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SCIENCE-GOSSIP. 



WATER-PLANTS AND THEIR WAYS. 



By H. B. Guppy, M.B. 



{Continued from page 147.) 



THEIR THERMAL CONDITIONS. 



TIT" HEN in recent works relating to aquatic 

 organisms their conditions of existence have 

 been discussed at all, it is too often in terms of the 

 evolution theory. More frequently the evolutionist 

 has been so absorbed in his " hop-skip-and-jump " 

 amongst the forms of life, that he dismisses their 

 conditions of existence in a sentence or a phrase. 

 In this region of investigation, however, intellectual 

 capering is impossible, for the physicist who casts 

 over all nature his greedy eye, has already marked 

 it out. 



In the spring, in the rivulet, the brook, and the 

 river, in the splash, the ditch, the pond, and the 

 lake, we find a variety of thermal conditions, each 

 of which has a direct bearing on the station of a 

 plant and its area of distribution. The perennial 

 rill that carries off the ground-water to a depth of 

 some six feet will in this climate vary only about 

 fifteen degrees in its temperature during the year. 

 Larger springs, rising from depths of between 

 fifteen and twenty feet, according to the observations 

 of Bischof (Chem. and Phys, Geol.) fluctuate five 

 or six degrees in the twelve months ; whilst 

 powerful springs ascending from depths of between 

 forty and fifty feet usually vary about a degree on 

 either side of the mean annual temperature of the 

 locality. Whilst, therefore, a spring-fed stream 

 exhibits at its sources a small yearly range and 

 scarcely perceptible daily fluctuations, as it proceeds 

 on its way it gradually enlarges its extreme range 

 of temperature and displays marked daily changes. 

 These will increase up to a certain distance, 

 when the stream comes under the control of the 

 agencies regulating the air temperature, and there 

 the influence of the spring is eliminated. It then 

 possesses the thermal conditions of an ordinary 

 surface stream. The determination, therefore, of 

 the limit of spring-influence is a matter of import- 

 ance, since with it is bound up the question of the 

 changes of temperature, whether in a year or in a 

 day, a subject of moment to aquatic organisms. 



Beginning with a spring-brook, which is two or 

 three feet across and a few inches deep, my 

 observations around Kingston show that in 

 summer it has to run a distance of 200 or 300 

 yards before its daily variations of temperature 

 "are beyond the reach of the equalising influence 

 of the spring. The amplitude of such variations 

 will then be about three-fourths that of the 

 daily rise and fall of the temperature of the air 

 in the shade. In winter, when the heating-power 



of the sun is much less, the distance traversed 

 by the brook before it acquires an independent 

 daily range will be increased to 350 or 400 yards. 

 Whilst, therefore, in an ordinary brook the 

 influence of the spring is confined to the 

 first few hundred yards of its course, in a stream 

 of any size the springs may extend their influence 

 for miles. I gather from M. Rissler's obser- 

 vations on the Asse, a stream eight or nine feet 

 broad and from twelve to eighteen inches deep, 

 which runs into the Lake of Geneva after rising 

 from unusually cold springs at the base of the Jura 

 Mountains, that after many vicissitudes on their 

 passage its waters are not beyond the control of 

 the springs at Caleves, about four kilometres from 

 their source (Ann. Soc. Met. de France, xvii.). This 

 is indicated by the water being more than three 

 degrees cooler than the air in the early morning in 

 summer ; whereas an ordinary stream of this size, 

 freed from the control of springs, would have been 

 some degrees warmer. The Ach, at Memmingen, 

 in its limited range of the monthly mean tempera- 

 ture (fifteen instead of about thirty degrees), 

 affords evidence of the marked influence of springs 

 after a course of three kilometres (Geogr. 

 Abhandl. Wien, v.). The same may be said of the 

 Kennet, at Marlborough (ten miles from its source), 

 where the monthly means between April, 1889, 

 and March, 1890, ranged only twelve degrees 

 (Brit. Assoc. Report, 1891). The Wandle affords 

 another remarkable instance ; and it would be 

 exceedingly interesting to determine in both these 

 rivers the limit of spring influence. In the case of 

 the Wandle, we have the important series of 

 observations made by Mr. Bayard, and discussed 

 by Mr. Cushing (Proc. Croydon Nat. Hist. Club, 

 1891), and it is to be hoped that the former 

 gentleman will be able to treat the matter from 

 this particular standpoint. 



A promising field of inquiry is to be found in the 

 methodical investigation of the effects of springs 

 on plants, both in the water and on the slopes 

 around. In a spring-brook, where the annual range 

 of temperature is only a few degrees, plants like 

 Ranunculus aquatilis. Nasturtium officinale, Callitriche 

 aquatica, etc., for ever thrive, seeming, in fact, as 

 regards their foliage, to display the greatest vigour 

 in mid-winter. One would expect to find curious 

 modifications in the life-history of a plant thriving 

 near the sources of the Wandle and the Kennet as 

 compared with its more orthodox behaviour further 



