304 BOTANY PART I 



unfertilised egg-cell, growth is suspended, but all vital activities are 

 not suppressed. 



At first sight it appears as if the resting condition during an 

 unfavourable season was caused thus. As a matter of fact, however, 

 periodic cessations of growth are found in many tropical trees ; while 

 temperature arid water-supply continue favourable, the leaf formation 

 does not proceed continuously, but is interrupted by resting periods, 

 so that there are several periods of active growth in the course of the 

 year. In our native plants also the entry upon a resting period is 

 in no way determined by the low temperature. 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. These 

 phenomena are not interpreted in the same way by all investigators. 

 On the one hand it is assumed that every periodicity in the growth 

 of a plant is determined by a periodicity in the environment which 

 need not be in the supply of moisture and warmth, but may concern, 

 for example, the absorption of nutrient salts. On the other hand it 

 may be assumed that plants possess a periodicity depending on 

 internal causes, and that they become adapted to the seasonal 

 changes in countries where such 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. 



The Oak, Beech, Apple, and Pear retain their resting period in the sub-tropical 

 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 gradually accustom them- 

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

 become nearly evergreen in the first generation and completely so in the second. 

 It does not appear to be known how the periodicity of the unfolding of its buds 

 has been affected. 



Commencement of Development. The termination of the resting 

 condition and the resumption of growth often depends only on the 

 establishment of general conditions for growth. In other cases the 

 resting condition is more fixed but may be sometimes shortened by 

 particular stimuli. 



The germination of seeds takes place as a rule when the general 

 conditions for growth are present, especially the necessary temperature, 

 supply of oxygen, and water ; but examples are not wanting in which 

 special stimuli are requisite. Such special stimuli, usually provided 



