292 LYCOPODIALES 



which Spring placed first of all was L. Selago ; and though this may not 

 be actually the most elementary living species in the genus, still it is the 

 best known of those which show a low degree of differentiation. 



The plant of L. Selago is shrubby, with dichotomously branched axes, 

 bearing numerous leaves of approximately equal size and simple form 

 (Frontispiece). There is usually a sterile region at the base of the plant: 

 this is followed by the well-known alternating sterile and fertile zones, 

 the length of which corresponds with a high degree of exactitude on the 

 several branches. They are stated to be determined by successive seasons, 

 the middle region of each year's increment of growth being fertile. These 

 zones are not definitely marked by any distinction of the leaves themselves, 

 but by the presence or absence of sporangia : nor are they strictly delimited 

 in this respect, for occasionally a single sporangium may be found in an 

 otherwise sterile region. About the limits of these zones sporangia of 

 smaller size may be found, which sometimes remain closed when all those 

 near them have dehisced. These are those incomplete sporangia which 

 have already been referred to in Chapter XIII., in connection with the 

 argument for sterilisation as affecting the balance of the sterile and fertile 

 regions. The condition thus seen in L. Selago is shared in more or less 

 complete degree by about 40 living species, which constitute the section 

 Selago : they are mostly ground-growing plants. An examination of them 

 shows that while most of them, .have, like Z. Selago, a sterile basal region 

 of considerable length, still in certain species (L. compactum, Hook., and 

 L. Trendlla Sodiro) sporangia have been found in the leaf-axils down to 

 the base of the mature plant : this has been noted also, but less completely, 

 in L. firmum, Mett., and L. rigidum, Gmel. Unfortunately these species 

 are unknown in the embryonic state, so that it is impossible to tell how 

 early in the individual life the formation of sporangia actually begins ; 

 but practically the whole of the mature plant is a fertile strobilus. The 

 incomplete differentiation of the sterile and fertile zones is seen in all the 

 40 species : isolated sporangia are frequently found in an otherwise sterile 

 zone, and occasionally sterile leaves occur in a fertile zone : these facts, 

 together with the occurrence of incompletely developed sporangia at the 

 limits of the zones, and the very uniform character of the leaves whether 

 sterile or fertile, have their direct bearing on the theory of sterilisation 

 enunciated in Chapter XIII. 



Some ten other species were grouped by Baker under the heading 

 Sub-Selago, and are characterised by having the sterile leaves a little 

 different from the fertile, but passing into them gradually, while the 

 sporangia are aggregated into indistinct terminal spikes. All the species 

 thus grouped have a sterile basal region : above this follows a recurrence 

 of sterile and fertile zones, as in Selago (L. Dalhousiatanum) \ reversion 

 from the fertile strobilus to a permanently sterile state is more common 

 (L. carinatum, gnidioides). A progressive diminution of size of the fertile 

 leaves upwards is seen : it is sometimes gradual (L. squarrosum\ but 



