Affinities of the Palaeozoic Seeds of the Co no stoma Group. 45 
secretory nature may be due to the same internal causes that have so fre- 
quently rendered vestigial structures secretory among living plants. The 
f blow-off’ layer and the soft apical tissue of Conostoma oblongum may be the 
remnant of a once much more extensive tissue comparable to the sarcotesta 
of the medullosean series. The closest analogy which modern plants offer 
appears to be the megaspore of Pilularia . Here the mucilaginous layer which 
invests the megaspore serves to attract and retain the sperms; above the 
archegonium the mucilage forms a deep funnel, which becomes filled with 
spermatozoids. 1 In Conostoma the mucilage layer, as in Pilularia , reaches its 
maximum development at the apex. In Conostoma oblongum the epidermis 
split up the flanks of the free apical lobes, as is seen in PL I, fig. 6, bl. ; the 
expanding mucilage must thus have found its way into the micropyle and in 
the space between the apical lobes. If the seeds were retained till after 
pollination this mucilage may well have acted as a drop mechanism com- 
parable to that of the present-day Taxus. If, however, as might have been 
the case, the seeds were first shed, perhaps the mucilage played a part analo- 
gous to Pilidaria in capturing and nourishing the male cells. Our know- 
ledge, however, of the functions of mucilage, even in recent plants, is so 
incomplete as to render the problem in fossil plants extremely difficult. 
In Conostoma anglo-germanicum and Gnetopsis the ‘blow-off’ is not ex- 
foliated even in specimens showing pollen-grains. We probably have then 
in all these seeds to deal with a common physiological cause, and any value 
the layer may have had in certain cases is to be regarded as a secondary 
adaptation. 
VIII. Conclusion and Summary. 
The facts recorded in the foregoing paper go to prove that the seeds 
of the palaeozoic epoch showed, within certain well-defined limits, a con- 
siderable degree of diversity in mechanism. 
When regard is had to the dominance which seed-possessing plants 
afterwards attained, it is hardly surprising that the seeds of Coal Measure 
times should have shown unmistakable indications of modification and 
elaboration in a variety of different directions. 
This diversity, as it affected the apex of the seed, is fully illustrated 
in Text-figs. 12 and 13. Whilst the actual parts involved are in funda- 
mental agreement- — lagenostome, plinth, and a compound integument — the 
detailed relations of these parts are altogether different. In Physostoma the 
large lagenostome was enveloped in the lobes of the integument, which 
collectively formed what may well have been the precursor of the micropyle 
in this group of seeds. In Lagenostoma these arms were united into a 
chambered ‘ canopy ’, which whilst investing the lagenostome, was over- 
topped by the orifice of the latter, which thus had direct access to the sur- 
1 Campbell : Mosses and Ferns, p. 425, ’1905. 
