2l6 THE PLANT WORLD 



To trace the origin, development and differentiation of the 

 sporophyte is, therefore, to investigate the origin of a land 

 flora. 



The strobilar hypothesis begins with a primitive aqu:itic 

 form in which there are post-sexual cell divisions which lead 

 to the multiplication of germ.s. It is upon these germs, ac- 

 companying alternating drying and wetting of the habitat, 

 that selection and variation play. Thus the biological con- 

 ditions attending the transition from water to land naturally 

 encourage some form of "amphibious alternation." Where 

 alternation of generations occured, therefore, the sexual or 

 asexual phase, became intercalated in the course of the descent 

 of successive gametophytes. Once the asexual phase is 

 formed its differentiation takes place through progressive 

 sterilization, the segregation of spores into pockets, and the 

 origin of appendicular organs. This progression is associated 

 with a need of a larger number of spores, or with their pro- 

 tection, or their nutrition, but the production of spores re- 

 mains the most complex plant forms, as it is in the primitive 

 sporophytes, the most important function. 



"A prototype, which was probably a prevalent, though 

 perhaps not a general one for the Pteridophytes, may then 

 be sketched as an upright, radial, strobiloid structure, con- 

 sisting of a predominant axis, showing continual apical 

 growth, and bearing relatively small and simple appendages." 

 From this primitive type the author evolves the more complex 

 types of sporophytes by natural process as suggested above 

 and, by extensive reference to anatomical and embryological 

 works, as well as to researches in palaeontology, places his 

 hypothesis on what appears to be safe ground. It is this wide 

 reference to other branches of the science that serves to bring 

 into harmonious relation, and give especial point to widely 

 separated facts and to place these together as a logical 

 hypothesis, as presented in so masterly a manner by its 

 author. — ^W. A. C. 



