THE DICOTYLEDONES 



1683 



plants have proceeded so far that it is very difficult to recognize the original 

 organs from which the structures have been produced, and in the vegetative 

 state many of them might be mistaken for Thallophytes (Fig. 1542). The 

 minute seeds on germination give rise to an axis which is devoid of any 

 primary root. From this primary axis is budded off endogenously a creeping 



Fig. 1542. — Moineia ueddeliana. A large represent itive of 

 the Podostemaceae. {After Baillon.) 



thallus which is more or less root-like. Its form and mode of growth vary 

 greatly in different genera. In Podostemon it is more or less filamentous, 

 creeping over rocks to which it is attached by exogenously developed 

 haptera. Secondary shoots arise endogenously from the thallus and may 

 produce minute leaves, and subsequently the flowers. These flowers are 

 hermaphrodite, achlamydeous and enclosed in a spathe. There is a variable 

 number of stamens and a bicarpellary gynoecium with two loculi and 

 numerous ovules with axile placentation. The plants live only in very fast 

 running water and grow submerged on rocks in rivers. Naturally in such 

 very greatly specialized plants the systematic position is subject to dispute 

 and the inclusion of them in the Saxifragales is tentative. Hutchinson 

 relegates them to a separate order, Podostemales. 



Akhough the family is widespread in the tropics, the individual species 

 have often a very restricted distribution and many occur only in a single 

 locality. They provide an interesting problem in evolution. 



