170 Trow. — Observations on the Biology and 
so far as they are known, are confined to the sporophyte 
and to a stage in development which we may call sporogenesis. 
In the Thallophytes we have frequent cases of polymorphism, 
as in the Uredineae. Where the number of forms is reduced 
to two and these alternate with one another, we have what is 
frequently called an alternation of generations. Such instances 
are in reality, as a rule, only special cases of dimorphism of 
the gametophyte. They occur in Cyst opus, Cutler ia, Achlya , 
Saprolegnia , &c. Many of the Thallophytes are, however, 
monomorphic, and they may in that case be represented by 
gametophytes, as in Fucus , or by a plant-body with asexual 
reproduction alone, as in Agaricus. There may be a true 
antithetic alternation of generations in such forms as those 
met with in the Florideae and Ascomycetes. It is possible 
that the Uredineae present us with a specially interesting 
condition — that in which we may have three generations 
taking part ; — viz. asexual gametophyte, sexual gametophyte, 
and sporophyte. Such, at any rate, is the logical consequence 
of the acceptance of De Bary’s view as to the systematic 
position of this group of plants. Whether the alternation of 
generations is strictly homologous need not concern us greatly. 
Alternation of generations may have arisen independently 
several times as is clearly the case with heterospory, secondary 
thickening, sex, &c. 
Strasburger’s view that the thallus of Fucus may be a 
sporophyte deserves special attention. To my mind it is 
negatived from every point of view but that of karyology. 
We do not know enough of the nucleus in plants, however, to 
enable us to use it exclusively as a means for deciding com- 
plex phylogenetic questions. What we do know should 
make us very sparing of its use. Moreover, as we shall see, 
there is no necessity, even from the point of view of karyology, 
for such a topsy-turvy treatment of the Thallophytes. We 
may, upon the safer ground of comparative morphology, have 
no hesitation in regarding the plant-body of Fucus as a game- 
tophyte. Reducing-divisions occur, as we have said, amongst 
Thallophytes, in the gametangia of Fucus , Dictyota, Achlya , 
