248 The Phenomena of Morphogenesis 



from roots and one which has been rather widely discussed is whether the 

 early primordia of new organs are "indifferent" in nature and may pro- 

 duce either buds or roots, or whether they are determined from the first 

 to form one or the other. A bud is a young shoot and has the rudiments of 

 leaves, which the root primordium does not have, and it should therefore 

 presumably be easy to tell one from the other. This seems often to be 

 difficult, however, in the very early stages, and many instances where 

 root primordia have been reported to change into shoots, and vice versa, 

 may be due to erroneous observation. Dore (1955) has studied the origin 

 of young primordia in the regeneration of horseradish roots and finds that 

 these are produced as organized meristems in close association with the 

 scars of lateral roots and that they originate in the phellogen of the main 

 root. He is certain that at the beginning they are capable of developing 

 into either roots or shoots. That this is so is suggested by the fact that the 

 ratio of buds to roots, where they can be definitely distinguished, is not 

 constant but varies with conditions, as though neutral primordia were 

 being tipped in one direction or the other. If this should finally prove to 

 be the case and the existence of truly indifferent primordia be established, 

 useful material would be provided in which to examine the very early 

 stages in the differentiation of these two structures which soon become so 

 dissimilar. This reminds one of the case in Selaginella reported by Wil- 

 liams (1937). Here an "angle meristem" near the tip will normally pro- 

 duce a rhizophore, a structure somewhat intermediate between root and 

 shoot and which finally grows downward and forms typical roots. If the 

 main axis of the shoot is decapitated, however, the young primordium 

 which would have produced a rhizophore will now grow upward into a 

 typical shoot. Mention has been made earlier (p. 71 ) of cases where a 

 young leaf primordium, if isolated by deep cuts from the meristem tip, 

 will grow into a bud-like structure. 



Cuttings from Other Parts. A variety of phenomena of regeneration has 

 been described in typical leaves and in cotyledons, scales, and carpels, as 

 well as in inflorescences, flowers, and fruits. In contrast to the axial portion 

 of the plant, leaves are organs of determinate growth, and the restoration 

 of lost parts by them is thus somewhat different from the process in 

 axial structures. It is especially common in succulent leaves. 



The restoration of roots or shoots on leaves detached from the plant 

 and with petiole placed in soil has often been observed ( Stingl, 1908; and 

 Yarwood, 1946). In such cases, after adventive roots and buds have been 

 formed on lamina or petiole, various anatomical changes may be observed, 

 especially a marked increase in the vascular tissue of the petiole. Further- 

 more, instead of being disposed in an arc, as in normal petioles, this tissue 

 often enlarges to form a complete vascular ring. The petiole thus becomes 

 structurally as well as functionally a stem. Winkler ( 1907o) reviewed cases 



