CHAPTER XIV 



SEASONAL PHENOMENA IN THE LIFE OF PLANTS 



103. The Dormant Period of Plants. Autumnal Maturation 

 of the Buds and Wood of Trees. Shedding of Leaves. — In the 



evolution of plants, the periodic alternation of the seasons 

 of the year with a periodic return of winter, which stops the 

 growth of plants and kills their tender organs, has left a deep 

 impression on the life habits of perennial plants, especially of 

 woody species. Periods of intensive growth in summer alter- 

 nate with an almost complete suppression of all vital processes 

 in winter. It is only plants native to the perpetually humid 

 tropics that are capable of vegetating uninterruptedly through- 

 out the year. It is precisely this peculiarity that has made them 

 suitable as greenhouse crops. Plants of a temperate climate 

 stop growth during winter, shed their leaves and sometimes all 

 aerial shoots, and then enter into a state of rest, or dormancy. 



One must not imagine that in this state there is a complete 

 cessation of all vital processes. Resting organs, e.g., the buds 

 of trees and the tubers and rhizomes of grassy perennials, respire 

 during complete dormancy. Transformations of reserve sub- 

 stances also occur. For instance, in the twigs and buds of trees, 

 starch gradually disappears and is replaced by sugars and 

 fats, leading to an increase of frost resistance. Toward the 

 spring, the reverse metabolic changes may take place. In 

 general, all vital activities manifest themselves during dormancy 

 with the exception of growth, which does not take place, though 

 the external conditions may be quite favorable to it. One 

 cannot make the potato tuber sprout in September and October 

 even under proper temperature and moisture conditions, while 

 in spring it will develop in a cold dry room. 



The period of dormancy very often may be separated into 

 two stages: the stage of meristematic growth and that of elonga- 

 tion. Such a rest period may occur even in annuals, especially in 



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