130 DISPERSION OF POLLEN BY THE WIND. 



which they afterwards convey to the stigmas of other flowers. Subsequently, 

 however, the conditions are reversed, the supply of honey is exhausted and 

 insects stay away; but, on the other hand, the filaments bearing the anthers 

 have elongated, the pollen-sacs are consequently exserted above the mouth of 

 the corolla, the pollen contained in them is laid bare, and, at the proper time, is 

 blown away by the wind to the stigmas of younger blossoms. Plants of the 

 kind thus appear to have a second contrivance in readiness in case the first 

 fails, so that in any circumstances the object of flowering may be attained. This 

 is indeed a matter of urgent necessity. How easily may it happen that insects 

 are kept away for a long time by unfavourable weather or that they pay but 

 a few visits. Most plants, therefore, take the precaution to provide that under 

 such circumstances the expenditure of energy involved in the production of 

 flowers shall not have been in vain. 



It would be inconsistent with the plan of this book to discuss here all the 

 remarkable adaptations which have been evolved for the purpose of providing 

 a supplementary means of dusting the stigmas with pollen in the event of an 

 absence of insects, but it is necessary to make preliminary mention of this 

 one arrangement whereby many flowers, originally entomophilous, subsequently 

 become anemophilous, because it enables us to determine the proper degree of 

 significance to be attached to the division of plants into anemophilous and 

 entomophilous species. 



As would naturally be expected, it is, speaking generally, only pollen which 

 is of dusty or floury consistency that is transported by the wind. If it is true, 

 as gardeners assert, that the pollen of Azaleas, which oozes from the anthers in 

 the form of sticky fringes, has on occasion been torn away and conveyed to the 

 stigmas of neighbouring flowers by the wind, the occurrence can only be looked 

 upon as accidental. In ninety-nine cases out of a hundred the viscid strings, if 

 detached by the wind, would not be conveyed to the stigma of another flower, 

 but would adhere to the outside of the calyx and petals, or to the leaves and 

 stem, and would there perish. The same remark applies also to pollen-cells 

 which are bound together into little lumps by oil and viscid substances, or by 

 acicular processes on the outer layer of the cell-membranes. Only in the rarest 

 instances are they carried by the wind to the stigmas of flowers in the vicinity. 

 These are primarily adapted to becoming attached to the bodies of winged insects. 



All the more remarkable, therefore, is the fact that in certain water-plants 

 the pollen, though cohering in sticky masses, is blown by the wind on a kind 

 of little boat to the stigmas which are raised above the surface of the water. 

 The phenomenon was first observed in the case of Vallisneria spiralis, an 

 aquatic plant which grows in still water, and is widely distributed in Southern 

 Europe. It is, for example, very luxuriant in the ponds, canals, and shallow 

 inlets along the shores of the Lake of Garda, and we will select it as an illustration 

 in the account which follows. The reader is requested first of all to look at the 

 figure on p. 667 of vol. i. It represents a plant living under water with strap- 



