[ 260 ] 



CHAPTER XXI 



ABSORPTION OF WATER AND TRANSPIRATION 

 CURRENT IN HYDROPHYTES 



ONE of the unfortunate results, which followed the 

 publication of The Origin of Species, was the acutely teleo- 

 logical turn thus given to the thoughts of biologists. On the 

 theory that every existing organ and structure either has, or has 

 had in the past, a special adaptive purpose and " survival value," 

 it readily becomes a recognised habit to draw deductions as 

 to function from structure, without checking such deductions 

 experimentally. Many points in connexion with the study of 

 aquatics, and, notably, the whole subject of the absorption of 

 water by such plants, have suffered profoundly from this ten- 

 dency. Two of the most conspicuous anatomical characters of 

 hydrophytes, as compared with land plants, are the relatively 

 small amount of cuticle 1 on the surface of the epidermis, and 

 the poor development and lack of lignification of the xylem. 

 From these facts it has been lightly concluded that submerged 

 plants, being able to absorb water over their entire surface, have 

 simply dispensed with the transpiration current from root to 

 leaf which is universal among land plants, and that their roots 

 have lost all function except as attachment organs. These ideas 

 have become text-book platitudes, and may still be found even 

 in the writings of professed physiologists 2 , despite the fact that 

 they have been, to a large extent, refuted by a series of experi- 

 mental investigations by different observers, the first 3 of which 



1 Cuticle, though small in amount, is invariably present on the epi- 

 dermal walls of aquatics. See Geneau de Lamarliere, L. (1906). 



2 See for example Hannig, E. (1912), where the author speaks of 

 submerged plants " bei denen kein Transpirationsstrom existiert." 



3 Unger, F. (i 862). For a recent discussion of the subject see Snell, K. 

 (1908). 



