Regeneration 247 



acids in root formation has made "root hormones" of importance in horti- 

 culture. 



Bud formation, also, is influenced by physiological conditions. Miller 

 and Skoog ( 1953 ) report that tobacco-stem segments, in sterile culture, 

 form buds much more readily if adenine is present and that indoleacetic 

 acid reduces their development. Both results, presumably, are due to 

 effects on nucleic acid metabolism. Ruge (1952) and a number of earlier 

 workers observed that a functioning chlorophyll apparatus is necessary 

 for the successful rooting of most cuttings. Shoots with variegated leaves 

 or in the dark root poorly. Whether this is owing to the production by 

 the leaves of food or of a growth substance is not clear, but van Overbeek, 

 Gordon, and Gregory ( 1946fo ) believe that the main function of leaves in 

 the rooting of cuttings is simply to supply nutrition. 



Hereditary tendencies may also influence the character of regeneration, 

 as in the conversion of axillary buds into tubers in the potato (Isbell, 



1931). 



Root Cuttings. Under natural conditions shoots are produced by roots 

 rather infrequently and chiefly in woody plants. In many cases roots may 

 be used as cuttings, however. Here the restoration of lost structures by 

 the production of adventitious roots or shoots occurs much as in the stem, 

 shoots tending to be restored at the basal (proximal) end and roots at 

 the apical ( distal ) one. Most growth is from callus. Roots show a greater 

 tendency to form adventitious buds than do stems. Naylor ( 1941 ) finds 

 that both structures arise from meristematic tissue produced by paren- 

 chyma cells in the younger phloem and not from the cambium. The polar 

 development of regenerating structures on fleshy roots has been studied 

 by various workers (p. 124). 



Way ( 1954 ) investigated regeneration on apple roots of different sizes. 

 In some varieties the larger ones ( 8 to 12 mm. in diameter ) produced only 

 shoots and the smaller (3 mm.) only roots. When both were formed, the 

 zone of shoot production ( at the proximal end ) extended farther distally 

 in the wider roots, and that of root production (at the distal end) ex- 

 tended farther proximally in the narrower ones. Way interprets these dif- 

 ferences as due to auxin gradients, with different concentrations in large 

 and small roots. 



Buds on roots are usually endogenous in origin. In Bryophylhim they 

 arise from the subepidermal layer (Ossenbeck, 1927), and Rauh finds 

 them originating at the scars of the delicate branch roots. In Aristolochia 

 and the Podostemaceae they grow from the cortex. Carlson ( 1938 ) reports 

 that in the orchid Pogonia an adventitious shoot arises by enlargement 

 and division of the surface and cortical cells at the tips of lateral roots. 

 This forms a swelling in which a bud develops. 



An important morphogenetic question concerned with regeneration 



