210 Proceedings of the Royal Physical Society. 
dress bearing only submerged linear leaves. In a perennial 
spring, or in the upper waters of a river, like the Wandle 
at Hackbridge—that is, entirely under the control of such 
springs, the floating rosettes remain through the winter. 
Then, again, there is a form of Ranunculus aquatilis that 
in winter possesses only submerged linear leaves. Shortly 
before it flowers in the spring season, it produces broad, 
rounded floating leaves, and the difference between the plant 
in its winter and summer dress is very striking. The form, 
hk. fluitans, that is found in running streams, and has no 
floating leaves, might be induced to develop them if exposed 
to the surface-heating of a pond. 
Everything in plant-life is behindhand in a river in com- 
parison with a pond. The seeds floating in the drift germinate 
some weeks later, and the flowering and fruiting processes 
are, in a corresponding degree, postponed. The temperature 
of 50°-0 is, in this climate, a critical temperature, as the year 
advances, for aquatic plant-life, not only as regards germina- 
tion, but also as concerns the growth of the bud. It was 
attained by the Thames in 1893 on March 31st, and in 1894 
on March 29th; whilst, in the marginal shallows of the 
ponds, it was reached in the middle of February. It is in 
these marginal shallows that floating seed-drift collects, and, 
as a result, the floating seeds germinate much earlier in a 
pond than in a river, and the floating buds, bulbs, and root- 
stocks commence their growth at a correspondingly early 
date. On account, also, of the relatively great surface- 
heating of a pond, the flowering process is greatly forwarded. 
During the hot summer of 1893 the highest temperature 
attained by the Thames was 75°, and for twenty-five days 
its temperature was 70° or over. In the cooler summer of 
1894 the maximum Thames temperature was 73°°3, and the 
days during which the river maintained a temperature of 
70° and over were ten in number. During hot weather, 
when the river-temperature was 70°, it was not uncommon, 
in the case of ponds filled with submerged plants, to find a 
surface-temperature of over 80°. Though this surface-heating 
is not exhibited by ponds covered with the large floating 
leaves of such plants as our water-lilies, our Liimnanth, and 
