3IO AFFINITIES [ch. 



The Droseraceae possess one curious little floating water 

 plant, Aldrovandia vesiculosa. Its flowers are aerial and of the 

 type characteristic of the family, but it is rootless, and its 

 anatomy is much simplified. 



The Podostemaceae have been placed in the most various 

 systematic positions, but botanists seem now to regard them 

 as showing some affinity with such forms as Nepenthes'^ and the 

 Saxifragaceae. The carpels present numerous points of simi- 

 larity with those of the latter family, e.g. the gynaeceum is 

 hypogynous, with a bicarpellary ovary, two free styles and a 

 number of ovules on a thick placenta connected with the outer 

 wall by a thin septum, while the ovule is anatropous, with a 

 straight embryo and no endosperm^. The most modern view is 

 to regard the Podostemads as an old phylum lying near the 

 Rosales and Sarraceniales^. 



The Crassulaceae, which presumably belong to the same 

 plexus as the Podostemaceae, though typically xerophytic, 

 include certain aquatic forms belonging to the genus TiHaea 

 {Bulliardd), 



Several families containing a few aquatic plants are to be 

 found in the same cycle of affinity as the Caryophyllaceae ; the 

 plants in question are characterised by their inconspicuous 

 flowers, which suggest reduction from a more highly deve- 

 loped type. Montia jontana (Portulacaceae), which occurs in 

 Britain, generally lives submerged. In the heat of summer, 

 however, the shoots often become exposed, but the thickish 

 stem and leaves do not collapse in drought in the manner 

 characteristic of submerged plants. The Portulacaceae include 

 many succulent xerophytes, and it has been suggested that 

 Montia is descended from ancestors of this type, and that, in 

 spite of adopting the water life, it has retained to its own 

 advantage certain xerophytic characters^. As a water plant 

 descended from a xerophilous stock, it may perhaps be compared 

 with Tillaea aquatica. The Elatinaceae, which show affinities 



1 Gardner, G. (1847). 2 Warming, E. (1888). 



3 Willis, J. C. (1902). 4 Focke, W. O. (i 893I). 



