CHAPTER XII 

 PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 



94. Normal Cycle of Development of Higher Plants. Annual 

 and Perennial Plants. Interrelations between Growth and 

 Development. — The Ufe 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 ; and very large trees, some attaining an age of several 

 hundred years, and others living several thousand years, like 

 Sequoia in California. It should be emphasized here that there 

 is a great difference between animals that 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 are then superseded by new^ ones. 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 perenniaj plants have the capacity 

 of rejuvenating annually. They therefore have no definite 

 length of hfe. Some of their parts die each year 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 indis- 

 putable symptoms of aging. It is rather the definite relationship 

 between the vegetative growth of a plant and its reproduction 

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



duration of its life. 



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