CHAPTER XII 



REPRODUCTION 



99. The Normal Life Cycle of Higher Plants. Annual and 

 Perennial Plants.— The life cycle of a higher plant, as well as 

 of any organism, begins with the primary division of the fertilized 

 egg cell and ends with the death of the individual. The early 

 part of the life cycle is characterized by growth and the development 

 of organs. This is succeeded by the stage of reproduction, which is 

 followed by senescence and death. 



The length of life of plants varies within broad limits. If only 

 the higher plants are considered, there may be found ephemerals, 

 like Stellaria media, which complete their development within a few 

 weeks; very large trees, attaining an age of several hundred years, 

 as oaks and linden; and those living several thousand years, as 

 Sequoia in California. It should be emphasized here that there 

 is a great difference between animals which have attained senility 

 and plants in the same state. In an animal organism, almost all 

 its tissues and organs are of the same age as the organism itself, 

 while in a thousand-year-old tree all its active parts, the leaves, 

 buds, tips of shoots, and roots, serve only for a short time and are 

 then superseded by new ones. Thus, for instance, the leaves of a 

 century-old oak tree are only a few months old. Only the inner- 

 most layers of the trunk of the tree, which usually are dead, may 

 have originated in the distant past. 



As a rule, trees and other perennial plants have the capacity 

 of rejuvenating annually. They, therefore, have no definite length 

 of life. Some of their parts die annually and are again restored. 

 The cells of the embryonic regions of a thousand-year-old tree 

 are as young as those of a year-old seedling. At any rate, it has 

 not yet been possible to discover in many the indisputable symp- 

 toms of aging. It is, rather, the definite relationship between the 

 vegetative growth of a plant and its reproduction, therefore, which 

 is characteristic of its life cycle, and not so much the total duration 

 of its life. 



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