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 Selaginella. 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 L. 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. 1 One 

 point of difference is in the part played by the lower of the two tiers 

 of cells of the embryo : in Lycopodium 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 Sela- 

 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 L. Selago and in S. spinulosa. 

 The similarity of the embryogeny of the genus Selaginella 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 Isoetes seems to differ radically from 

 that of Selaginella. 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 representative of the suspensor : what remains in Isoetes 

 may be held to correspond to the product of the two upper embryonic 

 tiers only. 2 



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 B) : 

 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 Selaginella : the variations seen in 

 Isoetes 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 Isoetes. The hypobasal tier here forms the foot only : all the 



1 On this point the facts have been better made out in /_. phlegmaria and L. clavatuni 

 than in L. Selago ; but the facts for the latter, so far as they go, are consistent with the 

 same conclusion. 



2 See Campbell, Mosses and Ferns, p. 545, etc. Also Kienitz-Gerloff, Bot. Zeit., 

 1881, p. 761. 



