xxii] TEMPERATURE 275 



regulating the depth at which plants can grow. In deep lakes, 

 in which the thermometer at 10 metres below the surface 

 registers about 12 C. in summer, the higher plants are not 

 found at a greater depth than 6 metres. In peat-bog lakes, 

 however, the temperature of the lower layers is unusually high 

 (17 C. to 21*^ C. at 10 metres) and, in these lakes, plants may 

 be found even at a distance of 1 3 metres from the surface. 



When we compare the aquatics of hot and cold countries, 

 we do not find structural differences corresponding to the 

 differences of temperature; there is, in fact, a remarkable 

 uniformity in the general organisation of water plants, whether 

 they live in tropical or temperate climates. On the other hand, 

 they differ markedly in their life-cycles, since those in warm 

 surroundings vegetate continuously, while those which have to 

 pass through a cold season show the special features associated 

 with hibernation, which we have discussed in Chapter xvii. 



We owe to Guppy the discovery that the rarity, in this 

 country, of the flowering and fruiting stages in the life-history 

 of certain hydrophytes, is due to thermal conditions. He has 

 shown, for instance, that Ceratophyllum^ requires almost tropical 

 temperatures for the maturation of its fruit, and that Lemna 

 gibba^ does not flower except in water which is heated, during 

 the summer, to a degree unusual in this country. For many 

 water plants, however, the temperature of optimum vegetative 

 growth is decidedly low^. It has been recorded, for instance, 

 that, in the case of a certain canal near Manchester, which is 

 kept tepid by the entry of hot water from various mills, the 

 vegetation does not develop with any luxuriance. A Pondweed, 

 Potamogeton crispus^ grows in this canal as a dwarfed variety, 

 especially near spots where warm water enters*; critical experi- 

 mental work would, however, be required before we could feel 

 certain of the fact that this result is due to temperature alone. 



To some aquatics, the fact that lakes and rivers remain in 

 summer cooler than the surrounding atmosphere, may be a 



1 Guppy, H. B. (1894I). 2 Guppy, H. B (18942). 



3 Goebel, K. (1891-1893). 4 Bailey, C. (1884). 



182 



