310 Lang . — The Prothallus 
hypocotyledonary stem the first leaf is brought to the light, 
and is capable of carrying on the work of assimilation ; it is 
almost immediately followed by the second leaf, which is 
apparent before the embryo has broken through the pro- 
thallus. The embryology of L . Selago agrees in detail with 
that of L. Phlegmaria , save in the period of formation of the 
wall separating the two tiers ; no appearance of a protocorm 
is recorded for this species. 
The main differences then between the embryology of 
these species with saprophytic prothalli, which grow a short 
distance below the surface of the substratum, and that of 
the L. cernuum type, are accounted for by the absence of a 
protocorm. The reasons given above in favour of the proto- 
corm being a primitive organ, taken together with the simi- 
larity between the L. Selago prothallus and those of the 
L. cernuum type, support the view that this stage has been 
omitted from the development of these young plants. Some 
direct evidence in favour of this is further afforded by 
the occurrence of a rudimentary protocorm bearing rhizoids 
in L. Phlegmaria 1 . While the absence of a protocorm 
stands in relation with the subterranean habit of the 
prothallus, the comparatively small foot must be put into 
relation with the fact that the first leaf is able to reach 
the light, and by its assimilation renders the young plant 
independent of the prothallus. 
There remains for consideration the type of embryology 
presented by L. clavatum , annotinum, and complanatum, 
which all agree in this respect. The early segmentation is 
the same as in L. Selago , and here also the suspensor and 
two tiers are recognizable. The stem-apex, first leaves, and 
first root originate from the tier furthest from the suspensor ; 
this, however, remains for a considerable period as a small- 
celled meristem in which no differentiation of members is 
apparent. Before this takes place, the foot, derived from 
the middle tier, has undergone great enlargement so that 
the embryo for a time is almost spherical. With the appear- 
1 Treub., Ann. Jard. Bot. Buit., viii. p. 32. 
