. Elementary biology; an introduction to the science of life . Fig. 136. Polymorphic flowers of purple loosestrife {Lyth7-ui)i) In species having three forms of flowers the best seed- production seems to result from the pollenation of a pistil by pollen from a stamen of the corresponding length, which must necessarily be from a different flower. (After Darwin) may bend as it grows, dumping some of the pollen from the anthers onto the stigma. In other cases the anthers are placed above the stigma, so that the pollen is brought to the latter organ by the action of gravity. There are many plants
. Elementary biology; an introduction to the science of life . Fig. 136. Polymorphic flowers of purple loosestrife {Lyth7-ui)i) In species having three forms of flowers the best seed- production seems to result from the pollenation of a pistil by pollen from a stamen of the corresponding length, which must necessarily be from a different flower. (After Darwin) may bend as it grows, dumping some of the pollen from the anthers onto the stigma. In other cases the anthers are placed above the stigma, so that the pollen is brought to the latter organ by the action of gravity. There are many plants in which the stigma regularly pushes through the ring of anthers and thus becomes pol- lenated. In other plants this kind of pollenation takes place only under special conditions, as in extreme dampness or extreme drought. 358. Close pollenation and cross pollenation. Any process that results in the transfer of pollen from the anther of a flower to the stigma of the same flower is called close pollenation. This designation is used to distinguish the process from cross pollenation, in which pollen is carried from the anther of one flower to the stigma of another flower (of the same kind, however). There are many plants in which close pollenation is impossible. 359. Obstacles to close pollenation. There are three sets of conditions in plants that interfere with close pollenation. I. Space 7'elations. The relative position of stamens and pistils within the flower may make close pollenation impossible. Or the
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