DIV. ii PHYSIOLOGY 301 



They were not longitudinally-united halves, however, but inner and 

 outer layers of the growing point were formed of tissues of the two 

 different species (cf. pp. 307 and 86). These have therefore been termed 

 periclinal chimaeras in contradistinction to the sectorial chimaeras in 

 which longitudinal segments are evident. 



Cytisus Adami and the Crataegomespili are also periclinal chimaeras. 

 True graft hybrids in which a mingling of the specific characters in a 

 single cell has resulted from grafting are as yet unknown. 



Solanum tubinyese has the dermatogen of the Tomato, while the internal 

 tissues are those of the Nightshade. The converse is the case for Solanum Kolreu- 

 terianum. In S. proteus the two outer layers are. from the Tomato and .the 

 remainder from the Nightshade, while S. Gaertnerianum affords the converse 

 condition (Fig. 265). In a corresponding fashion the dermatogen in Cytisus 

 Adami is derived Ifrom Cytisus purpureus and the internal tissues from Laburnum 

 vulgar e. In one of the Bronveaux hybrids (the form Asnieresii} a core of Crataegus 

 is covered by the epidermis of Mespilus ; the other form (Dardari) has two or 

 more enveloping layers from Mespilus. When adventitious shoots are developed 

 from a single layer, these have the pure specific characters proper to the layer 

 without any trace of admixture with the other symbiont. 



Nothing is known with certainty of the mode of origin of periclinal chimaeras, 

 but it can hardly be doubted that the growing points of these adventitious shoots 

 are composed of cells derived from the two components, the one forming the core 

 and the other the surface layers. WINKLER'S contention that there were also 

 true graft hybrids is doubtful, and this author's own investigations show that 

 the change in chromosome number in these plants is susceptible of another 

 explanation. Further, the association of specifically different nuclei in the one 

 cell, so long as they do not fuse, does not constitute a true hybrid but only a 

 chimaera. Such a mixo-chimaera, which can again separate into its components 

 vegetatively, has been experimentally produced in Phycomyces nitens by 

 BUUGEFF ( 616 ). 



III. The Course of Development and its Dependence 

 on External and Internal Factors 



The course of development consists of a succession of processes 

 which tend to be repeated in the same order in any particular kind of 

 plant. Observations in nature suffice to show that this succession 

 must be capable of modification. Deviations from typical form which 

 are spoken of as monstrosities are not uncommonly met with. It is 

 one of the objects of developmental physiology to ascertain the causes 

 of such monstrosities, to produce them experimentally, and thus to 

 arrive at some insight into the causes of normal development. 



Although there are at present few of the phenomena of develop- 

 ment which can be controlled experimentally, the results obtained force 

 the conclusion upon us that THE TYPICAL COURSE OF DEVELOPMENT 



IS ONLY ONE AMONG A NUMBER OF POSSIBILITIES, THE OCCURRENCE 

 OF WHICH IS DETERMINED BY A PARTICULAR COMPLEX OF CAUSES. 



