358 LYCOPODIALES 
of the parent prothalli is so marked; the difference being between a 
free-growing, self-nourishing bisexual prothallus in Lycopodium, and an 
endosporic, unisexual, storage prothallus in Seéagine/la. The early form 
and structure of the embryo, consisting of a pluricellular suspensor and 
two tiers of cells of the embryo, is virtually the same in both cases. In 
the later development the nearest similarity is between Z. Se/ago and 
S. spinulosa: there is in both a marked elongation of the hypocotyl, with 
the first root originating laterally near its base: there is the same absence 
of any determinate foot: and as a rule the same origin of a first cotyledon 
laterally, with the apex of the axis between it and the next-formed leaf. 
The axis thus originates in both in close relation to the intersection of 
the primary segmentation-walls of the upper tier of the embryo! One 
point of difference is in the part played by the lower of the two tiers 
of cells of the embryo: in Lycofodium it remains in close relation with 
the suspensor, and may be more or less swollen into a foot in some 
species; but it does not elongate, or form any permanent part of the 
embryo, the whole of which originates from the upper tier. But in Seda- 
ginella the lower tier elongates to form the greater part of the hypocotyl, 
while the first root originates from its base. The position of the root 
relatively to the other parts is otherwise alike in Z. Se/ago and in S. spinulosa. 
The similarity of the embryogeny of the genus Se/aginella to that of 
Lycopodium is thus established by comparison of species both of which 
are held to be primitive in their respective genera, on the basis of com- 
parison of their mature sporophytes. 
At first sight the embryogeny of /svefes seems to differ radically from 
that of Se/agine/la, notwithstanding that the endosporic prothallus is so 
similar in both cases. The key to the difference is first the inversion 
of the embryo, as compared with Selaginella, and secondly, the entire 
absence of any represefitative of the suspensor: what remains in Jsoetes 
may be held to correspond to the product of the two upper embryonic 
tiers only.? 
The first division of the zygote is by a wall more or less inclined to the 
axis of the archegonium, but occasionally almost including it (Fig. 191 8): 
this indeterminate position of the ‘basal wall” is theoretically important, 
as bearing on the inversion of the embryo in the archegonium as com- 
pared with that in Lycopodium or Se/aginella: the variations seen in 
Zsoetes suggest how that inversion may have come about. The two tiers 
thus initiated are usually called the hypobasal and the epibasal; but the 
octant divisions commonly seen in other embryos are not always clearly 
defined in Jsoe‘es. The hypobasal tier here forms the foot only: all the 
1On this point the facts have been better made out in LZ. phlegmaria and L. clavatum 
than in Z. Sedago; but the facts for the latter, so far as they go, are consistent with the 
same conclusion. 
2See Campbell, Mosses and Ferns, p. 545, etc. Also Kienitz-Gerloff, Bot. Zevt., 
1881, p. 761. 
