240 The Phenomena of Morphogenesis 



terminal leaflet of the pinnately compound leaf of Bryophyllum (Kalan- 

 choe ) was cut off when the leaf was very small, it was partially regener- 

 ated. Other cases of such regeneration have been reported, but the leaf, 

 doubtless because of its generally determinate growth, shows meri- 

 stematic reconstitution much less readily than does root or stem. 



Gametophytes sometimes display this type of regeneration. Albaum 

 (1938rt), confirming earlier workers, found that if the ordinary prothal- 

 lium of certain ferns is cut transversely the anterior portion, near the 

 meristematic notch, will re-form the typical heart-shaped structure again. 

 In the posterior region this does not happen, but new adventive pro- 

 thallia are produced from the cut surfaces (p. 121). Meyer ( 1953) reports 

 that the meristematic region in the notch of the prothallus, and particularly 

 its apical cell, seems to inhibit the formation of other apical cells; for if 

 the prothallus is divided lengthwise into three parts, the central one, 

 which includes the apical cell, will regenerate its lost portions whereas 

 the two lateral pieces will each first produce a new apical cell and then 

 proceed to develop into typical prothallia. True reconstitution of a meri- 

 stematic region thus seems to be limited to very early developmental 

 stages. In later ones, even though the tissue may still be meristematic, it 

 has lost some of its morphogenetic potencies and injury will result either 

 in simply a wound reaction or in the production of new adventitious 

 growing points rather than in a remolding of the old one. 



Reconstitution of Tissue Patterns. A somewhat different type of recon- 

 stitution occurs where the structure that is removed is not a meristem but 

 a part already differentiated, at least to some degree. To restore the dis- 

 turbed tissue pattern involves a more complex process and is rarely as 

 complete as the reconstitution that occurs in a meristem. Where mature 

 or nearly mature cells are part of this pattern, some of these cells must 

 evidently become embryonic again and assume a new function in the 

 reorganized system. There are some remarkable examples of this which 

 provide particularly interesting morphogenetic problems. 



The first step in such a reconstitution is healing of the wound itself. 

 Wound reactions differ with the type of plant and the conditions. Cells 

 near the wound surface generally become more active and soon, under 

 the influence of wound hormones, cell division is initiated parallel to the 

 surface. A phellogen here develops which forms a layer of protective cork 

 over the wound. In many cases this is all that happens. Sometimes a callus 

 is formed here and from it primordia of roots and shoots may develop. 

 The phenomena of wound reactions have been reviewed by Bloch ( 1941, 

 1952). Fourcroy (1938) has discussed the same subject and particularly 

 emphasizes the accelerating influence of wounding on differentiation and 

 its effects on vascular anatomy. 



In many plants, however, the tissues under the wound may be reorgan- 



