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CHAPTER XXII 



THE INFLUENCE OF CERTAIN PHYSICAL 

 FACTORS IN THE LIFE OF WATER PLANTS 



THE physical conditions, under which water plants have 

 their being, differ widely from those which affect land 

 plants. We have already considered the special features of the 

 gaseous exchange and the water supply due to life in a liquid 

 medium instead of in the atmosphere; it now remains to discuss 

 the influence of certain other factors especially temperature, 

 illumination and gravity upon plants growing in water. 



When the thermal conditions of land and water plants are 

 compared, the chief difference is found to be the smaller range 

 of temperature variation both diurnal and seasonal which 

 aquatics are called upon to endure. Though the truth of this 

 statement is universally recognised, it is based upon relatively 

 few exact observations, and further detailed field work is much 

 needed upon the temperature variation in different types of 

 waters, and the relation of this variation to vegetable life. A 

 notable beginning in this direction has been made by Dr 

 Guppy^, to whom we owe many original observations on the 

 bionomics of aquatics. He has shown that during a summer 

 day and night, when the range of shade temperature in the air 

 may be about 1 1 C, the range in the water of a river, such as 

 the Thames at Kingston, may be as little as about o-8 C. The 

 smaller the stream, the greater the range of variation ; a little 

 brook, two or three feet across and only three or four inches 

 deep, may show a variation in 24 hours of about 8 C, that is 

 to say, about three-quarters of the range in the air, but ten times 

 the range in the river. Irrespective of the size of the body of 



^ Guppy, H. B. (1894^) ; the results in this paper are given on Fahren- 

 heit's scale, but in the present chapter they are quoted in Centigrade 

 terms for the sake of uniformity. 



A. w. p. i8 



