84 PLANTS 



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forms show a decided tendency to undergo their usual series 

 of life processes and accompanying change of character, even 

 though the seasonal changes of conditions fail to occur at the 

 appropriate time. For example, the tropical plants present an ; 

 expanse of leaf surface throughout the year, although the older ; 

 leaves are continually falling, because new ones are as constantly 

 developing. The deciduous perennials are characteristic of 

 temperate latitudes, where there is alternately winter and sum- 

 mer. In the latter case there is evidently a relation between 

 the fall of the leaf and the seasons. But the leaves do not fall 

 only after they have been killed by a frost. Rather, they die, 

 and physiological connection with the plant body is cut off 

 before the time for serious frosts arrives. Otherwise not only 

 would the leaves be killed, but the plant itself might suffer 

 serious injury. 



198. For the plant to retain its leaves during the snows of 

 winter would also expose it to the danger of being overloaded 

 and crushed by sheer weight of the snow. 



199. Another example of this principle of the independence 

 of the plant of the direct conditions of its environment is found 

 in the period of rest required by seeds and other reproductive 

 bodies, such as bulbs, which normally remain quiescent during 

 the winter and resume their growth with the recurrence of the 

 warmth of spring. But the rest is taken whether or not the win- 

 ter conditions supervene. 



200. Such adaptations of the plant are not responses to 

 changing external conditions. The plant undergoes changes 

 which anticipate the corresponding changes in the conditions. 

 Such adaptations must be regarded as habitual responses. 



20 1. Some other adaptations of the plants to rigorous 

 climates are, for example, such modifications of the leafy shoot 

 as the rosette and the creeper. By hugging the earth plants 

 of this type avoid the great exposure to cold which a freer 

 method of growth would entail. 



