236 The Phenomena of Morphogenesis 



velops into two or more parts (Fig. 9-1). These all grow for a time but 

 usually only one survives and develops into the embryo of the seed. A 

 portion of the original embryo thus reconstitutes a complete whole. This 

 recalls the not infrequent cases among animals where one fertilized egg 

 produces two or more individuals (as in identical twins) or where a 

 single blastomere, experimentally isolated, will form a whole. 



Fig. 9-1. Cleavage polyembryony 

 in Torreya. This group of young 

 embryos have all come from a sin- 

 gle fertilized egg by cleavage. 

 (From Buchholz.) 



It is with the terminal meristems of the older plant axis, however, that 

 most of the experimental work on regeneration has been carried on. In 

 roots it is generally agreed by observers that if only the extreme tip is 

 removed, about y 2 to % mm., a new growing point will regenerate di- 

 rectly at the wound surface from the underlying tissue of the plerome. If 

 a little more is cut off, regeneration is only partial and chiefly by the for- 

 mation of new growing centers in the outer portion of the root. If still 

 more is removed, true reconstitution ceases and a callus is formed with 



