9" 



ix] FLOWERS AND SEEDS 121 



flowers, to members of the Podostemaceae in which anemo- 

 phily or autogamy is associated with gradually increasing dorsi- 

 ventrality. To this subject we shall return 

 in Chapter xxvii. Cleistogamous flowers are 

 also sometimes produced (Fig. 82). 



The peduncles of the Podostemaceae 

 contain little water-conducting tissue, and, 

 possibly in correlation with this, the seed- 

 development proves to be of a decidedly 

 xerophilous type^ an illustration of the 

 conservatism of the reproductive organs of 

 aquatics and their tendency to retain terres- 

 trial characters. By disappearance of nucellar 

 tissue, a cavity is formed beneath the embryo- 

 sac which, at the time of fertilisation, is filled 

 with fluid. This cavity is bounded by the 

 strongly cuticularised inner wall of the inner 

 integument and the suberised cells of the 

 chalaza. It is open only on the side towards 

 the developing embryo, and is described as 

 "an ideal water reservoir." Mucilage is often 

 present in the neighbouring cells of the 

 inner integument and this may perhaps form 

 an additional protection against loss of water. 



The seeds of the Podostemads are often small and numerous. 

 Those oi Rhyncolacis macrocarpa are about as large as the largest 

 known pollen-grains (e.g. those oiMirabilis). The seeds of this 

 species often germinate when caught in some cranny of the 

 parent, so that the old plant may support a number of seedlings. 

 The embryo is strictly rootless, but haptera grow out from the 

 hypocotyl^. 



The morphology of the Tristichaceae and Podostemaceae 

 positively bristles with problems for the botanist, but great 

 caution has to be exercised in dealing with them, since it must 



^ Magnus, W. and Werner, E. (1913). 

 2Goebel,K.(i 891-1893). 



Fig. 82. Podostemon 

 Barberi, Willis. Clei- 

 stogamic flower in 

 spathe {spa.), the 

 front of which is re- 

 moved to show the 

 gynaeceum {g), the 

 single stamen (st.) and 

 one of the two thread- 

 like organs represent- 

 ing the perianth or 

 staminodes {per.). 

 rSimplified from 



Willis, J. C. (1902).] 



