370 SENESCENCE AND REJUVENESCENCE 



but it apparently holds for at least many other plants as 

 well. Propagation by cuttings may be continued for a large 

 number of generations and probably indefinitely in many 

 plants, and some plants are not known to reproduce sexually in 

 nature. 



Many of our cultivated plants have been bred agamically, 

 either wholly or to a large extent, for a long period of years. The 

 banana is one of the most conspicuous examples, the sugar cane 

 another, and in various species of willow and poplar and many 

 plants grown from bulbs or tubers, e.g., the potato, the agamic 

 method of propagation is the usual one. 



Mobius ('97) has brought together a large number of these cases, 

 and has c onsidered particularly those in which agamic propagation 

 for a longer or shorter time was apparently followed by the deteriora- 

 tion or the dying out of the stock. In many cases parasitic diseases 

 are responsible for this result, in other cases climatic or other ex- 

 ternal factors, and Mobius concludes that there are no grounds for 

 believing that agamic propagation necessarily results in an aging, 

 deterioration, and death of the stock. 



Mobius has also discussed the facts bearing on the question 

 whether continued agamic propagation may lead to loss of the power 

 of gametic reproduction and concludes that, in at least most cases, 

 gametic reproduction is prevented by external factors and agamic 

 reproduction takes its place. It is an undoubted fact that plants 

 which do not reproduce sexually usually show a high degree of 

 agamic reproductive capacity of one form or another. While it is 

 not possible at present to analyze most of these cases, they all fall 

 readily into line with the view that gametic reproduction is char- 

 acteristic of a certain relatively advanced stage of the life history 

 of the individual, and that the individual cannot attain this stage 

 under conditions which bring about a continued or periodic breaking 

 up, physiologically speaking, into new individuals. If conditions 

 in nature or under cultivation favor continued vegetative growth, 

 new individuations continually occur and the reconstitutional 

 changes connected with this continued agamic reproduction prevent 

 any individual from attaining the condition of maturity. Or the 

 conditions may decrease the degree of individuation of the species 



