THE "SELAGO" CONDITION 165 



With this progressive sterilisation, and the consequent increase of the 

 vegetative region, the apical growth of the axis keeps pace : it secures the 

 initiation of additional sporophylls and sporangia to take the place of 

 those transformed or aborted, and as there is no theoretical limit to the 

 apical growth and branching, in such species as L. Selago the balance 

 can constantly be readjusted between the sterile and the fertile regions. 

 This combination of sterilisation and continued apical growth provides, 

 in a sense, a forward impulse, and it will be effective up to the limit of 

 physiological supply. That it is so is seen in the fact that at the apex 

 of any Lycopod strobilus imperfect sporangia are found, w r hich are to 

 be recognised as supernumeraries, showing the continued exuberance of 

 initiation beyond the power of the plant to bring to complete maturity. 

 We thus acquire the conception of a zone of reproductive activity or in 

 the Selago type it may be several interrupted zones limited below by 

 parts which are to be held as vestigial, and above by parts which are 

 supernumerary. By comparison of living species of Lycopodium it is seen 

 that the fertile zone is not always located at the same level on the plant: 

 it is sometimes preceded by a shorter, sometimes by a longer vegetative 

 region. There has probably been a phylogenetic shifting of the fertile 

 zone or zones : the biological significance of this is obvious, for any 

 advance of the fertile zone to a higher point, by abortion of sporangia, 

 while the sporophylls remain in a vegetative capacity as foliage leaves. 

 provides a larger vegetative region below for purposes of nutrition. Such 

 a manner of advance has probably been effective in the evolution of the 

 Lycopods as we now see them. 



If the Lycopods stood alone in showing such features as those described 

 the facts would be of limited interest, but they do not ; conditions essentially 

 similar are seen in the sporophytes of other Vascular Cryptogams, though 

 varying in detail. The mature plant of Isoetes is virtually of the Selago 

 type : it bears fertile and sterile leaves intermixed : vestigial representatives 

 of sporangia are found in the position normal for sporangia upon many of 

 the sterile leaves ; further, the probability that the leaves actually sterile 

 are so by suppression is as strong here as in the case of Lycopodiinn 

 Selago. The mature plant is preceded by an embryonic vegetative phase, 

 with leaves bearing no sporangia ; but after the first sporangia appear, the 

 whole plant may be regarded as a strobilus, imperfectly differentiated, as 

 in the Selago type, into fertile parts and parts sterile by abortion or by 

 complete suppression. 



Similarly, in the Psilotaceae, the Selago condition, with irregular alter- 

 nation of sterile and fertile zones, is seen in both Psilotum (Fig. 87) and 

 Tmesipteris, while imperfect synangia are found about the limits of the 

 fertile regions. There is, however, a broad difference in form between the 

 simpler sterile appendages and the more elaborate fertile ones ; in this 

 respect the differentiation of sterile and fertile parts has proceeded further 

 than in the Lycopods. In the allied fossils, the Sphenophylleae, there 



