PHYSIOLOGICAL ASPECTS OF AGING IN PLANTS 383 



more or less well-defined juvenile and adult stages as a part of their normal 

 ontogeny. This aspect of the aging process may be illustrated by the English 

 ivy, Hedera helix. 



From seed the ivy grows as a vine with five-lobed leaves in a 2/2 rank- 

 ing. It is plagiotropic and crawls along the ground, rooting at intervals. If it 

 reaches a wall or tree trunk, it grows up the support, clinging to it with aerial 

 roots. Sooner or later the upper portion of the vine changes in growth habit. 

 Oval leaves with entire margins are produced in a 2/5, 2/6, or 2/7 ranking, 

 the growth is orthotropic, aerial roots are no longer produced, and this por- 

 tion of the plant blooms and fruits. 



Cuttings of the juvenile form of Hedera root readily and grow as vines, 

 eventually producing under suitable conditions the adult stage, usually at the 

 top of the plant. Cuttings of the fruiting portion of the plant root less readily, 

 but if rooted, grow as shrubs or small trees and retain the leaf characters and 

 habit of growth of the adult form. The arborescent form flowers and fruits; 

 its seeds yield the juvenile type of growth. All species of Hedera have juvenile 

 and adult stages and yield arborescent forms from the fruiting adult shoots by 

 vegetative propagation. The juvenile, or vine, stage of Hedera differs so much 

 in appearance from the arborescent form that the two forms might easily be 

 mistaken for different species instead of stages of the same plant. 



The arborescent Hedera has a considerable degree of stability. I have seen 

 at the Huntington Botanical Garden two arborescent specimens of a variegated 

 form of Hedera which are nearly 50 years old; these have never shown any 

 reversion to the juvenile or vine type of growth. 



There are five specimens of Hedera helix v. arborescens at Vilmorin- 

 Andrieux, Verrieres, France, three of which are now more than fifty years 

 old. They were started as cuttings of fertile branches and are growing on 

 their own roots. One which I have seen is 8 or 9 feet in height and 15 feet in 

 diameter, a large and vigorous shrub. These arborescent forms are very fruit- 

 ful, and I am told have never shown any tendency to produce the juvenile 

 form, even from the base. I have also examined a number of old arborescent 

 forms of ivy at the Royal Botanic Gardens at Kew and at Edinburgh, as well 

 as elsewhere. 



Instances of spontaneous reversion of the arborescent form to the juvenile 

 condition have been reported, and I have examined a few examples which 

 suggest that reversion may occur, but such reversion is the exception and not 

 the rule. 



The creeping fig, Ficus repens, also has a juvenile and adult type of growth, 

 and cuttings from the fruiting-stage grow as small shrubs. Some of the aroids 

 respond in a similar manner; the juvenile form has small, short-petioled leaves 

 and grows as a vine clinging closely to its support; with age large pinnatifid 

 leaves are formed; rooted cuttings of the latter develop a more or less erect 

 plant bearing little resemblance to the juvenile state. 



