296 Proceedings of the Royal Physical Society. 
abounded, a surface temperature of 82°, whilst the bottom, 
3 feet down, was 73°. Early in July I observed this plant 
flowering under closely similar conditions, with a differ- 
ence also of 9° between the top and the bottom, the surface 
temperature of 83° being 11° higher than in the case of the 
Thames. It would seem probable that though Myriophyllum 
is able to flower in the stream of the Thames, it would not 
often fruit. Though the initial temperature of germination 
for Myriophyllum spicatum proved in my experiments to be 
as low as 49°, I estimate that it needs a surface-temperature 
of about 70° to produce its flowering spike. This heat 
was reached on about twenty-one days in the Thames 
last summer, the highest reading being 75°. 
In the middle of June, in a pond 3 to 4 feet deep, on 
‘Oxshott Heath, luxuriant masses of Scirpus fluitans were in 
flower, the upper portions of the plants being in water of a 
temperature of 82°, whilst their lower parts were in water 
11° or 12° cooler. I have noticed Ceratophyllum demersum 
flowering abundantly in rivers like the Lea, but never in 
mature fruit, and after a careful comparison of my river and 
pond temperatures, I formed the conclusion that whilst this 
plant requires an average daily maximum temperature of 70° 
to produce its flowers, a warmth of 80° and over is necessary 
to mature its fruit, a condition to be found in this country 
in shallow ponds, where the plant fruits abundantly, but 
not in rivers, where they may flower, but do not, as far as 
I know, mature the fruit. The.thermal conditions for the 
maturation of the fruit occur in our shallow ponds, those for 
the flowering only are found in our rivers. 
Summing up the results of the comparisons of the Thames 
and the ponds in the vicinity of Kingston during last year, 
I may say that on sunny days the surface-waters of the 
ponds were, in March, 2° or 3° (F.) warmer than the river, 
in April 3° to 5°, in May and June 6° to 8°, and in July and 
August 6° to 12°. This goes to explain the fact, that during 
the past summer the Limnanth and the water-lilies of the 
ponds were, as regards fruiting and flowering, some weeks in 
advance of those of the river. It is to be remarked that 
the surface-heating is always more pronounced in a pond 
