GENERAL MORPHOLOGY 307 



outgrowths from the axis itself, which 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 Selaginelfas. 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 in 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. Zcit., 1902, p. 179. 



