384 ROBBINS 



Juvenile and adult stages have been distinguished for a number of trees 

 including apples, apricots, pecans, beech, oaks, Hevea, and citrus, as well as 

 others. Generally speaking, a seed-grown tree — if it has a distinguishable 

 juvenile stage — is juvenile for some years, but eventually the new peripheral 

 growth is adult, with the result that the tree is then made up of a cone in 

 the juvenile condition enclosed by a zone in the adult condition. The juvenile 

 portion of some trees retains its dead leaves in winter, but the adult parts 

 drop theirs. For such trees, beech and oak, for example, one can frequently 

 see specimens grown in the open which in late autumn or early winter have 

 held their leaves in the interior juvenile portion of the tree but have lost 

 them from the peripheral twigs. 



Although seedlings of most types of citrus are thorny, this tendency gradu- 

 ally declines in the peripheral portion of the plant with continued extension 

 of growth. Plants grown from buds taken from the base and inner portion 

 of such a seed-grown tree are thorny, while plants which develop from buds 

 taken from the peripheral portion are likely to be nearly thornless. In other 

 words, the budwood transmits the characters of the portion of the plant from 

 which it is taken and the tree growing from such buds continues to completion 

 the succession of changes which characterize the normal ontogeny of the 

 plant. Budwood from commercial varieties which have been propagated vege- 

 tatively for considerable periods yields trees which are virtually thornless. 



The situation in citrus, so far as juvenile and adult stages are concerned, 

 appears to be essentially similar to that of a number of other kinds of woody 

 plants. There are juvenile and adult stages which are transmitted through 

 vegetative propagation. Trees grown from seed are thornier, more upright, 

 and symmetrical in habit; are of greater vigor and fruit later and less pro- 

 fusely than trees which have been propagated as clones. 



Citrus differs, however, in one notable respect from a number of other 

 woody plants. Citrus seeds frequently contain, in addition to the zygotic em- 

 bryo, others which are derived from the nucellus. Strasburger (1878) found 

 that the supernumerary embryos arose from the nucellus and traced the chain 

 of events in which nucellar buds penetrated the embryo sac and developed 

 into embryos. The nucellus might be considered to be "adult" tissue since it 

 is located in a part of the plant where budwood would normally yield plants 

 with adult characters. The nucellar embryo could be regarded as a type of 

 vegetative reproduction analogous to a bud, cutting, or scion. We might 

 expect, therefore, that plants from nucellar embryos would skip the juvenile 

 stage and show merely adult characters. However, they have the same juvenile 

 characters as those which develop from zygotic embryos. 



Hodgson and Cameron (1938) compared two pairs of paper rind (St. 

 Michael) orange trees, one (young clone) obtained by budding from a nucel- 

 lar seedling, the other (old clone) by budwood from the tree which had pro- 

 duced the nucellar seedling. After nine years the young clone was more upright 



