induce the plant to resume its activities are usually fruitless. These facts must 

 be considered in the forcing of plants, and just as much so the circumstance that 

 the temperature optima for various developmental processes in the plant differ ('•'). 



In the case of seeds and spores it is much more apparent than in 

 the case of winter buds that the resting period depends on internal 

 causes. These organs are cut oflf from their water supply, and ultim- 

 ately shed as a result of the vital processes of the plant. Many of 

 them, even when favoui'able conditions of growth are supplied, are 

 uiia1)le to germinate at once ; but require first to undergo a long 

 period of rest. Periodic phenomena are also frequently met with 



