i66 



STERILE AND FERTILE REGIONS 



is, as a rule, a definite strobilus, which is fertile throughout ; but in .S. majus 

 .this is not clearly differentiated by form either at base or apex from the 



vegetative region. Such a condi- 

 tion shows an interesting analogy 

 with the less differentiated states 

 of Lycopodium. 



Among the Equisetales, Equi- 

 setum and Bornia have, as a rule, 

 a definite strobilus, composed 

 entirely of sporangiophores borne 

 on an axis, and clearly marked off 

 from the vegetative region which 

 precedes it in the ontogeny. But 

 in the Calamarians, as also in 

 the fossil known as Phyllotheca, 

 sterile leaf-sheaths are interspersed 

 between the sporangiophores, a 

 condition occurring also as an 

 occasional abnormality in Equi- 

 setum (Fig. 88). The morphology 

 of these cases will be more fully 

 discussed below; meanwhile it may 

 be held that while in Equisetum 

 and Bornia the differentiation of 

 the fertile strobilus from the vege- 

 tative region is more distinctively 

 marked, Phyllotheca or the Cala- 

 marians show some degree of 

 analogy with the Selago condition 

 seen in Lycopodium. 



The Ophioglossaceae provide 

 clear cases where the argument 

 of abortion leading to complete 

 suppression will apply ; for various 

 degrees of development of the 

 fertile spike may be found borne 

 upon the fully formed leaf, from 

 that fully matured to small 

 vestigial parts which do little more 

 than mark the place where the 

 normal spike would be inserted ; 



in other cases again the spike is entirely unrepresented. . The facts here 

 correspond to those in Lycopodium SeJago or in Isoetes, except as regards 

 the size and number of the parts concerned. Such a condition in an 

 Angiospermic flower would certainly be interpreted as abortion, and the 



FIG. 87. 



Shoot of Psilotum. Natural size. Showing " Selago " 

 condition in the bifurcate branch-system : the base is 

 vegetative : then follows a zone bearing synangia, then 

 a vegetative zone, and higher up a second fertile zone. 



