THE PER TILISA TION OF FL WERS. 325 



there are certain conspicuous differences between the pollen and its 

 quantity, and between the form of the stigma, &c., in wind-fertilised 

 and in sect- fertilised flowers. 



The pollen of the wind-fertilised plants is produced in far greater 

 quantity than that of the insect-dependent flowers. Then, also, the 

 former flowers open before the leaves are in full growth, in order that 

 the clouds of pollen may gain easy access to the pistils ; whilst their 

 stigmas are usually branched and bending (e.g., alder, wheat, &c.), so 

 as the more readily to intercept and detain the pollen in its wind- 

 flights. Allusion has already been made to the showers of pollen 

 emitted by coniferous trees, and it may be added here that bucketfuls of 

 pollen from conifers and grasses are occasionally swept off the decks 

 of vessels off North American coasts ; whilst North American lakes may 

 be covered over a considerable area of their surface by the yellow pollen 

 of the pines. Most of our cereals are presumably wind-fertilised ; 

 and the importance of light breezes in the early summer may there- 

 fore be a matter of consideration in respect of the full ears of 

 autumn. Hooibrenk and Kcernicke, in their practical suggestion, 

 carried out in Belgium and Germany, of drawing a rope across the 

 full-flowered ears so as to distribute pollen and cross-fertilise the 

 plants, seem therefore to have imitated nature's method. The question 

 of the wind-fertilisation of the cereals, it may be remarked, however, is 

 at present an open one, since some botanists elect to believe that the 

 wind-distributed pollen is simply the excess or useless pollen remaining 

 after fertilisation has been accomplished ; the actual agency in 

 scattering abroad the fertilising dust being said to be the sudden 

 extension and elasticity of the stalks of the stamens. 



That cross-fertilisation is the rule of nature, is a fact amply 

 demonstrated by the well-nigh endless contrivances in flower- 

 structure, form, appearance, and function, through which the inter- 

 change of pollen is brought about. Let us briefly glance at the out- 

 lines of such a study. Allusion has already been made to cases in 

 which a separation of stamens and pistil takes place as a normal 

 condition of many plants. Such separation may proceed to the 

 extent of placing stamens in one set of flowers, and pistils in another 

 set on the same plant ; or it may be illustrated by the more complete 

 isolation of these organs, so that in the latter case we find all the 

 flowers on one plant to be " staminate," and all the " pistillate " 

 flowers to be borne on another plant. The lesser nettle, for instance, 

 has its stamens and pistils in different flowers on the same plant, as also 

 have the oak (Figs. 212, 213), melon, cucumber, maize, hop, hazel, 

 carex, &c. The greater nettle, on the other hand, bears on one plant 

 none but staminate flowers, and on another plant none but pistil- 

 bearing flowers ; whilst hemp, willow (Figs. 210, 211), the variegated 

 laurel (Attatba Japonica\ palms, &c., also illustrate the complete 



