241 
Algal Flora of the Tropics . 
the group of the blue-green Algae also plays a great part in the aquatic 
algal flora of the tropics (cf. p. 243). Amongst all the different groups 
of Algae probably none present so many unique features as do the 
Cyanophyceae. It is only necessary to recall the complicated cytology, 
the curious structure of the investment, the very characteristic propagation 
by means of motile filaments or hormogonia, and the peculiar heterocysts 
found in so many members of the group. Extensive study of the 
Cyanophyceae leads one to the gradual conviction that one is dealing 
with a very ancient phylum of the vegetable kingdom, which has in part 
retained a number of primitive features, although along other lines it has 
become highly specialized. There is little fossil evidence in support of this 
point of view, but so little is known about fossil Algae generally (except 
some of the calcareous forms) that the absence of such evidence cannot 
be regarded as in any way a proof against the ancient character of a group. 
A few doubtful fossils (e. g. Zonatrichites , Bornemann, and Gloioconis 
Borneti , Renault) have been referred to Cyanophyceae, and certain oolitic 
structures have been regarded as the result of the activity of members 
of this group, but these data are very fragmentary. Seward 1 discusses 
their possible presence in former periods as follows : ‘ Although our exact 
knowledge of fossil Cyanophyceae is extremely small, it is probable that 
such simple forms of plants existed in abundance during the past ages 
in the earth’s history. Several writers have expressed the opinion that 
the blue-green Algae may be taken as the modern representatives of those 
earliest plants which first existed on an archaean land-surface. The living 
species possess the power of resisting unfavourable conditions in a marked 
degree, and are able to adapt themselves to very different surroundings. 
Their occurrence in hot springs proves them capable of living under 
conditions which are fatal to most plants, and suggests the possibility of 
their occurrence in the heated waters which probably constituted the 
medium in which vegetable life began V In view of the exceeding 
abundance of this group of Algae in the tropics one can scarcely resist 
the assumption that their habitats in these regions are more in accord with 
their former conditions of existence than the habitats of temperate regions. 
A considerable number of Cyanophyceae have indeed become acclimatized 
to the changed conditions in these latter parts of the world, but the group 
is certainly prevalently tropical. Moreover, it was only in the damp 
tropics that many forms were able to leave the water and live a more or less 
terrestrial life ; after all, the tropical subaerial habitats are semi-aquatic in 
the conditions they offer. The abundance of the blue-green group in the 
tropics is not the only evidence for the view that these regions of the 
world are more like their habitats in former epochs of the earth’s history. 
1 Fossil Plants, vol. i, Cambridge, 1898, p. 131. 
2 Cf. also Warming, Okologische Pflanzengeograpkie. German Edition, Berlin, 1896, p. 158. 
