THE ACCESS OF THE POLLEN. 301 



dioecious plants. More than half a century ago, Spallanzani found 

 that the pistillate blossoms of Hemp may produce fertile seed with- 

 out the concurrence of jJoUen ; and recently Naudin and Decaisne 

 have confirmed the fact by experiment, and from seeds produced 

 without fertilization have raised a second generation of plants, the 

 pistillate individuals of which, kept from all access of pollen, have 

 themselves ripened seeds with perfect embryos.* Two or three 

 dioecious Euphorbiaceous plants are known to produce good seed 

 under the same circumstances, and Naudin has shown it freely to 

 occur in Bryony. Still these are very exceptional cases, and are all 

 confined, so far as known, to dioecious plants. Ordinarily the access 

 of pollen of the species to the ovules is necessary to the production 

 of the embryo. 



572. The Access of the Pollen to the pistil is secured in a great 

 variety of ways and adaptations. In hermaphrodite blossoms the 

 relative length and position of the stamens and stigmas are com- 

 monly so adjusted that the pollen may fall directly upon the 

 stigma, the anthers being usually higher than the stigmas when 

 the flower is upright, and shorter when it is nodding. Sometimes 

 pollen is projected upon the stigma by transient and often sudden 

 movements, either mechanical, as in Kalmia, or spontaneous and 

 vital, as in the Barberry (to be mentioned in another place). Some- 

 times fertilization takes place in the bud, where the parts are in 

 apposition, or the anthers are kept in contact with or proximity to 

 the stigma, as in papilionaceous flowers by the enclosing keel-petals, 

 and in the Fumitory family by a close-fitting little sac formed of the 

 united spoon-shaped tips of the two inner petals confining the an- 

 thers to the stigma. Very often the pollen is conveyed from the 

 anthers to the stigma by insects, searching for honey or nectar ; and 

 there are many species in which fertilization seems absolutely to 

 depend upon the agency of insects ; such, for instance, as those of 

 Aristolochia, Asclepias or Milkweed, and many jilants of the Orchis 

 family. In dioecious and many monoecious plants, with widely sep- 

 arated blossoms, fertilization is mainly dependent upon insects, pass- 

 ing from flower to flower, and upon winds and currents. And the 

 immense quantity of pollen which many such plants produce com- 

 pensates for the greater distance of the passage, and greatly dimin- 

 ishes the chance of failure. The air of a Pine forest in flowerinor- 



* Comptes Rendus, Vol. 43, 1856, and Hooker's Jbaraa/ of Botany, 1857, p. 53. 

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