272 ABSORPTION OF WATER [CH. xxi 



habit, such as Potamogeton natans. Again, it is found that all the 

 Nymphaeaceae have their young leaves clothed with mucilage, 

 with the one exception of Nelumbo, the Sacred Lotus. In this 

 plant, on the other hand, the epidermal cells become cuticula- 

 rised relatively early, and thus are able to exert a protective 

 function. The mucilage of the Waterlilies may reach extra- 

 ordinary proportions. In Brasenia Schreberi (pettata) 1 , for 

 instance, the thickness of the layer of slime coating the petioles 

 and flower-stalks may exceed the diameter of the organ itself. 

 Such an abnormal development can scarcely be regarded as a 

 useful adaptation, and it is probably safest to look upon the 

 production of mucilage, both in this and other aquatics, as a 

 mere by-product of the plant's metabolism, any useful purpose 

 that is served being purely secondary. There are certain cases 

 which are particularly difficult to explain on the adaptational 

 view. In Ceratophyllum^^ for instance, in which the growing 

 point and young leaves are cuticularised, curious mucilage hairs 

 occur, but do not seem to give rise to any protective layer. Again, 

 the trichome-diaphragms, formed across the intercellular spaces 

 in the petiole of Nymphaea lutea, are clothed with mucilage, 

 although they are not in contact with water but with the internal 

 atmosphere 3 . 



The problems in relation to water which confront a terres- 

 trial plant, all hinge upon the difficulty of obtaining a constant 

 and adequate supply. In the case of submerged aquatics, on the 

 other hand, the supply is permanently excessive, and the plant 

 can only live successfully in this milieu if it possesses the knack 

 of controlling and regulating its absorption and elimination in 

 such a fashion that a steady upward stream is ensured, while 

 the tissues, especially those that are young and delicate, are 

 preserved from supersaturation. 



1 For a detailed account of the mucilage of this plant see Schrenk, J. 

 (1888), and Keller, I. A. (1893). See Fig. 20, p. 38, for the structure 

 of the mucilage-secreting hairs. 



2 Strasburger, E. (1902). 3 Raciborski, M. (18942). 



