of Lycopodium clavatum, L. 31 1 
ance of the rudiments of the members of the young plant in 
the region derived from the terminal tier, another difference 
of detail is manifest. For in this case the apex of the stem 
is first apparent, followed by a pair of leaf-rudiments the 
position of which is not constant ; these are succeeded by 
a second pair at right angles to the first. Later the first root 
appears. These differences seem to stand in relation to the 
depth at which the prothallus grows. The young plant, on 
account of this, is dependent on the prothallus until it has 
attained a much greater size than in L. Selago and L. Phleg- 
maria. This explains the need for the large spherical foot 
with its special absorbent layer. Further the arrangement 
of the first leaves covering the apex serves the purpose of 
protecting the latter on its way to the surface. 
The known forms of Lycopodium prothallus having thus 
been compared with one another, the evidence which the 
resemblances and differences between them afford as to the 
relationship of the species of that genus must now be con- 
sidered. In doing this it is necessary to bear in mind that 
the problem is complicated by the need of adaptive modifica- 
tions of the prothalli to the diverse conditions under which 
they live. In the preceding pages the relation that appears 
to exist between the conditions of existence and particular 
characters of the , prothallus and young plant has been 
repeatedly referred to. The importance to the species in the 
Vascular Cryptogams of satisfactory adaptation of the gameto- 
phyte to its environment can hardly be overestimated. For 
the establishment of the sporophyte in a new situation is 
entirely dependent on the gametophyte having been able to 
come to maturity and produce sexual organs, so that the 
localities of the plants are determined more by their suitability 
for the gametophyte than for the sporophyte, except in so far 
as the latter is spread by vegetative propagation. Since the 
struggle for existence tells largely upon plants in their 
attempts to seize upon fresh situations, it becomes all-im- 
portant in the case of a Vascular Cryptogam that favourable 
modifications should take place in the prothallus. It is thus 
