200 SOME BRITISH PLANT GROUPS 
reached by the abandonment of the aquatic mode of 
pollination by means of swimming spermatozoids, as 
still found in the Maidenhair Tree (Ginkgo), Cycads, 
Ferns, and groups lower in the scale, and the adoption 
instead of pollination through the medium of the air, 
“which” (to quote Mrs. Arber’s happy phrase) “has 
won for them the freedom of the land.” The Seed 
Plants, then, achieved their wonderful abundance and 
variety owing to the highly stimulating conditions 
offered by a terrestrial existence; we must assign to all 
the existing types a long terrestrial ancestry. How, 
then, about the water plants whose leaves and flowers 
so decorate our lakes? There seems no doubt* that 
they are species which have left the land to resume 
the aquatic habits of their remote ancestors. With few 
exceptions they retain the aerial mode of pollination 
which is the pride of the specialized land plants. The 
pressure of competition has probably driven them into 
the water, where they descend as far as the lessening 
light-supply will allow. Some—presumably the earliest 
to take to an aquatic life—have all their relations to 
keep them company, the remote ancestor which 
adopted an aquatic habit being now represented by 
many species, or even by many genera. In other cases 
a terrestrial genus or order has few or only a single 
aquatic representative. It may be assumed that in 
such a case the aquatic habit has been recently 
acquired. The great majority of water plants send 
their flowers up above the surface to be pollinated by 
wind or (more rarely) by insects. It may be noted 
that few of the more highly evolved groups of Seed 
* See AGNEs ARBER: ‘‘ Aquatic Angiosperms: the Significance 
of their Systematic Distribution,’’ Journal of Botany, 1919, p. 83. 
