704 CONCLUSION 



themselves of the sporangia. It is well known to have been initiated along 

 several distinct phyletic lines : well-ascertained cases are seen in the ligulate 

 Lycopods (Figs. 23, 24; 165, 166; and 170), in the Calamarians (Fig. 210), 

 and in the Hydropterideae, while it is quite possible that the heterospory 

 which preceded Seed-formation in the Pteridosperms may also have been 

 independently initiated. The innovation is closely connected with the 

 sacrifice of a proportion of the potential germs for the better nutrition 

 of the rest : this has already been seen to occur in various homosporous 

 types such as the Psilotaceae (p. 417), and Equisetwn (p. 380), though 

 the spores produced in these plants show no differentiation in size, or 

 apparently of sex. The condition seen in Calamostachys Casheana (p. 381) 

 is but little removed from this : here, however, heterospory is clearly present, 

 but not far advanced from that homosporous state where sacrifices for 

 nutritive purposes are seen : the megaspores appear relatively small and 

 numerous, as they are also in some of the heterosporous Lycopods, such 

 as Lycopodites Suissei, with 16 to 24 in each sporangium. In Selaginella 

 itself the number of the megaspores is smaller, and may vary from 8 in 

 S. apus, through the common number of 4, to sometimes a single one, 

 as in S. rupestris. The latter condition is found also in the Hydropterideae, 

 and it is the state which is commonly seen in all the higher Seed-Plants. 

 The facts indicate with no possible uncertainty that a progressive reduction 

 in number of the spores, which prove on germination to be female, has 

 taken place, till finally a single, large, well-nourished spore is the sole 

 product of each megasporangium. 



Such changes, however effective they may be in the successful establish- 

 ment of the new individual, through the concentration of the nutritive 

 store conveyed from the parent plant in a few enlarged megaspores, or 

 in only a single one, are nevertheless intra-sporangial : they rarely affect 

 other parts. It is true that in Azolla abortive primordia of microsporangia 

 accompany the megasporangium, as though their correlative diminution 

 followed on the great enlargement of the megasporangium ; but this case 

 is exceptional among heterosporous plants, and thus it is seen that the 

 introduction of heterospory does not necessarily bring far-reaching effects, 

 but involves a readjustment of the available nutritive material within the 

 single sporangium, and its concentration round few centres, or only a 

 single one, in place of many. 



It is different, however, with the other, and much more effective 

 innovation, vi/., the Seed-Habit. This also was initiated along more than 

 one line of descent, though it may still be a matter of doubt whether 

 it became permanently effective in more than one distinct phylum. It 

 will suffice here to quote the cases of incipient seed-like habit of the 

 Lycopodiales, seen in Lepidocarpon Lomaxi, and in Miadesmia, in which 

 the megasporangium, with its single megaspore retained within it, is covered 

 in by an integument, leaving a micropylar slit or pore : the whole 

 structure, together with the sporophyll to which it is related after the 



