244 The Phenomena of Morphogenesis 



near the meristem. Regenerative tissues of this sort are particularly prom- 

 ising material for a study of cytological differentiation. 



These are examples of the relatively simple reconstitution of a tissue 

 pattern. A much more involved one is that described by Vochting ( 1908 ) 

 in his classic studies of regeneration in kohlrabi. If a young and growing 

 tuber of this plant was cut transversely at about half way from the tip to 

 base, care being taken not to injure the leaves on the basal portion, the 

 cut surface of this portion soon began to swell and by the end of the sea- 

 son had developed a convex, rounded structure which in some cases 

 restored the general form of the normal tuber except that no leaves de- 

 veloped on its surface. Internally, however, the complex system of bundle 

 connections in the reconstituted half was hardly to be distinguished from 

 that of the original portion. This provides the most remarkable example 

 so far described of a structure already well differentiated internally which 

 proceeded to reorganize itself and reconstitute, in almost its original form, 

 a large mass of tissue. There was meristematic activity here, following 

 the dedifferentiation of much of the structure near the cut surface, but it 

 was diffuse meristematic activity like that of a leaf or fruit rather than 

 that of a localized growing point. Studies on regeneration in this very 

 promising material have been largely neglected in the half century since 

 Vochting's description of his work was published. 



Restoration 



Most regeneration in plants is not due to the reorganization of em- 

 bryonic regions but to the onset of meristematic activity in regions ad- 

 jacent to the place where loss has occurred. This leads to the production 

 of substitute structures that restore the original whole by indirect means. 



These processes are examples of compensatory correlation (p. 98) in 

 which the balance of the organism is restored after being disturbed. As 

 redifferentiation after injury often throws light on the problems of differ- 

 entiation and tissue pattern, a study of the restoration of lost structures 

 offers a useful means of analysis of developmental potencies and the proc- 

 esses of correlation. 



In many cases, as the result of wounding, a callus is produced (p. 288) 

 at the cut end of a stem or root. This often originates from the cambium 

 but may come from other tissues. From such calluses primordia of roots 

 and shoots commonly arise. This is the most frequent type of regeneration 

 in the higher plants and underlies the horticultural arts of multiplication 

 by vegetative propagation, chiefly the rooting of cuttings. This field has 

 been reviewed from the botanical point of view by Priestley and Swingle 

 (1929) and Swingle (1940, 1952). 



Almost every plant organ has been used as a cutting— stem, root, leaf, 

 hypocotyl, floral axis, and flowers— and all have been found to have some 



