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_ REVUE BRYOLOGIQUE 87 
seems almost a truism, that all the varieties of Æ. aduncum have 
been evolved from 4. aduncum itself. When it is suggested that 
the var. intermedium reverts through the var. polycarpon to the 
var. falcatum, Uhe possibility of such an origin in these forms 
cannot be denied, although no real evidence of this is fortheoming. 
The mere fact of two varielies growing in different parts of the 
same pond is hardiy convincing proof that one originated from 
the other. It is true that many of the varieties (as often occurs 
with aquatic plants) are very variable, a fact which Mons. 
Rènauld has impressed upon all who have attempted Lo follow 
his able classification of these plants in the «Muscologia Gallica ». 
ln this respect the Harpidia offer no exception to the other groups 
of critical species, either of the higher or lower natural orders. 
Although wetter or dryer conditions materially afect the size and 
appearance of the plants grouped under /1. aduncum, my own 
abservations do not lead me to consider that the vars. typicum, 
intermedium and pseudofluitans are convertible by such edaphic 
factors one into the other. The problem is much more complex. 
Hardly anything is known of the ecology and phylogeny of the 
mosses. Some of our named forms; the vars. orthophyllum 
__ and falcatum, for instance, which occur under several species, 
seem hardly worth maintaining. But who can say why some 
mosses have straight leaves and others falcale, ones? In 
some species they are never falcate, in others always 50 
‘and in a few apparently sometimes falcate and sometimes 
Straight, Until we know more as Lo the causes determining the 
various morphological features it seems very desirable to retain 
as named forms those exhibiting unusual characters, especially 
when they show some degree of permanence under varying condi- 
tions over considerable areas. In support of his view as to the 
conversion of one variety of Æ. adunceum into another, M° Ingliam 
offers the suggestion that the production of simple or pinnate 
stems is merely a question of ecological conditions. At first sight 
there appears some plausibility in the idea that the pinnate bran- 
ching of stems is due to lateral illumination, caused by the plants 
becoming stranded during dronght, and falling into a horizontal 
. position ; and that elongate, little-branched stems are produced 
by erect growth in deep water. It is quite true that H. pseudoflui- 
tans (usually a submerged plant) often presents in its stem a long 
unbranched apical portion, and this peculiarity may be due as 
suggested, to an effort to reach the light. But the most pinnate 
form of {. aduncum that I have met with, and which I distributed 
through the British Moss Exchange Club in 4897, grew entirely 
"RE 
