LEAVES IN SEASONS 155 



number, and man}? plants exhibit flowers and 

 fruit as well during the 3^ear. Such favorable 

 conditions for growth are found only in certain 

 circumscribed areas, as a large proportion of 

 the earth's surface within the tropics has a 

 supply of moisture during one part of the 

 year wholly insufficient for the needs of grow- 

 ing vegetation, and on the approach of this 

 dry season the plants in rich regions discard 

 all or a greater part of their leaf surfaces. 

 This shedding of leaves is not attended by 

 many of the more prominent features of au- 

 tumnal leaves, how^ever. 



A portion of the year in the temperate zone 

 is characterized by a protracted low temper- 

 ature which is unfavorable to even the simpler 

 forms of activity of protoplasm, and renders 

 the presence of a great expanse of leaf surface 

 not only useless but dangerous to plants 

 growing in those zones, and provision is made 

 for the economical disposition of the foliage. 



Plants growing in regions with this alter- 

 nation of seasons have modified the primitive 

 rhythm of protoplasm in such manner that Yearly rhythm 

 they manifest annual periods of rest and ac- 

 tivity. While this yearly period has been ac- 

 quired in somewhat recent time perhaps, j^et 

 it is most firmly fixed in the constitution of 

 the plant as may be demonstrated if an at- 

 tempt is made to grow a deciduous tree or 



