xx] LACK OF OXYGEN 255 



leaves may, for instance, be very thin, but extensive in area, as 

 in the case of the submerged leaves of the Waterlilies, or they 

 may be sub-divided into hair-like segments, as- in Myriophyllum , 

 etc.^ In certain Podostemaceae belonging to the genus Oenone^y 

 curious hair-like outgrowths, rich in chlorophyll, are developed 

 on the leaves (Fig. 81, p. 119). These outgrowths, from their 

 presumed analogy with the breathing organs of water animals, 

 have been called gill-tufts (Kiemenbiischel), though it has not 

 been proved that they possess a respiratory function. The 

 negatively geotropic roots of Trapa natans^^ the Bull Nut (Fig. 

 160, p. 247), provide another example of a finely divided sub- 

 merged organ, by means of which gaseous exchange can readily 

 take place. The intimate contact achieved between these organs 

 and the water, probably assists not only assimilation but also 

 respiration. 



It is true that dissolved air is richer in oxygen than atmo- 

 spheric air, about one-third of its volume consisting of this 

 element, but the essential point to bear in mind is that the 

 total volume of air held in solution in water at ordinary tempera- 

 tures is so extremely small that in a litre of water the maximum 

 amount of oxygen present is 10 cubic cms., as compared with 

 more than 200 cubic cms. in a litre of atmospheric air 4. The 

 result is that water plants have considerably less oxygen at 

 their disposal in each unit volume of the surrounding medium 

 than is the case with land plants^; as far as hydrophytes are 

 concerned, oxygen is a rare and precious commodity. 



Thus, on account of the poverty of the medium in this 

 element, no plant can be a successful aquatic unless it has a 

 special capacity either for obtaining an adequate oxygen supply, 

 or for husbanding it when obtained. 



Every green plant forms oxygen as a by-product of carbon 



^ See Chapters xi and xii. 



2 Goebel, K. (i 891 -1893) and Matthiesen, F. (1908). 



3Goebel, K. (i 891 -1893). 



* Regnard, P. (1891) ; see also Forel, F. A. (1901). 



