THK '^SELAGO" CONDITION [65 



With this progressive sterilisation, and the consequent UlCreMC of the 

 vegetative region, tlu- apical giowth of tin- axis krcps pace: it secures the 

 initiation of additional sporophvlls and sporangia to take- the place ol 

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

 apical growth ami branching, in such species as L. Selago the halanee 

 Can constantly hi- readjusted between tin- sterile and the fertile regions. 

 This combination of sterilisation and continued apical growth provides, 

 iii a sense, a forward impulse, and ii will be effective up to the limit of 

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

 ot am Lvcopod slrohilus imperfect sporangia are found, which arc to 

 he recognised as Supernumeraries, showing the continued exuberance ol 

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

 \\ e thus acquire, the conception of a zone of reproductive activity or in 

 the Selago t\pe it may be several interrupted zones limited below by 

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

 supernumerary. l',y comparison of living species of AmyW/Ww it is seen 

 that the fertile zone is not alwa\s 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 phvlogcnctic shifting of the fertile 

 /one or /ones: the biological significance of this is obvious, for any 

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

 while the spoioplulls remain in a vegetative capacity as foliage Lea 

 provides vegetative region be-low 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 inteiest, but they do not; conditions essentially 

 similar are si-en in the sporophytes of other Vascular Cryptogams, though 

 \arying in detail. The mature plant of Jsoctcs is virtually ol the .SV/r/^v 

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

 of sporangia are lomul 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 Lycopodium 

 .SV/r/v". The mature plant is preceded by an embryonic vegetative ph.; 

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

 whole plant m. iv be regarded as a strobilus, imperfectly differentiated, as 

 in the Scla^o type, into fertile parts and parts sterile by abortion or by 



complete suppression. 



Similarly, in the 1'silotaceae, the Selago condition, with irregular alter- 

 nation of sterile and fertile /ones, is seen in both rsilithnn (Fig. 87) and 

 Twt'.\rf>ft-ris. \\hile imperleet lynanglS are found about the limits ot the 

 fertile regions. There is, howe\ei, 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 Lyeopods. In the allied fossils, the Sphenophvlleae, there 



