ECOLOGICAL 189 



WATER POLLINATION.— Some peculiar features are exhibited 

 by aquatic plants. Thus in the sea-grass (Zostera), the pollen-grains 

 are pecuhar in being long filaments, and the stigma is also filament- 

 ous. The pollen- threads have the same specific gravity as the sea- 

 water and can float at any level. This elongation of the pollen- 

 grains occurs in not a few aquatic plants. In Najas the female 

 flowers are near the bottom, and the very much simplified male 

 flowers are nearer the surface. The pollen-grains are almost spherical 

 and so heavily weighted with starch grains that they sink when 

 they are shed and are thus caught 6n the lower female flowers. 



The tape-grass VaUisneria is an interesting example of those 

 water plants in which the pollination is effected at the surface. The 

 male flowers are low down on a spike enclosed in a spathe; the 

 female flowers are solitary and float when ripe on the surface on the 

 end of long slender stalks. When the male flowers are ripe, the 

 inflorescence breaks off, floats up to the surface, and sets the indi- 

 vidual florets free. They drift about passively and sometimes collide 

 with a female flower, transferring to the pistil a little clump of 

 coherent pollen-grains. After pollination there is a remarkable spiral 

 coiling of the stalk of the female flower, which brings the fruit to 

 the bottom, where it ripens. In the American Pondweed (Elodea) 

 there is a somewhat similar sequence ; but in this case it is the great 

 elongation of the ovary that brings the female flower to the surface, 

 where pollination occurs. This plant was brought from America to 

 Britain in 1842, and has become over-abundant in many canals and 

 rivers in Europe. As no male flowers occur in the Old World, the 

 multipUcation must be purely vegetative. It is effected by the 

 breaking off of twigs. 



POLLINATION BY INSECTS— We have mentioned Camerarius 

 ( 1665-172 1), who proved experimentally that fertile seeds cannot 

 be formed without the co-operation of pollen, and also insisted that 

 the anthers and the ovaries were the male and the female sex- 

 organs of the flower in no figurative sense. This conclusion was 

 confirmed and deepened by Koelreuter (1733-1806), who discerned 

 that genuine, i.e. animal-like, fertilisation must occur in flowering 

 plants, and made many important experiments in hybridisation. As 

 he also recognised the role of insects in carrying pollen and the use 

 of nectar as an inducement to the insects to visit the flowers, he 

 leads us naturally to vSprengel, who linked him to Darwin. 



Christian Konrad Sprengel (1750-18 16) was a keen observer of 

 the inter-relations between flowers and insects. In his Newly 

 Discovered Secret of Nature in the Structure and Fertilisation of 

 Flowers he expounded three conclusions: (i) that many of the 

 characteristics of flowers, such as nectaries, markings, and shapes, 

 are adaptive to the visits of insects which secure cross-polHnation ; 



