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 Lguésetum (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 Sutsset, 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 thatgin Azol/a 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, viz., the Seed-Habit. This also was initiated along more than 
one line of descent, though it may still bé 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 Lepzdocarpon 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 
