276 PHYSICAL FACTORS [CH. 



drawback, and it has been suggested in this connexion that the 

 development of anthocyanin, which is so frequent in hydro- 

 phytes, may be an adaptation for heat absorption 1 . In con- 

 sidering the general question of the pigmentation of water 

 plants, however, it must be remembered that some of the most 

 striking examples may possibly represent pigmented races 

 derived from the normal specific form by the loss of an in- 

 hibiting factor; on this view, they are comparable with certain 

 coloured varieties well known among terrestrial plants, and 

 there is thus little reason to suppose that their pigmentation 

 bears any relation to the aquatic milieu. Nymphaea lutea, var. 

 rubropetala* for instance may perhaps be compared with the 

 chestnut-red variety of the Sunflower, while a form of Castalia 

 alba z ^ which has been described as bearing rose-purple flowers, 

 may be analogous to the red variety of the white Hawthorn. 

 But, apart from such cases, there are certainly indications that 

 anthocyanin is formed by water plants with special facility. The 

 leaves of the Lemnaceae, Hydrocharis, Limnanthemum, and 

 certain Nymphaeaceae, are often more or less pigmented. The 

 Podostemaceae 4 also, are apt to develop anthocyanin in their 

 surface cells. 



There is, indeed, little room for doubt about the liability 

 of water plants to produce red and violet pigment, but the 

 attempt to explain this fact is fraught with difficulty and con- 

 fusion. The simple teleological explanation which assumes that 

 the development of anthocyanin is an adaptation for the absorp- 

 tion of heat rays, is probably far too facile; the fact that the 

 Podostemads, growing in the tropics, in water which maintains 

 a constant high temperature, very frequently produce these 

 pigments, seems to tell against such a view. The few observa- 

 tions which the present writer has been able to make, do not 

 seem to harmonise with any general statement about the adapta- 

 tional distribution of red and violet pigments in water plants. 

 For instance, in the Forest of Dean (September, 1910) Peplis 



1 Ludwig, F. in Kirchner,O. von, Loew, E. and Schroter,C. (i 908, etc.). 



2 Caspary, R. ( 1 86 1 ). 3 Fries, .(1858). 4 See p. n 3. 



