328 MR. H. B. GUPPY ON THE 
throw off the water from the first, and do not acquire this power 
at the surface. If there is any impediment to rising, the sunken 
fronds begin to bud at the bottom, and through the buoyancy 
thus obtained they sooner or later appear on the top. Usually, 
however, the sunken winter frond is independent of its own 
efforts in floating up. The bubbles, evolved in quantity through 
the excitement of vegetable activity on the mud of the pond, 
carry them up in numbers, and once at the surface their 
power of throwing off the water keeps them afloat, and in 
calm dry weather they will, in a few days, acquire independent 
buoyancy. 
The time when the winter fronds of Lemna polyrrhiza begin 
active budding is generally in April; but this is mainly deter- 
mined by the temperature of the water, and though the amount 
of sunshine received is the same, the fronds in the shallows will 
always be more advanced in the growth of the summer plants 
than those in the cooler and deeper water. 
In considering the temperature at which the winter fronds 
begin to bud into the summer plants, it is as well to remember 
that an estimate of the initial temperature of budding or of ger- 
mination of a water plant, to be of any practical value, should be 
brought into some kind of relation with the thermal régime of a 
ditch, pond, or river *. This subject can only be touched upon 
here. Rivers, on account of their lack of surface-heating, their 
small daily range of only one or two degrees (Fahr.), and their 
comparatively uniform rise of temperature with the advance of 
spring, afford very valuable opportunities of ascertaining the 
temperature of budding or germination of floating plants or seeds. 
Here a single forenoon observation gives the day’s mean tempe- 
rature. With ponds we get the disturbing influence of daily 
range, not very marked it is true; in the centre of a pond 
it would be only five or six degrees, but considerable at the 
margin, where it would amount to some twelve or more degrees. 
A comparison of the daily mean temperatures of a river and 
a pond is, therefore, theoretically open to objection; but in 
practice inconsistent results are not obtained. 
From my Thames observations I estimated the initial tempe- 
rature of active budding in the river, in the case of the winter 
fronds of Lemna polyrrhiza, at about 55° Fahr., sometimes more 
* Tam dealing with this subject in a series of papers, of which the first 
was read before the Royal Physical Society of Edinburgh in January. 
