x.] FLORAL DEGENERATIONS 87 



character. Not only are they reduced in size 

 to a few inches only, but the structure of 

 the flowers has often become degenerate as 

 well : as of Nardus strict a (fig. 19) with only 

 one stigma ; Sagina apetala, etc. ; Potentilla 

 Tormentilla with four, instead of five petals ; 

 Senebiera didyma, which is apetalous and with 

 only two stamens, etc. In all cases, we may 

 safely conclude that their depauperised con- 

 dition compared with their larger mesophytic 

 allies is simply due to impoverishment, yet 

 these characters are fixed and hereditary. 



Aquatic plants undergo similar degradations. 

 First, whatever be their size all which have 

 submerged parts are greatly degraded in their 

 histological elements, as we shall see in con- 

 trasting Monocotyledons with Dicotyledons ; 

 but besides these general features there are 

 several little plants which are comparable with 

 the depauperated land plants mentioned above. 

 Thus in the Cruciferce there is Subularia aquatica, 

 in Plantaginece, Littorella lacustris ; Elatine, 

 and Callitriche of Haloragece. Not only is the 

 vegetative system degenerated, but the flowers 

 of the genera, Hippuris, Myriophyllum and 

 Callitriche^ are in various stages of degradation. 

 Lastly, Lemna and Azolla represent the very 

 ultimate conditions it would seem possible to 

 attain. Yet in every case the degraded characters 

 are hereditary. 



