326 MR. H. B. GUPPY ON THE 
surface owing to their power of repelling water after being 
floated up by bubbles. As the winter advanced, the fronds lost 
their dark hue and the distinctness of their nerves; and after the 
beginning of the year the plants could not, in general form 
and appearance, be distinguished from those of Lemna minor. 
Budding, which had been suspended during the winter, began 
briskly early in February, similar fronds being produced; but 
up to the fourth week of April they exhibited only a slight 
sponginess in appearance, and would have been taken for 
vigorous forms of Lemna minor. Up to the time of writing, it 
was evident from the temperature observations made at the 
ditch and in my vessels, that for the development of the gibbous 
character the winter plants require, for a week or more, an 
average daily maximum temperature at the surface of the water 
of not much, if at all, under 70° Fahr. 
It would seem from the above that the form in which Lemna 
gibba passes the winter in this climate depends a good deal on 
the thermal conditions of the locality it frequents in summer. 
Where the water is cool and there is little or no surface-heating, 
as in a spring-fed brook or in a river, the gibbous plants would 
never flower and probably also would not develop special winter 
plants, but would float unimpaired through the winter. In the 
warm and stagnant water of a shallow ditch the plants would 
most often flower; but the flowering process, by exhausting 
their vitality, would result in the intervention of a peculiar 
winter form. That the intervention of a special winter form 
depends indirectly on a high summer temperature seems a little 
paradoxical; but my temperature observations indicate that 
there is much to learn about the habits of this plant. 
Lemna polyrrhiza, like Lemna gibba, has a winter form, but 
the resemblance there ends. Whilst with Lemna gibba the 
winter fronds, after detachment, bud in the autumn and pass the 
winter as complete plants, which attain in the spring the peculiar 
gibbous form, with Lemna polyrrhiza the winter frond remains 
solitary and produces, by budding, the summer plant without 
ultimately becoming incorporated with it. When the winter 
trond of Lemna polyrrhiza has performed its office, it becomes 
sickly, sinks, and dies. 
The description of the solitary, dark green, reniform, and 
rootless fronds that enable Lemna polyrrhiza to pass the winter 
in this climate without seeding, will be found in Hegelmaier’s 
