I INTHODUCTION 9 



case by no means a common colour. Such flo\Yers if 

 kept under water, by flood or otherwise, sometimes 

 become cleistogamous (see above) : as, for instance, in 

 Ranunculus aquatilis, Alisma natans, Svhularia 

 aquatica, and Illecebrttm verticillatum. 



To a second group belong certain species in which 

 the flowers raise themselves into the air, but which are 

 not large or conspicuous, and which are presumably 

 wind-fertilised : as, for instance, Myriophyllum, Potamo- 

 geton (Pondweed), and some Sparganiums. 



In a third group the flowers as well as the leaves are 

 submerged. Callitriche is an intermediate form ; some 

 species have aerial, others submerged flowers. The 

 flowers are very simple, arranged in the axils of the 

 leaves — the males reduced to a single stamen, the 

 females to a pistil. 



As typical of this group I may mention Cerato- 

 phyllum demersum. The flowers are surrounded by a 

 whorl of minute bracts : the female consists of a small 

 ovary with a single style ; the male of from 12 to 20 

 oblong sessile anthers. The pollen, as in all submerged 

 flowers, has no extine. 



Zostera, the Grass-wrack of our shallow seas, has the 

 small flowers enclosed in a sheath at the base of certain 

 leaves, which do not materially difi'er from the rest, but 

 are somewhat smaller. They are arranged in two rows, 

 with alternately male and female flowers. They are 

 protogynous, and the pollen is peculiar, being thread- 

 like, but without motion. 



Lastly, there is a small group, of which Vallisneria is 

 the best-known form, and which in our flora is repre- 

 sented by Euppia. The flowers are very simple, and 

 consist of two bilocular anthers and an ovary with four 

 carpels. The spadix or stalk on which they grow is at 

 first short. The anthers open first, and eject the pollen, 

 which, being lighter than water, at once rises to the 

 surface. Immediately afterwards the spadix lengthens 

 rapidly, carrying the female flower to the surface, where 

 it meets the floating pollen. In Vallisneria the male 



