164 STERILE AND FERTILE REGIONS 



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

 in the axils of the leaves ; but alternate fertile and sterile zones, merging 

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

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

 be styled the Selago condition, as it is seen conspicuously in Lycopodium 

 Selago (Frontispiece). In others, and especially in those in which the 

 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 difference 

 of form between the sterile and the fertile leaves, but the relation of the 

 sporangia to the latter is the same as in the Selago form. It seems 

 natural to conclude that the Selago type is the more primitive, and the 

 definitely strobiloid type the derivative. 



The question in either 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. The 

 clew is given by examination of the basal limits of the fertile zones in 

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

 fertile, imperfectly developed sporangia are often found, occupying the 

 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 Angiospermic flower, they will be 

 held to be vestigial representatives of sporangia, normally present, and 

 actually initiated, but not completely developed. Passing from these 

 to the vegetative region, where no vestigial sporangia are present, though 

 the arrangement, character, and in the Selago type even the form of 

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

 whether these are not essentially sporophylls, in which the 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 

 suppression, 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. The 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- 

 tionary factor. 



Such sterilisation as that believed to have occurred in l.vcopodiitm in 

 the course of descent has been experimentally induci-d by Goebel in 

 Selaginella? by cutting off young strobili, and treating them as cuttings : 

 the sporangia of the upper region aborted, and the sporophylls of the 

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

 result theoretically contemplated may follow from experiment. 



1 Organography, p. 



