2 45 
Algal Ancestry of the Higher Plants . 
which only the prostrate portion is developed, and there is the same 
diversity of habit as in the Chastophorales, though a terrestrial series 
is lacking. In many of the forms with a well-developed base, there 
is a tendency for the asexual (unilocular) sporangia to appear as 
lateral branches from the base of the upright threads, whilst the 
gametangia (plurilocular sporangia) are borne near their apex; this 
is well seen in Myrionemn vulgare 1 (Fig. 2, a). It will be noticed 
that this arrangement is just the reverse of what obtains in 
Trentepohlia. 
Throughout the Phseosporeas the thallus betrays a more or less 
marked differentiation into prostrate and erect portions, although 
this is very much obscured in some cases. It appears, however, to 
be a safe assumption that this type of construction is the primitive 
one for the whole group. 
The cases of alternation among the Brown Algae that have been 
fully studied are those of Cutleria, Zanardinia and Dictyota, and a 
consideration of these will suffice. In Dictyota , the two generations, 2 
indubitably identical in morphological construction as they are, 
could in no way be supposed to have arisen from different parts of 
an ancestral thallus; rather, they appear as two similar individuals, 
one of which has become sporogenetic, the other gametogenetic. 
The creeping base seems to have become completely suppressed in 
this case, although traces are seen in allied forms. Dictyota 
furnishes us with an instance of strictly homologous alternation, and 
the writer would put this case in a special category and does not 
regard it as directly comparable to the kind of homologous alternation 
that is supposed to have arisen in the line of evolution of the 
Pteridophyta. Zanardinia 3 seems to display the same kind of 
alternation as Dictyota, although here, possibly, it is the creeping 
system that has persisted and that has furnished the two homologous 
individuals. The two cases just cited are probably instances of the 
numerous modifications in the method of alternation which we may 
expect to find in so plastic and primitive a group as that of the Algae. 
The case of Cutleria is different. Here, apparently, we have a 
good instance of antithetic alternation, but appearances are 
deceptive. The life-cycle, whose cytology has been investigated by 
1 cf. Oltmanns, loc. cit., p. 383, fig. 235 ; Sauvageau in Ann. sci. nat., Bot., 
8 ser., V, 1898. 
’ cf. especially Lloyd Williams, in Annals of Botany, XVIII, 1904, pp. 141, 
183 ; Hoyt, in Bot. Gaz., L, 1910, p. 55. 
* cf. Yamanouchi, in Bot. Gaz., LVI, 1913, pp. 1—35. 
