506 INTRODUCTION TO PLANT GEOGRAPHY 



the so-called ' aerenchyma ', which often dominate their anatomy, 

 extending through their bodies right down to the roots. These 

 last have a particularly difficult problem of aeration for respiration 

 in the oxygen-poor ooze. 



Finally, although some annual aquatics occur, such as Naiads 

 {Najas spp.), most higher types are perennial, either living through 

 any unfavourable season apparently unchanged or else dying down 

 or, if pleustonic, often descending to the bottom. Even in cool- 

 temperate regions, many shallow-water types appear to continue 

 some photosynthetic activity in winter under the ice — ^at least as 

 long as this is not covered by a darkening layer of snow. 



Further Consideration 

 For additional details : 



P. S. Welch. Limnology, second edition (McGraw-Hill, New York etc., 



pp. xi + 538, 1952). 

 F, RuTTNER. Fundamentals of Limnology, translated by D. G. Frey 



& F. E. J. Fry (University of Toronto Press, Toronto, Ont., 



pp. xi + 242, 1953). 



F. R. MouLTON (ed.). Problems of Lake Biology (American Association 



for the Advancement of Science, Washington, D.C., pp. 1-142, 1939). 



Useful accounts of particular aspects are to be found in the appropriate 

 parts of : 



A. F. W. ScHiMPER. Plant-geography upon a Physiological Basis, transl. 

 and revised edition (Clarendon Press, Oxford, pp. xxx -j- 839 and 

 maps, 1903). Especially useful in this connection is the ' third ' 

 German edition, Pflanzengeographie auf physiologischer Grundlage, 

 revised by F. C. von Faber (Fischer, Jena, vol. I, pp. xx + 588, 

 and vol. II, pp. xvi -f- 589-1613 and maps, 1935). 



V. J. Chapman. An Introduction to the Study of Algae (Cambridge 

 University Press, Cambridge, Eng., pp. x + 387, 1941)- 



G. M. Smith (ed.). Manual of Phycology (Chronica Botanica, Waltham, 



Mass., pp. xii + 375, 1951). 

 K. E. Carpenter. Life in Inland Waters : with especial reference to 



animals (Sidgwick & Jackson, London, pp. xviii + 267, 1928). 

 G. E. Hutchinson. A Treatise on Limnology : vol. I, Geography, Physics, 



and Chemistry (Wiley, New York, pp. xiv + 1015, 1957). 



