sect, ii PHYSIOLOGY 283 



specific weight as the surrounding water, are easily set in motion by 

 the slightest currents, and are thus brought into contact with the 

 stigmas. 



In the case of the submerged water-plants, Vallisneria, Elodea, 

 and species of Enhalus, found in the Indian Ocean, the pollination is 

 accomplished on the surface of the water. Thus, for example, the 

 male flowers of Vallisneria, after separating from the parent plant, 

 rise to the surface of the water, where they open and float like little 

 boats to the female flowers, which, by the elongation of their spirally 

 coiled flower- stalks, ascend, at the same time, to the surface of the 

 water, only to become again submerged after fertilisation. 



In the great majority of Phanerogams pollination is effected by 

 means of animals. By enticing in various ways insects, birds, or 

 snails, plants are enabled not only to utilise the transporting power 

 but also the intelligence of animals in the service of pollen -con- 

 veyance. The pollination is then no longer left to chance ; and as 

 the transport of pollen to the sexual organs becomes more assured, 

 the necessity for its formation in such enormous quantities as in ane- 

 mophilous plants is obviated. For the most part, such plants (Fig. 

 219) are adapted to pollination by insects (Entomophily). For 

 their nourishment, plants offer not only the sugary sap, which, as 

 nectar, is excreted from different parts of the flowers, but also the 

 pollen itself, which furnishes a nitrogenous food material and which, 

 together with the honey, is kneaded by bees into bee-bread. As 

 additional means of enticement, and to attract animals from a distance 

 to the nectar offered by the sexual organs, special perfumes and 

 conspicuous colours have also been developed. The attractive-appa- 

 ratus of plants is generally formed by the coloured floral leaves ; by 

 the outer floral leaves or calyx (Nigella, Aconitum), or by the perianth 

 (Lily, Tulip), or as an extra-floral show apparatus, by the hypsophyllary 

 leaves and parts of the shoot, which do not belong strictly to the 

 flower (Astrantia major, Bichardia aethiopica, Melampyrum, Dalechampiaj 

 Bougainvillea Sjpectabilis). The pollen of the entomophilous, in contrast 

 to that of the anemophilous plants, is not a dry powder, but its grains 

 are stuck together with an oily mucilaginous fluid ; in other cases, 

 they are held together by their rough outer surfaces and can only be 

 removed from the anthers by animals. The structure of the flower is 

 so contrived, as Christian Conrad Sprengel first pointed out in 

 1793 in his famous work on the structure and fertilisation of flowers 

 (" Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung 

 der Blumen "), that the pollen grains must necessarily become attached 

 to certain parts of the body of the animal visiting it in search of food, 

 and so be conveyed to the sticky or hairy stigma of other flowers. 

 The remarkable variety of means employed to secure pollination, and 

 the wonderful adaptation shown by the flowers to the form and habits 

 of different insects, border on the marvellous. In addition to the 



