HABITS OF SPECIES OF LEMNZ. 327 
work, As pointed out there, though destined for the winter, 
they are largely produced in summer, and, as before remarked, 
they never came under my notice growing in such numbers 
from the mother plant as in the first week of July in the tepid 
water of a stagnant ditch where Lemna gibba was in flower. 
This budding process goes on until about the end of October, 
when the summer plants all die. My temperature observations 
during the autumns of 1892 and 1893 on the Thames above 
Kingston and on several ponds in that neighbourhood, all plainly 
indicate that when the surface temperature in the day falls short 
of 50° Fahr. rapid decay and death of the summer plants ensue. 
The winter fronds of Lemna polyrrhiza, when first detached, 
nearly always sink; but occasionally they cannot be sunk, such 
fronds being rather thicker than the average. Whilst some pass 
the winter afloat, others spend it at the bottom. Hegelmaier 
speaks of them as all remaining under water; but they vary 
much in this respect with the character of the weather at the 
time of their growth, and with the other conditions under which 
they are developed. They are to be found constantly in winter 
amongst the floating seed-drift of the rivers Lea and Thames, 
and of the ponds around Kingston ; and through the past winter 
they floated in great numbers on one of the Home Park ponds. 
In a fine calm autumn with plenty of sunshine, the sunken 
winter fronds in the shallow water are very liable to be floated 
up by air-bubbles, and they acquire independent floating powers 
at the surface. In such weather, also, the mother plants 
die slowly, and their winter offspring remain sufficiently long 
attached to be able to float of themselves. On the other hand, 
in rainy and windy autumns the disturbance of the surface of 
the water brings about the detachment of the winter fronds 
before they can float, and they all go to the bottom. 
During two winters I have kept under observation both the 
winter fronds that could not be sunk, and those that remained at 
the bottom. Various agencies assist in floating up the sunken 
fronds in spring. When exposed in a clean glass vessel to 
moderate sunshine, they are carried up by bubbles of their own. 
forming, and, throwing off the water, they emerge at the surface. 
For the first few days they can always be sunk at a touch, but 
after that they float of themselves. Without the sun, these 
fronds usually remain at the bottom, become sickly, and die. 
Those that bave remained under water all the winter are able to 
