10. 1 STI'kli K AND I'KkTILK KKC.IONS 



the initial vegetative stage is soon closed by the appearance of sporangia 

 in the axils of the leaves; hut alternate Irrlile ;ind sterile- /ones, merging 

 into one another imperceptibly as regards lorm, succeed one another at 

 irregular intervals throughout the upper region of the plant. This may 

 be styled the Scfago condition, as it is seen conspicuously in /jv/'/W////// 

 Selagp (Frontispiece). In others, and especially in those in which Un- 

 fertile region appears in the form of a definite terminal strobilus, the 

 initial vegetative phase is more extensive, though still essentially similar to 

 the strobilus in its construction ; there is, however, a prevalent dilfcrenec 

 of form between the sterile and the fertile leaves, but the relation <>i UK- 

 sporangia to the latter is the same as in the Sc/w form. It seems 

 natural to conclude that the Sclago type is the more primitive, and t In- 

 definitely strobiloicl type the derivative. 



The question in cither of these cases is, what genetic relation has 

 existed between these sterile and fertile regions which are so similar in 

 plan, but differ in the absence or presence of the sporangia. Tin- 

 clew is given by examination of the basal limits ot the lertile zones in 

 either case; for here, at the point of transition from the sterile to Un- 

 fertile, imperfectly developed sporangia are often found, occupying tin- 

 place normally taken in the fertile region by those fully developed. 

 Applying to these the same argument as in the case of an imperfectly 

 developed ovule or pollen-sac in an Angiospeimie flower, they will be 

 held to he vestigial representatives of sporangia, noimally present, and 

 actually initiated, but not completely developed, Passing from these 

 to the vegetative region, where no vestigial sporangia are present, t hough 



UK- arrangement, character, .ind in tin- St'/iig<> type even the loim of 



the leaves is the same as in tin- fertile region, the question arises 

 whether these are not essentially sporophylls, in which tin- sporangia 

 are completely suppressed? The result of a broad consideration of the 

 question will be an answer in the affirmative. The facts indicate that 

 in the simple Lycopod type progressive sterilisation has been effective, 

 and that it has involved the partial abortion, or even the complete 

 <uppression, of whole sporangia; the result is that leaves originally in 

 the race fertile have become sterile, and have thus contributed to the 

 enlargement of the vegetative region. Tin- fact that the Selago condition 



is seen represented in certain Lycopod fossils of the Coal period is 

 important evidence of the validity of [this progression as an early evolu 

 nonary factor. 



Such sterilisation as that believed to have occurred in / r.v/W////// in 



the course of descent has been experimentally induced by Goebel in 



:;inel/a> 1 by cutting off young strobili, and treating them as cuttings; 



the- sporangia of the upper legion ahoited, and the sporophylls of the 



newly formed parts of the shoot developed as foliage leaves. Thus the 

 result theoietieally contemplated may follow from experiment 



P- (l ^7- 



