Regeneration 235 



REGENERATION IN THE HIGHER PLANTS 



Among vascular plants there is a much higher degree of differentiation 

 than in lower forms and as a consequence the processes of regeneration 

 are more complex. In these plants we may recognize, for convenience, 

 three rather different types of regenerative activities. Reconstitution, or 

 regeneration proper, includes those cases in which, as in animal regener- 

 ation, there is a reorganization of the embryonic tissue by which its orig- 

 inal structure is re-formed. This is usually limited to truly embryonic 

 regions, such as growing points and young embryos, and to structures 

 where there is a reorganization of the tissue pattern by dedifferentiation 

 and subsequent redifferentiation. Restoration describes the wide range 

 of cases where missing tissues or organs are replaced through meri- 

 stematic activity arising in adjacent regions. This may result from the 

 activation of dormant buds or primordia already present or in the forma- 

 tion of new ones such as occurs in the origin of new roots and shoots in 

 the familiar processes of vegetative propagation. Reproductive regener- 

 ation, or vegetative reproduction, involves the separation, by natural 

 means, of a part of the vegetative body from the rest and its establishment 

 as a new plant, a process which often occurs in the lower groups. Similar 

 cases are those where plantlets develop on the leaves and drop off to form 

 new individuals. These are all specialized instances of the ability of the 

 plant, under favorable conditions, to produce a new whole from a part 

 of its body, an ability that comes from the totipotency of its various 

 members. 



Reconstitution 



This process, the reorganization of living material by which the normal 

 structure is restored when disturbed by outer circumstances, is relatively 

 uncommon in plants, since truly embryonic conditions persist in them for 

 only a relatively short time before changing into a mature state where 

 reorganization is difficult. Such reconstitution as does occur is of two 

 sorts. In one, truly embryonic or meristematic tissues may be reshaped 

 into a new whole. In the other, tissue already well along toward maturity 

 may undergo a certain degree of dedifferentiation and redifferentiation 

 so that its structure is reorganized and the original pattern, at least in 

 part, reconstituted. 



Meristematic Reconstitution. Among the simplest cases, and one which 

 not infrequently occurs in nature, is cleavage polyembryony. In many 

 conifers (Buchholz, 1926), though less commonly in angiosperms, the 

 early embryo rudiment, carried down into the endosperm at the tip of 

 the suspensors and still consisting of only a few cells, divides and de- 



