354 Bower . — On antithetic as distinct from 
logons generations which are fundamentally alike as regards 
descent ; the differentiation may be simply as regards the 
character of the reproductive organs which they bear, or 
there may also be a differentiation of them as regards form, 
which, though clearly seen in some animals (e.g. the Medusae), 
is among plants of more rare occurrence. As in the case of 
antithetic alternation, so also may the homologous alternation 
be correlated with changes of external condition, and in some 
of the more protean forms the interdependence would appear 
to come out clearly: a series of examples will now be dis- 
cussed by way of illustration of what is meant by homologous 
as distinct from antithetic alternation. 
Taking first the Siphoneae, this protean family will prove 
most instructive. Though not the simplest of these plants 
in its sexual process, Vaucheria has a comparatively straight- 
forward life-history : the zygote after a resting period ger- 
minates directly into a new Vaucheria plant, which may 
reproduce itself by brood-cells throughout a series of genera- 
tions, which are similar to one another : finally on plants 
resembling these in every respect except in the reproductive 
organs which they bear, the oogonia and antheridia are pro- 
duced and fresh zygotes formed. Now here is a series of gen- 
erations similar in all essentials except one : there is no reason 
to regard them as showing a true or antithetic alternation, 
but they should all be recognised as potential gametophytes 1 , 
a differentiation of them inter se having taken place to the 
extent that some produce only brood-cells, others sexual 
organs : in a similar manner the gametophyte of a Marchantia 
or Tetraphis may for a series of generations reproduce itself 
by gemmae (gametophytic budding) and the last generation 
bear sexual organs. If we are to recognise the sporophyte 
at all, it is in Vaucheria represented only by the zygote : 
tion, in view of the more striking phenomena of antithetic alternation in plants, 
while on the other hand Zoologists are apt to regard the latter as on a par with 
alternation in animals. Both these dangers are avoided by retaining the words 
in their original sense. Compare De Bary, Fungi, p. 125. 
1 ‘Potential oophores’ is a term applied to the corresponding generations of 
Coleochaete by Vines, Lectures on Physiology, p. 632. 
