HABITS OF SPECIES OF LEMNE. 329 
and sometimes less, this being the mean temperature of the week 
preceding the budding. Imitating at home the thermal con- 
ditions of the central parts of a pond with a daily range of 
five or six degrees, my estimates of two different years were 51° 
and 53°, Observations in the shallow water of a pond, where 
the range is more than twice as large, gave me estimates of 
between 53° and 54° as the mean temperature of the preceding 
week. It will thus be seen that the results obtained are much the 
same whether in the shallows or centre of a pond, or in a river. 
Since, however, it is at the margins of ponds that these winter 
fronds gather in the spring, I will deal more particularly with 
this part of a pond, not only as regards the daily mean tempe- 
ratures, but also with regard to the run of the daily maximum 
temperatures. When, as is usual at the beginning of April, the 
temperature of the water rises to 60° in the day, and the daily 
mean is 53° or 54°, the winter fronds commence to bud briskly. 
When the daily maximum rises to 65°, the summer plants are 
half developed. When it rises usually to 70°, as happens in 
May, the fully-formed plants abound: this represents a daily 
mean of 62° or 63°, and it is noteworthy that it is not until the 
temperature of the river Thames reaches 60° that the summer 
plants are found on its waters, the river lending its testimony to 
the evidence of the pond. 
Coming to Lemna minor, my notes contain nothing that in 
any way supplements Hegelmaier’s description of their habits. 
During three winters, both at home and in the ponds, they have 
floated to the spring. In three successive springs in my green- 
house, they began to bud many weeks before the winter fronds 
of Lemna polyrrhiza, in the same vessel, showed signs of com- 
mencing. At the margin of a pond they begin to bud actively 
when the mean maximum temperature of the water for a week 
reaches 45°, which represents a mean temperature of about 40° 
for that period. 
The seeds came under my notice in February 1891 and 1894, 
floating amongst the seed-drift of ditches, ponds, and rivers, sub- 
sequently germinating at the surface, and producing the typical 
plant. On February 2nd of this year there were a few seeds 
germinating in a pond where I was recording the temperature, 
an observation which enables me to say that the thermal con- 
ditions requisite for germination are the same as above given for 
the budding of the fronds in early spring. The budding and 
germination were going on side by side in the same pond. 
LINN. JOURN.—BOTANY, VOL. XXX. 2a 
