

GENERAL MORPHOLOGY 309 



potentially a fertile strobilus, in which the vegetative and reproductive 

 systems are not differentiated from one another. This, together with its 

 prevalent absence of terminal branching, points out Isoetes as a near 

 approach in its general construction to the strobiloid type theoretically 

 primitive for the Lycopodiales : this it shares with the simplest Selago- 

 forms of Lycopodium. But it is with the dendroid Lycopodiales that 

 Isoetes shows common characters of the sporangia themselves : there is 

 also some similarity to them in the structure of its abbreviated but bulky 

 stock : on this also the very similar bifurcating roots are inserted, but in 

 Isoetes their origin is localised in the depressed grooves which traverse 

 the stock longitudinally, instead of their being borne on Stigmarian out- 

 growths, as in the fossils. The Isoetes plant is then like a partially 

 differentiated Lepidostrobus seated upon a Lepidodendroid base : in fact, 

 like a stunted Lepidodendron, with its preliminary vegetative phase very 

 short. Its mature shoot still carries on both vegetative and propagative 

 functions, and in this lack of differentiation a primitive character is to be 

 recognised. 



The account thus given of the general morphology of the mature 

 sporophyte in the Lycopodiales, living and fossil, shows the essential 

 identity of their plan of construction throughout the phylum, and how in 

 the two series, the ligulate and the eligulate, parallel conditions of differ- 

 entiation are represented. In both the structure of the shoot is essentially 

 strobiloid, with a constant numerical relation of the sporangium to the 

 subtending sporophyll. In both series the branching of the axis is primarily 

 by dichotomy, with a deviation in the more specialised types, and especially 

 in the higher ramifications to the monopodial branching : but in certain 

 simple -types branching is rare, or even absent. The shoot is fixed in the 

 soil by roots, formed chiefly, or even exclusively, at the base of the axis 

 in the simpler types; but in the more specialised they may be formed at 

 various other points on the shoot-system, or on outgrowths from it of an 

 indeterminate character. In both series there is evidence of abortion of 

 sporangia, leading to a segregation of definite tracts of the shoot-system 

 devoted to nutrition and to propagation respectively : in the higher types 

 the strobilus becomes a definite cone of limited growth, clearly marked 

 off from the lower vegetative region : the production of spores is thus 

 deferred in the individual life, and a more lengthy vegetative phase 

 intercalated before that event. This progressive differentiation is best 

 illustrated in the eligulate series, which is also the more primitive in 

 respect of its homosporous condition. We are thus led by comparison 

 of the Lycopodiales, living and fossil, to contemplate as a fundamental 

 type of their shoot a simple unbranched strobilus with unlimited 

 apical growth, bearing un differentiated leaves, and having one sporangium 

 associated with each leaf. This may not improbably have been the 

 primitive type from which, by branching, by formation of a root-system, 

 by differentiation of the sterile from the fertile region, and, finally, by 



