162 



STERILE AND FERTILE REGIONS 



As this in my opinion has not yet been accorded its proper place in the 

 evolutionary story, I propose to consider it at some length. 



Imperfectly developed parts have played an important role in arguments 

 on Evolution. On the Zoological side especially they have been used 

 as weighty evidence. Similarly, in Botany they have been the basis of 

 discussion : in the morphology of the flower, abortive stamens, carpels,, 

 pollen-sacs, and ovules have been cited as foundations for elaborate 

 argument. Where present in normal position the existence of an abortive 

 stamen or staminode has been habitually held to be sufficient indication 

 of the previous existence of a fully developed stamen in the ancestral line; 

 and on such evidence natural affinities have been traced and accepted, 



c 



Fie;. 86. 



A, median section of young sporogonium ot Anciint iiinbrosioidcs. The internal mass 

 of cells of the sporogonial head (" archesporium ") is already differentiated so as to 

 indicate the sterile elaterophore, and the outer fertile region. B, the same, older : the 

 indications of sterilisation have extended outwards, and it is only the peripheral fringe of 

 cells (shaded) which will be sporogenous. C, transverse section of the same, x 150. 



usually without question. But floral morphology has gone further : com- 

 parative study has led to the conclusion that in certain ancestral lines 

 of descent parts have existed, which in the individuals of the present day 

 are entirely unrepresented by any vestigial growth. This condition of 

 complete disappearance of a part or parts has been styled "ablast," as 

 distinct from "abortion," where the incompletely developed part has an 

 objective- existence. Eichler maintained that the conditions distinguished 

 as "abort" and "ablast" are not essentially different in kind, but only 

 differ in degree. He points out that abortion itself is not susceptible of 

 objective proof, and it may be remarked incidentally that it is this fact 

 which has prevented the full recognition of the part which it has played 

 in the origin of the sporophyte. Speaking of the relations between partial 

 abortion, where a vestigial structure is present ("abort"], and complete 

 suppression ["ablast"], Eichler remarks (Bluthendiagramme^ p. 52) that in 





