Flowering in Woody Plants • 125 



of carbohydrates is by no means unequivocally supported by the 

 evidence at present, but it is attractive in view of Wetmore's (1953) 

 observations, discussed in Chapter Five, that juvenility and maturity 

 in fern leaf forms, and hence in the apex producing them, are 

 clearly correlated to sucrose supply. On the other hand, more 

 specific mechanisms of a hormonal nature may be involved in the 

 flowering of trees. 



In view of the work with herbaceous plants leading to the 

 florigen hypothesis, it is surprising how few experiments have been 

 published on the flowering responses of young scions after grafting 

 to mature, flowering plants. Sax (1958a) indicates that this tech- 

 nique is common among tree breeders, but that there is no conclu- 

 sive evidence for its effectiveness. Furr et al. (1947) found it com- 

 pletely ineffective in Citrus. In this connection, results of the 

 opposite kind of graft are also of interest. Freely flowering branches 

 from mature trees have been grafted on young stocks in order to 

 facilitate seed collection. Although Huber (1952) reports this tech- 

 nique as successful in poplar (Populas), there are cases in which 

 mature scions on young stocks revert to a nonflowering condition 

 after several years (see Fraser, 1958). Whether this reflects an insuffi- 

 cient supply of flower-promoting factors (florigen, carbohydrates) 

 from stock to scion, or the movement of inhibitors, or some other 

 relationship, is not known. 



The entire problem of juvenility is obviously closely related 

 to the subject matter of this chapter. It is particularly relevant 

 with regard to woody plants, but also probably important in herbs. 

 This problem has attracted relatively little attention in recent 

 years, but the interested reader should consult Sinnott (1960) for 

 a consideration of the literature. One striking if somewhat atypical 

 example, related to flowering, is provided by ivy (Hedera). The 

 young plant is a vine, with lobed leaves and aerial roots. After 

 10 or 12 years it produces branches that grow upward, bearing 

 entire leaves and no aerial roots. Only these branches are capable 

 of flowering. If they are cut off and rooted they grow into erect 

 shrubs that may become very large and rarely if ever revert to the 

 juvenile vine condition, although shoots produced from the base of 

 old shrub (or arborescent) forms may be juvenile— a phenomenon 

 observed also in apple and other trees with distinct juvenile forms 

 (see Sax, 1958a). Recent work by Robbins (1957, 1960) has shown 



