AGE CYCLE IN PLANTS AND LOWER ANIMALS 241 



dedifferentiation and rejuvenescence occur. In such cases, however, 

 the slight degree of individuation determines that reproduction shall 

 be almost continuous during vegetative existence; consequently 

 there is but little possibility of differentiation and senescence. 

 Under such conditions the plant as a whole may remain physiologi- 

 cally young for an indefinite period, simply because new individ- 

 uations from parts of pre-existing individuals occur very frequently. 

 Even in those algae and fungi which consist of a single multinucleate 

 cell, the localization and development of a new branch unquestion- 

 ably brings about some degree of reconstitutional change, for it 

 involves a local increase in the rate of growth. It is the continued 

 reorganization which keeps such plants from growing old under 

 such conditions. 



In some of these lower plants certain parts, usually those which 

 bear the spores, become more highly individuated than the rest of 

 the plant and consequently undergo a greater degree of differen- 

 tiation and usually undergo a more or less continuous senescence 

 and die of old age, while the less highly individuated and so less 

 differentiated portions may continue to live and remain young 

 indefinitely. Many of the fungi, and particularly the mushrooms 

 and related forms, are cases in point. The mushroom itself is the 

 more highly individuated spore-bearing portion of a plant whose 

 vegetative form consists of thread-like branching hyphae, which 

 are merely strings of like cells attached end to end. The mush- 

 room passes through a definite course of development and differ- 

 entiation, attains maturity, ceases to grow, and finally dies, 

 but the vegetative hyphae may continue to grow indefinitely 

 without any perceptible progressive morphological or physiological 

 change. 



The course of plant evolution from the lower to the higher forms 

 is characterized by an increasing differentiation in the vegetative 

 plant body and a more and more definite limitation of vegetative 

 reproduction, at least under the usual conditions, to certain parts 

 or tissues of the plant which remain undifferentiated and physio- 

 logically young for a long time or indefinitely, while the other parts 

 undergo differentiation, senescence, and, it may be, death. In 

 the mosses and ferns the regions which retain their youth and 



