386 BOBBINS 



juvenile scions on adult stock; the new growth of the juvenile scions was 

 not adult. This suggests that something is transmitted from the juvenile to 

 the adult which induces the meristem of the adult to develop juvenile parts. 

 Pursuing this idea Frank and Renner have reported that when rooted cuttings 

 of the juvenile and adult stages of ivy were grown together in the same bottle 

 containing a mineral nutrient solution, the adult produced juvenile growth. 

 If this is correct, it suggests that the juvenile form of ivy produces substances 

 which are excreted, are absorbed by the adult, and induce it to return to a 

 juvenile condition. Frank and Renner found also that treatment of the adult 

 cuttings with cold or X rays caused rejuvenation. 



Beissner and others believed that juvenile meristems could be stabilized. 

 They reported that cuttings of Thuja, Chamaecy parts, and Juniperus taken 

 from juvenile shoots, especially from those in the axes of the cotyledons, grew 

 into plants with juvenile foliage which, by continued vegetative propagation, 

 became "fixed" juvenile forms. However, Woycicki has recently reported a 

 failure to "fix" juvenile forms of Thuja by Beissner's procedure. It is probable 

 that the juvenile forms of Thuja, Chamaecyparis, and Juniperus are mutants 

 rather than stabilized or fixed forms in Beissner's sense. 



I believe that mutations and chimeras can be eliminated as explanations for 

 the adult condition since it occurs regularly in the normal ontogeny of the 

 plant and is not transmitted through the seed. I think we can agree that the 

 origin of the juvenile and adult stages must be looked for in the activity of 

 the apical meristem which changes or is changed, at least functionally, with 

 the age of the plant as measured from the seed in such a way as to produce 

 the adult physiology and morphology. 



What causes the changes in the meristem? The explanations which have 

 been offered fall into two more or less clearly marked categories. One assumes 

 that the differentiation of a meristem is determined by the materials which 

 come to it from the balance of the plant. The meristem itself does not change 

 during the life of the plant — it does not age. The morphology and physiology 

 characteristic of the juvenile stage as contrasted with those of the adult 

 result from differences in the quantity and quality of the materials which 

 reach the meristem as the plant ages. Mineral salts, carbohydrates, nitrog- 

 enous substances, or growth substances and the balance between them have 

 been suggested at one time or another as the factors which detemine whether 

 a meristem produces juvenile or adult characters. 



Another hypothesis assumes that meristems themselves age, that they 

 change during the life of the plant. There are young meristems and adult 

 or old meristems. While the activity of each is affected by the materials which 

 come to it from the balance of the plant, the response to any given substances 

 differs because the juvenile meristem is basically different in character from 

 the adult meristem. Evidence to support each of these viewpoints can be 

 presented. I am not prepared to accept the extreme view of Molisch, which is 



