PHYSIOLOGICAL ASPECTS OF AGING IN PLANTS 385 



and much thornier, especially in the lower part of the tree. The old clone bore 

 two years earlier; the young clone had twice the top volume but less fruit. 

 Frost (1938) reported similar results in a comparison of trees grown from 

 budwood from young nucellar seedlings of old varieties with those developed 

 from buds obtained from ordinary orchard trees. 



Frost concluded that the regularly occurring developmental changes were 

 not genetic, were not caused by infectious diseases or by the accumulation of 

 inert material in the cells. He considered that they were temporary but per- 

 sistent modifications of the meristematic cells ; these modifications were trans- 

 mitted through vegetative propagation and perpetuated for indefinite periods 

 even in rapidly dividing cells. They were erased in the formation of zygotic 

 or nucellar embryos. He suggested that with repeated cell divisions changes 

 might have occurred in important cell proteins. These modified proteins were to 

 some degree self-propagated and were perhaps located in the plastids. 



Swingle (1932, 1933) referred to the juvenility of nucellar embryos as an 

 "extraordinary recapitulation of a stage in ontogeny already passed by the 

 generation which furnishes the nucellar buds," considered that the nucellar 

 bud developing in the embryo sac underwent rejuvenation, called the phe- 

 nomenon "neophysis," and suggested that the cause might be hormone-like 

 substances located in the embryo-sac apparatus. 



Various criteria have been used to distinguish between the juvenile and the 

 adult stages. Among these are leaf shape, production of pigment (usually 

 anthocyanin), phyllotaxy, shedding of leaves, ease of rooting of cuttings, 

 thorniness, growth habit, and flowering. The juvenile condition is marked 

 by the production of more anthocyanin, retention of leaves during the winter, 

 greater ease of rooting of cuttings, more thorniness, and lack of flowering. 

 No single kind of plant shows all these distinctions between the juvenile and 

 the adult condition, and no one character is a sure and certain indication that 

 a change from juvenile to adult has occurred. Most authorities consider that 

 the greater ability of cuttings to root is most closely associated with juvenility. 



The most striking feature of this phenomenon is that both the juvenile 

 state and the adult are transmitted by vegetative propagation. While vege- 

 tatively propagated offspring from the juvenile stage proceed to complete 

 the normal ontogeny, vegetative propagations from adult shoots continue to 

 grow as adult for indefinite periods of time, rarely showing for some kinds of 

 plants spontaneous reversion to the juvenile condition. By gametic reproduc- 

 tion the plant is returned to the juvenile stage. 



Of particular interest to the problem of the juvenile and adult stages are 

 the recent results obtained with ivy by Doorenbos and those reported by 

 Frank and Renner. Doorenbos grafted 5-cm. scions of mature branches on 

 rooted cuttings of the juvenile form. The new growth of the adult scions in 

 many instances was juvenile; the effect was more marked if leaves were left 

 on the juvenile stock. No effect was noted for the reverse grafts, namely, 



