68 INTRODUCTION 



{b) Pitfall flowers, Fpf. 

 (c) Pinch-trap flowers, Fpt. 

 ((/) Deceptive flowers, Fd. 

 (c) Hover-fly flowers, Fh. 

 8. Small-insect flowers, Sm (Miiller). 



I. Water-pollinated Plants, Hydrophilae (Hy). 



It is comparatively seldom that water serves as the agent for carrying pollen to 

 the stigmas of flowering plants. It is characteristic of the pollen of many plants 

 that bloom in water, that the grains should not possess an outer coat (extine). The 

 specific gravity of the pollen (or of the male flowers) is either about equal to that of 

 water, or it is less. In the former case, pollination is eflfected under water 

 {Hyphydrogamy), in the latter case it takes place at the surface of the water 

 {Epkydrogamy). Naias, e. g., belongs to the former group of water-flowers, for 

 its pollen-cells (owing to enclosed starch-grains) are rather heavier than water, 

 so that they sink down and are caught by the female flowers. 



The process is somewhat different in the case of Ceratophyllum. Long after 

 Vaucher, in his ' Histoire physiologique des plantes d'Europe' (1841), had described 

 the fertilization of species of this genus by pollen swimming about in the water as 

 'kornige Materie' (granular matter), Ludwig (1881) observed that the stamens are 

 made up of a lower part next the short stalk, consisting of two lateral anther-lobes, 

 and an upper part, the 'Auftrieb ' (float), composed of air-containing tissue, and pro- 

 duced into two little spines. This arrangement makes the stamen specifically lighter 

 than the water, and it consequently rises to the surface. During this ascent 

 the anthers dehisce, and the large pollen-grains, which are of the same specific 

 gravity as the surrounding medium, are dispersed through the water in 'which 

 the plants grow, so that the female flowers are necessarily fertilized. 



Pollination of flowers on the surface of the water is a more frequent occurrence. 

 In Callitriche autumnalis the pollen is lighter than the water, and therefore rises 

 to the surface, where fertilization results. The same is the case in Ruppia spiralis, 

 in which the stalk of the female inflorescence stretches up spirally to the surface 

 of the water, where the female flowers are fertilized by pollen-grains that have 

 also floated up. The relations are similar in Vallisneria spiralis, as also in the 

 allied Enalus acoroides, native to the Indian Ocean. In these plants the female 

 flowers ascend as before on spirally wound stalks to the surface of the water, while 

 the entire male flowers become liberated from the plant, and also rise to the 

 surface, after which the pollen is discharged, and the female flowers are fertilized 

 by it. After this has been effected, the spirally wound flower-stalk of the female 

 flower coils up again, so that the fruits mature under water. In its native country 

 (America) the male flowers of Elodea canadensis also float and pollinate the 

 female flowers that' reach up to the surface. Only female plants of this species 

 occur in Europe (since 1836). In Zostera marina ('sea-grass'), the two filiform 

 forked stigmas grow out from the flower-sheath in the first stage of flowering, 

 and are pollinated by swimming pollen that has been shed from older inflorescences. 

 In the second stage of flowering all the anthers of the inflorescence dehisce simul- 



