112 ] 



CHAPTER IX 



THE LIFE-HISTORY OF THE TRISTICHACEAE 

 AND PODOSTEMACEAE 1 



A^L the families of aquatics hitherto considered are 

 represented in our own country; some of them, e.g. the 

 Potamogetonaceae, show a marked preference for temperate 

 regions, while others, e.g. the Lemnaceae, seem equally at home 

 in both the hotter and colder parts of the world. The Tristi- 

 chaceae and Podostemaceae, however, whose life-history we 

 propose to touch upon in the present chapter, are, with rare 

 exceptions, confined to the tropics. That they are essentially 

 plants of hot regions, is indicated by the statement of Dr Willis 2 

 that the forms living in the low-country of Ceylon and S. India 

 inhabit water which maintains a very constant temperature of 

 80 F. (27 C.). The two families together form an anomalous 

 group, characterised, as regards their morphology, by remark- 

 able variety, but agreeing, as regards their ecology, in one 

 singular feature a preference for inhabiting water which flows 

 rapidly or even torrentially over a rocky substratum. This 

 peculiarity, sometimes rendered more noticeable by reason of 

 the striking colour of the plants, has been observed from the 

 earliest time at which Podostemads became known to botanists. 

 The first recognition of a member of this group as the type of a 



1 General accounts of these plants will be found in Gardner, G. (i 847), 

 Tulasne, L. R. (1852) and Warming, E. (1881, 1882, 1888 and 

 1891). They have only recently been divided into these two families 

 (Willis, J. C. 19 1 5 1 ), and many authors still refer to them all as Podo- 

 stemaceae. 



2 Willis, J. C. (1902). This interesting memoir has been largely drawn 

 upon in the present chapter; it contains a bibliography of previous work. 

 See also Willis, J. C. (I9H 1 ), (iqiS 1 ) and (1915*) and Matthiesen, F. 

 (1908). 



