124 The Phenomena of Morphogenesis 



Various investigators (Klebs, 1903; Kiister, 1904; Freund, 1910; and 

 Ursprung, 1912) found that local differences in water or oxygen may 

 affect root production and thus obscure the inherent polar tendency. 

 Only Plett (1921), who studied phenomena of internode polarity in 

 410 species, has attempted to explain the variability in distribution of 

 roots and shoots on the basis of the anatomy of the plant from which the 

 cutting was taken. He found that shoots from axillary buds regenerate in 

 a polar fashion, as do adventitious roots that arise endogenously. Ad- 

 ventitious buds growing from callus or superficial regions of the cortex, 

 however, are generally distributed rather irregularly, a fact which sug- 

 gests that the inner layers of the stem have stronger polar tendencies 

 than do cortex and callus tissues. 



Root Cuttings. Cuttings of roots behave in polar fashion. Dandelion, 

 chicory, and sea kale have been studied most frequently in this regard. 

 Shoots are commonly regenerated at the basal or proximal pole (the end 

 next the shoot) and roots from the apical (distal) pole. This polarity 

 is maintained even when the root cutting is grown in an inverted posi- 

 tion (Fig. 6-4). Wiesner (1892c) made the observation, often confirmed 

 since, that, in relatively short pieces of root, shoots regenerate at both 

 ends. This was also seen by Neilson-Jones (1925) and, in stem cuttings, 

 by Fischnich (1939). If the growing roots were continually trimmed off 

 from the apical end, shoots finally appeared there. Czaja ( 1935) produced 

 roots at both ends by trimming off tissue from the basal end. Centrifuga- 

 tion toward the shoot pole results in bud formation at the root pole, as 

 does enclosing the base in sealing wax (Goebel, 1908). These results 

 are now interpreted as due to the effect of auxin (p. 392), which tends to 

 move toward the root apex. A high concentration of it tends to produce 

 roots and a low one, shoots. This has been shown clearly by Warmke 

 and Warmke (1950). Callus develops more vigorously at the proximal 

 pole. As early as 1847 Trecul reported that, in root cuttings of Madura, 

 buds and roots showed polar distribution and were formed endogenously 

 but that in Ailanthus, where the buds arose in the cortex, polarity was 

 much less evident. This agrees with Plett's findings in stem cuttings and 

 emphasizes the more intense polar behavior of the inner tissues. 



Leaf Cuttings. Leaves when treated as cuttings behave quite dif- 

 ferently from stems and roots and show a somewhat different type of polar 

 behavior, evidently related to the fact that they are organs of determinate 

 growth. In most cases, regeneration of both roots and shoots occurs at the 

 leaf base near the cut end of the petiole. Hagemann ( 1931 ) performed 

 inversion experiments on various species. In certain cases he found that 

 wound stimulus or water affects regeneration. In Achimenes, shoots were 

 thus obtained from the apical cut surface and roots from the base under 

 certain conditions, but Hagemann concluded that, in general, polarity as 



