324 MR. H. B. GUPPY ON THE 
drought. It is conceivable that during changes of climate the 
Lemnw might long find a refuge in the vicinity of perennial 
springs. 
Coming to the individual species, I will first refer to Lemna 
gibba. The plants with gibbous fronds as a rule die long before 
the winter, and new plants similar in form appear in April or 
May. Now and then, however, in mid-winter, a few healthy 
gibbous plants are tv be found in ditches, ponds, and rivers, 
which bud freely in the spring. A number of plants, kept 
through the winter of 1892-93 in a room without artificial heat, 
retained their buoyancy and gibbous form, and were quite healthy 
when they budded in the spring. The temperature of the water 
was for some weeks under 40° Fahr., and for ten consecutive 
days it was under 36°, the plants being inclosed in ice for a day 
or two. Evidently, therefore, low temperature is not of itself 
injurious to the gibbous plants; and since, as shewn below, they 
die in numbers iv the tepid water of August after flowering in 
July, their death before winter is due to the exhaustion of their 
vitality and not to the fall of temperature. 
Assuming that this plant is sometimes able and at other 
times unable to float through the winter in the gibbous form, it 
becomes a question, when the plants disappear in the autumn, 
whether the new plants in the spring are produced from seed, or 
whether, as in the case of Lemna polyrrhiza, there is a special 
winter form. Hegelmaier inclined to the latter view in the text 
of his work (page 64), and although the winter form of the 
plant had, in a sense, escaped his observation, in his supplement 
he welcomed the discovery of Dr. van Horen in this respect. 
The following notes bear on this matter :— 
In the first week of July, 1893, I found in the almost stagnant 
water of a ditch, connecting the Diana Pond with the other 
large ponds in Bushey Park, a truly enormous quantity of Lemna 
gibba, abundautly in flower, and mixed with a large amount of 
separate thin flat fronds, just like those of Lemna minor, and 
also in flower. Mingled with them were a great number of the 
plants of Lemna polyrrhiza, not in flower, but producing the 
winter fronds to an extent I have never seen before or since. 
Beneath the floating mass of duckweed, which was two or three 
inches thick, flourished Lemna trisulca, but not in flower. The 
temperature of the surface of the ditch in the afternoon of 
Juiy 7th was 87° Fahr., and at the bottom, eight inches down, 
