Consequently when insects visit the flowers of either form (for the stamens in this species occupy the same position in both forms), they will get their foreheads or proboscides well dusted with the coherent pollen. As soon as they visit the flowers of the long-styled form they will necessarily leave pollen on the proper surface of the elongated stigmas; and when they visit the short-styled flowers, they will leave pollen on the upturned stigmatic surfaces. Thus the stigmas of both forms will receive indifferently the pollen of both forms; but we know that the pollen alone of the opposite form causes fertilisation.

(Figure 3.5. Long-styled form of L. perenne var. Austriacum in its early condition before the stigmas have rotated. The petals and calyx have been removed on the near side. (3/3. I neglected to get drawings made from fresh flowers of the two forms. But Mr. Fitch has made the above sketch of a long- styled flower from dried specimens and from published engravings. His well-known skill ensures accuracy in the proportional size of the parts.)

In the case of L. perenne, affairs are arranged more perfectly; for the stamens in the two forms stand at different heights, so that pollen from the anthers of the longer stamens will adhere to one part of an insect's body, and will afterwards be brushed off by the rough stigmas of the longer pistils; whilst pollen from the anthers of the shorter stamens will adhere to a different part of the insect's body, and will afterwards be brushed off by the stigmas of the shorter pistils; and this is what is required for the legitimate fertilisation of both forms. The corolla of L. perenne is more expanded than that of L. grandiflorum, and the stigmas of the long-styled form do not diverge greatly from one another; nor do the stamens of either form. Hence insects, especially rather small ones, will not insert their proboscides between the stigmas of the long-styled form, nor between the anthers of either form (Figure 3.5), but will strike against them, at nearly right angles, with the backs of their head or thorax. Now, in the long-styled flowers, if each stigma did not rotate on its axis, insects in visiting them would strike their heads against the backs of the stigmas; as it is, they strike against that surface which is covered with papillae, with their heads already charged with pollen from the stamens of corresponding height borne by the flowers of the other form, and legitimate fertilisation is thus ensured.

Thus we can understand the meaning of the torsion of the styles in the long- styled flowers alone, as well as their divergence in the short-styled flowers.

One other point is worth notice. In botanical works many flowers are said to be fertilised in the bud. This statement generally rests, as far as I can discover, on the anthers opening in the bud; no evidence being adduced that the stigma is at this period mature, or that it is not subsequently acted on by pollen brought from other flowers. In the case of Cephalanthera grandiflora I have shown that precocious and partial self-fertilisation, with subsequent full fertilisation, is the regular course of events. (3/4. 'Fertilisation of Orchids' page 108; 2nd edition 1877 page 84.) The belief that the flowers of many plants are fertilised in the bud, that is, are perpetually self-fertilised, is a most effectual bar to understanding their real structure. I am, however, far from wishing to assert that some flowers, during certain seasons, are not fertilised in the bud; for I have reason to believe that this is the case. A good observer, resting his belief on the usual kind of evidence, states that in Linum Austriacum (which is heterostyled, and is considered by Planchon as a variety of L. perenne) the anthers open the evening before the expansion of the flowers, and that the stigmas are then almost always fertilised. (3/5. H. Lecoq 'Etudes sur la Geogr. Bot.' 1856 tome 5 page 325.) Now we know positively that, so far from Linum perenne being fertilised by its own pollen in the bud, its own pollen is as powerless on the stigma as so much inorganic dust.

The Different Forms of Flowers on Plants of the Same Species Page 43

19th Century English Literature

Charles Darwin

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Charles Darwin

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