344 Cleland. — T he Cytology and Life-history of 
a tetrasporic generation. A striking feature of the tetrasporic plant, in 
almost every case, is the fact that morphologically it is almost identical 
with the sexual plant. Galaxaura excepted (Howe, 1917), there have been 
reported no morphological differences between these two generations that 
cannot be correlated with the peculiarities of their reproductive organs ; and 
even in Galaxaura the differences between sexual and tetrasporic plants are 
small and not phylogenetically fundamental. There are no forms in which 
the tetrasporic generation appears to be in process of development from 
something simpler to something more complex. In other words, there is 
no evidence in the red algae pointing to a separate origin and gradual 
development in complexity of the tetrasporic generation comparable to the 
increasing complexity of the alternating sporophytic generation in the 
Bryophytes and Pteridophytes. 
Evolution in the red algae has probably been somewhat as follows. 
All of the red algae at one time were without tetraspores or cytological 
alternation of generations. They were all in agreement in the fact that 
reduction occurred at the first division of the zygote. Although none 
possessed tetraspores, some of them bore monospores, just as some of 
the non-tetrasporic plants to-day possess monospores. The present-day 
tetrasporic plants came about through a postponement of the time of 
reduction, at first perhaps to the germination of the carpospore and finally 
to the following generation. The failure of reduction to occur at carpo- 
spore germination meant that the next generation was diploid instead of 
haploid, as it otherwise would have been. Original monospores then 
probably became the seat of chromosome reduction, developing four tetra- 
spores by the cutting in of walls following the reduction divisions. The 
occasional appearance of abnormal or partly developed tetraspores on 
sexual plants indicates a close relationship between the monosporangium 
and the tetrasporangium, suggesting strongly a homology. All of the 
evidence so far seems to point toward this as the course of evolution in the 
Florideae. The morphological similarity of the sexual and tetrasporic 
plants, the sharp break between haplobiontic and diplobiontic types, the 
presence of incomplete tetraspores on sexual plants and possibly apogamous 
sexual organs on tetrasporic plants, all seem to favour this view. Cyto- 
logical investigation is much to be desired on other forms of the haplo- 
biontic group. It is possible that types may be found in which reduction 
takes place with the germination of the carpospore. Such forms would 
constitute an important link, clearly indicating the above as the probable 
course of evolution. 
We have, therefore, a method of evolution in the Florideae very 
different from that found in the Archegoniates. The situation in the 
Archegoniates . is simple. The sporophyte has been developed as an 
entirely new structure to bear the diploid phase. The sporophyte and the 
