Regeneration 237 



adventitious roots growing out from it. The early papers of Prantl (1874) 

 and Simon ( 1904 ) on angiosperms and of Stingl ( 1905 ) on gymnosperms 

 present the basic facts, which have been reviewed by Nemec ( 1905 ) . 



More recently Torrey ( 1957Z? ) has studied the regeneration of decapi- 

 tated roots grown in culture media and as affected by auxin. If an abun- 

 dance of auxin was present, the vascular cylinder of the new roots was 

 hexarch instead of the normal triarch. Such a root reverted to triarch 

 again if returned to the usual medium. Torrey interprets these changes 

 as due to the direct effect of auxin on the size of the meristematic tip, 

 the structure of which evidently is not determined by the mature tissue 

 farther back. 



If the young root is split lengthwise, scar tissue forms on the inner 

 portion of the cut surface but each tip will become reorganized into a 

 new and complete meristem and will finally reconstitute a normal root 

 (Lopriore, 1892). This argues against the idea that there is a single apical 

 cell in the root. Ball ( 1956 ) split the hypocotyl tip of a Ginkgo embryo 

 and found that the effect of this was evident for some distance upward 

 in the epicotyl in the differentiation there of a divided vascular cylinder. 



In the shoot meristem the situation is complicated by the presence of 

 leaf primordia. Only the terminal dome, about 80 /x back of the actual tip, 

 will be regenerated if it is removed. The earlier workers believed that a 

 new apex was formed here, as in the root, by direct growth from the cut 

 surface, and more recently Mirskaja ( 1929 ) has reported that this occurs 

 in Tradescantia. Most observers, however, have found that scar tissue 

 forms over the wound and that one or more new meristems arise at the 

 edges of this. 



Several plants have marked powers of meristematic reconstitution. In 

 the much reduced aquatic Podostemon ceratophyllum, if the tip of the 

 shoot is cut off a new one arises from a group of cells just back of the 

 cut surface in or around a vascular bundle. A decapitated root is recon- 

 stituted in much the same way (Hammond, 1936). In Zamio, a new shoot 

 will often grow out directly from the stump of an old one, usuallv from 

 the region of the central cylinder (Coulter and Chrysler, 1904). Such 

 simple and direct regeneration in plants is rare. 



Karzel ( 1924 ) and others split growing shoot tips and found that each 

 half regenerated more or less completely, depending on the species. 

 Pilkington (1929) split simply the terminal meristem itself and observed 

 the same result. From the tissue of dodder which remains within the host 

 plant after the external portion of the parasite has been experimentally 

 removed, Truscott (1958) observed the regeneration of a shoot meristem 

 which pushed out through the surface and developed into a normal 

 dodder shoot. 



Much experimental work on the regeneration of the shoot apex has 



