AGE CYCLE IN PLANTS AND LOWER ANIMALS 239 



process as truly as any other more specialized kinds of reproduction. 

 For convenience it may be distinguished from these as vegetative 

 reproduction. 



Agamic reproduction is, then, a characteristic feature of the 

 vegetative life of plants. The degree of individuation is so low that 

 growth leads very readily to physiological isolation of parts and 

 new individuation, and, without doubt, also, conditions which 

 decrease the metabolic activity of the dominant region accompHsh 

 the same result. Most of what is commonly called growth in plants 

 involves the formation of new phytoids. The new buds of each 

 season or active period in perennial plants are new individuals. 



THE VEGETATIVE LIFE OF PLANTS IN RELATION TO SENESCENCE 



It is important at the outset to distinguish clearly between the 

 occurrence of senescence in the plant as a whole and its occurrence 

 in single phytoids or parts. The vegetative propagation of plants 

 from cuttings, which in the case of some species — such, for example, 

 as the banana — has continued for hundreds of years, and the 

 capacity of the lower plants for indefinite vegetative growth under 

 proper nutritive conditions demonstrate clearly enough that the 

 hfe of the plant or some part of it may continue indefinitely without 

 any indication of aging. On the other hand, in many plants the 

 length of Hfe under natural conditions is more or less definitely 

 limited, though the hfe period may range from a few hours to 

 centuries in different forms. In the higher plants, particularly 

 in the woody forms, certain of the cells cease sooner or later to 

 divide, undergo specialization, show all signs of aging, and sooner 

 or later die, while others apparently remain young indefinitely. 

 Must we then conclude that some plants and not others undergo 

 senescence ? In all except the earHer stages of the life cycle the 

 old differentiated or dead cells usually constitute by far the larger 

 proportion of the plant mass, the young cells in which growth and 

 division occur being often but a minute fraction of the whole. Are 

 we then to conclude that some plants and not others, and some parts 

 of a plant and not others, undergo senescence and die of old age ? 



As regards metabohc condition, it is well known that plants 

 and plant organs in general show a higher rate of oxidation in 



