THE ANGIOSPERMAE 1293 



inflorescence appears to be considerably larger than the male. The pollen 

 grains are rounded tetrahedra, covered with a network of tubercles, and are 

 shed only after the female inflorescences on the same stem have already 

 passed maturity. In this way geitonogamy is prevented and the pollen is 

 blown by the wind to adjacent plants. 



II. Hydrophily or Pollination by Water 



Although it might be expected that water would provide a ready means 

 of transport, especially for aquatic plants, it is remarkable how few plants 

 may make use of this medium for pollination. Many submerged water 

 plants utilize wind pollination, raising their inflorescences to this end above 

 the water surface. Such are, for example, Potamogeton and MyriophyUum. 

 Possibly because damp pollen is liable to germinate prematurely or because 

 of the efi'ect of surface tension, we find hydrophily employed only in the 

 case of plants whose flowers are permanently submerged below the water. 

 A second type, in which pollination is eflFected at the surface of the water, 

 is somew'hat different and as will be explained later might be more correctly 

 regarded as a highly specialized type of wind pollination. 



Hydrophily cannot be regarded as a primitive mechanism. It is almost 

 certainly a secondary adaptation of a wind mechanism, used by plants which 

 have taken to an aquatic habitat from a terrestrial one. For successful polli- 

 nation to occur there must be a close correlation between the position of the 

 flowers and the specific gravity of the pollen grains. If the female flowers 

 are produced low down in the water then the pollen must have a specific 

 gravity equal to or slightly greater than that of the water. If the female 

 flowers develop at the surface of the water then the specific gravity of the 

 pollen must be less than that of the water, so that it shall rise, if liberated 

 below water, and float on the surface. These changes in the specific gravity 

 of the pollen grains appear to be regulated by the formation in them of one 

 or more large starch grains. 



{a) Pollination below Water Level (Hypohydrogamic) 



Two of the best examples of this type of pollination are provided by the 

 genera Naias and Ceratophyllum, both of which are represented in the British 

 Flora. 



Ceratophyllum demersum is monoecious (Fig. 1224). The male inflores- 

 cence consists of a cluster of 12-16 stamens with very short filaments, each 

 bearing a two-lobed anther which opens laterally by longitudinal slits. The 

 tip of the anther is prolonged into a double, horned process which is com- 

 posed of aerenchyma. When the stamens separate from the involucral base 

 to which they are attached, the aerenchyma serves as a float and raises the 

 anther to the surface of the water. The pollen grains are rounded and are 

 enclosed in a delicate intine, the extine being absent in this and other pollens 

 adapted to hydrophily. The pollen has just the same specific gravity as the 

 water, consequently, when the anther opens during the period when it is 



