Regeneration 245 



ability to restore lost parts. The axial organs-stem and root-are the ones 

 most commonly employed in the practices of propagation and have been 

 most thoroughly studied. 



Stem Cuttings. In stem cuttings of dicotyledons buds develop most fre- 

 quently at the apical end and roots from the basal one (p. 119) but this 

 polarity varies considerably. The buds may be the usual axillary ones, 

 many of which would not normally develop, or they may be accessory 

 buds. If these are absent, dormant primordia may grow. Carlson (1950) 

 has described the origin and distribution of dormant root initials on 

 willow shoots. Primordia may also develop anew, from callus or from 

 the normal tissues of the stem. Adventitious roots in young stems usually 

 come from the pericycle but in older ones they may have a deeper origin 

 in the vascular cambium (Plett, 1921). Mahlstede and Watson (1952) 

 found that adventitious roots in blueberry originate in cambium or phloem 

 and push out through vascular tissue, cortex, and epidermis. Priestley 

 ( 1926a ) stated a general rule that, of the two lateral meristematic regions 

 of the axis, the phellogen is more likely to produce buds and the vascular 

 cambium to produce roots. Morphogenetic problems here involved con- 

 cern the causes of the differentiation of dormant or "reserve" primordia 

 in particular places and especially the factors that first keep them dormant 

 and then stimulate their development in regeneration. 



Bud formation is frequent on hypocotyls and has been studied particu- 

 larly bv Rauh ( 1937 ) . In a few species these buds normally develop into 

 shoots. In other cases they may be present but do not develop and in still 

 others they may be induced only by the stimulus of regeneration, after 

 the decapitation of the hypocotyl. In Linum usitatissimum, the origin of 

 these buds has been traced, in decapitated hypocotyls, to single cells of 

 the epidermis ( Crooks, 1933; Link and Eggers, 1946a ) in which divisions 

 begin to appear. A group of cells is thus produced which develops into a 

 bud initial and finally into a shoot. Several buds may begin to grow, only 

 one of which becomes dominant. In undecapitated hypocotyls a few epi- 

 dermal cells may divide but they rarely produce buds. Bud development 

 is induced more readily in young hypocotyls than in older ones. After a 

 bud begins to grow, vascular strands differentiate which connect it with 

 the main vascular cylinder (Fig. 9-6). Van Tieghem (1887) described 

 similar bud development in the hypocotyl of Linaria, as did Bain ( 1940 ) 

 in cranberry. Such hypocotyls offer a good opportunity for the study of 

 cellular totipotency and the redifferentiation of vascular tissue. 



So-called "adventitious leaves" (really reduced shoots, Rauh believes) 

 are produced abundantly on the decapitated hypocotyl, or seedling tuber, 

 of Cyclamen (Boodle, 1920; Rauh, 1937) and develop there from sub- 

 epidermal cells. There are transitions here from simple leaves to fully 

 developed buds. The great number of these buds normally produced sug- 



