EMBRYOLOGY 345 
developed in more open soil suggests the origin of the phlegmaria-type : 
while its green sub-aerial forms are reminiscent of the cernuum-type. A 
plant which shows such plasticity is clearly not far removed from the self- 
nourishing condition of the prothallus, which was probably the primitive 
condition for them all. 
These remarks upon the curiously divergent development of the pro- 
thallus in the genus Lycopodium are a necessary preliminary to the study of 
the embryogeny in the genus; for it is impossible to understand the 
comparisons of the different forms of embryo without some knowledge of 
the prothalli which produce them. In all the species of Lycopodium in which 
the embryogeny is accurately known, an early stage of the embryo is 
found in which it consists of a suspensor, and of two tiers, each composed 
of four cells (Fig. 182). The first cleavages are variable in their succession, 
as is found to be the case also in other embryos ; 
but their position shows considerable constancy. 
It is stated that from the lower tier of cells, 
ze. that adjoining the suspensor, the structure 
designated the foot arises, while the upper tier 
gives origin to all the other parts of the 
embryo, and the correctness of the statement 
is borne out by numerous drawings. But after 
the first stages are past there is usually no 
sharp limit between the tissue composing the 
foot and that of the other parts: in the simplest 
cases it appears as though the foot were merely 
a region of tissue lying between the suspensor 
and the upper tier, rather than a definite organ 
or part. Functionally, the foot does not appear 
Fic. 182. 
Diagram illustrating the primary 
segmentation of the zygote in Lyco- 
podium. J, d=first segmentation 
wall which separates the suspensor, 
here cross-hatched. 4, 4 separates 
a lower tier (foot-tier) here dotted, 
from an upper tier (stem-tier) left 
clear; each tier consists at first of 
four cells. The wall 4, 4 corresponds 
to the wall IV.-IV. in Figs. 183, 186, 
and to wall II.-II. in Fig. 185. 
to be differentiated from the suspensor in the genus Lycopodium, and it 
shares with it the office of maintaining connection with the prothallus. Not- 
withstanding the initial similarity which thus rules in the embryos of the 
genus, the further steps of the embryogeny differ according to the different 
forms of prothallus above described; and it becomes a question which of 
the divergent types is to be held as the most nearly reflecting the original 
condition, and which as later and derivative. 
The type of Z. Se/ago may be taken first, since it does not show any 
high degree of specialisation in its variable gametophyte, while it has 
been seen above that its mature sporophyte is one of the least differen- 
tiated in the genus. Its early embryogeny, so far as is known, conforms 
to the usual type, as above stated. The foot originates from the lower 
tier, and the various parts of the embryo from the upper.! But the foot 
is only slightly developed. The upper tier of cells soon assumes a green 
colour and unsymmetrical form, owing to the lateral upgrowth of the first 
leaf or cotyledon, while the apex of the axis also originates early, near 
1Bruchmann, /.¢., pp. 97-103. 
