420 WILLIS : MORPHOLOGY OF THE PODOSTEMACBÆ 
but at length in the cases of extreme dwarfing seen in 
Dicræa, Griffithella, Hydrobryum, and Farmeria, we find 
the expanded green thallus the chief organ of assimilation, 
taking on more or less of the anatomical and morphological 
construction of a leaf, or rather of the flat thallus of many 
Bryophyta or Algæ. These two features, the dwarfing of 
the shoots and the expansion of the thallus, are evidently 
more or less correlated, but we are met by the usual diffi- 
culty, whether one or the other, or neither, may be regarded 
as cause or effect. 
The peculiar thalli just mentioned may be roughly divided 
into two groups, the algal forms of Dicræa and Griffithella 
(and some Podostemons to a less extent), and the more lichen- 
or liverwort-like forms of Hydrobryum and Farmeria. The 
extraordinary resemblance of many of these plants to algæ, 
especially the seaweeds of the rapidly moving or rising and 
falling water of the seabeach, has often been noticed, and 
some special cases of resemblance in detail have been pointed 
out above. Comparison of the general conditions of life 
given in the introduction with those which obtain for sea- 
weeds in such positions as are occupied by many Fuci, 
Himanthalia, Bostrychia, and many more, will shoTv that 
there is a considerable agreement in the two cases. That 
this is so is also suggested by the behaviour of the interesting 
algæ described by Goebel,'" found at the tidal mouths of large 
rivers (but in fresh, not brackish water, owing to the great 
size of these streams), where they are exposed to the air 
during part of the day, Bostrychia Moritziana is one of these 
forms, which in its morphological construction is closely 
analogous to Podostemon subulatus, and which also occurs 
in the rapids of the mountain streams of Guiana, occupying 
there the same localities as the Podostemacea, Oenone 
Imthurni, a plant with filamentous creeping roots, upon which 
are borne prostrate secondary thalli of shoot nature, but not 
* lieber einige Süsswasserflorideen ans Britiscli-Guyana ; Flora, 83, 
1897, p. 436 ; Eine Süsswasserfloridee aus Ostafrika ; Flora, 85, 1898, p. 65 ; 
and Organographie der Pflanzen, pp. 30-34, &c. 
