GENERAL MORPHOLOGY 307 



outgrowths from the axis itself, ^vhich have been regarded as rudimentary 

 rhizophores. It does not seem an undue strain of comparison to suggest 

 that in this basal knot is still to be seen, on a minimal scale, a living 

 representative of those larger growths known as the Stigmarian trunks. 

 These would thus be in their nature indeterminate outgrowths of the 

 hypocotyl, as are these rudimentary rhizophores ; but like them, strictly 

 localised in origin, instead of being dispersed over the branch-system, as 

 are the rhizophores in most modern Selaginellas. It is thus possible to 

 bring the general morphology of Lepidodendron into relation to that of 

 the modern Selaginella, a type which there is reason to believe itself 

 dated from the Carboniferous period. 



On the other hand, there are obvious relations between the dendroid 

 Lycopodiales and the living genus Isoetes : this type has been found 

 fossil in the Tertiaries, and back as far as the Lower Chalk, while in the 

 Trias the curious fossil Pleuromoia is represented : but there is no suffi- 

 cient evidence of the genus Isoetes having itself figured among the earliest 

 fossils. 



The plant of Isoetes consists of a short upright axis covered by relatively 

 large leaves (Fig. 155) : the axis is usually unbranched, though bifurcation 

 occasionally occurs, a fact that has its interest for comparison with the 

 Lycopods. 1 The leaves are essentially of one type, with broad base and 

 acicular upper part, while seated in a pit on the upper surface, at some 

 little distance from the base, is the ligule. The leaves may be either 

 sterile or fertile, and in some species there is a difference in size, the 

 sterile leaves being the smaller. The plant is heterosporous. Where the 

 leaf is fertile the large cake-like sporangium lies in a depression of the 

 leaf-surface, between the ligule and the leaf-base, that region being 

 elongated to accommodate it : in the sterile leaves it is shorter. An 

 examination of the sterile leaves of /. lacustris (and Wilson Smith made 

 similar observations in /. echinospora) shows that sporangia in various 

 degrees of abortion may be found upon them : in some of these spores 

 are developed, but in smaller numbers than the normal : other sporangia 

 may remain quite small, and produce no spores. Dissections show that, 

 in the majority of leaves that are apparently sterile, a rudimentary sporan- 

 gium is really present hi a normal position. It is stated that a regular 

 seasonal sequence is followed in the distribution of the megasporophylls, 

 the microsporophylls, and the foliage leaves : that the megasporangia are 

 borne on the first or outermost leaves of each annual increment, then 

 follow leaves with microsporangia, while the sterile leaves form the transi- 

 tion from one year's increment to the next. It is thus seen that in the 

 distribution of its sporangia Isoetes shows a condition similar to that of 

 Lycopodium Se/ago, but that the various degrees of their abortion are 

 better represented. *It follows from the facts that after the embryonic 

 stages are past in which no sporangia are produced the whole plant, is 



1 Solms Laubach, Bot. Zeit., 1902, p. 179. 



