I could detect no difference in the size of the pollen-grain or in the structure of the stigma in the plants which differed most in the above respects; and all of them, when protected from the access of insects, yielded plenty of seeds. Again, from statements made by Vaucher, and from a hasty inspection, I thought at first that the allied Anchusa arvensis and Echium vulgare were heterostyled, but soon saw my error. From information given me, I examined dried flowers of another member of the Boragineae, Arnebia hispidissima, collected from several sites, and though the corolla, together with the included organs, differed much in length, there was no sign of heterostylism.]

Polygonum fagopyrum (Polygonaceae).

(FIGURE 3.7. Polygonum fagopyrum. (From H. Muller.) Upper figure, the long-styled form; lower figure, the short-styled. Some of the anthers have dehisced, others have not.)

Hildebrand has shown that this plant, the common Buck-wheat, is heterostyled. (3/13. 'Die Geschlechter-Vertheilung' etc. 1867 page 34.) In the long-styled form (Figure 3.7), the three stigmas project considerably above the eight short stamens, and stand on a level with the anthers of the eight long stamens in the short-styled form; and so it is conversely with the stigmas and stamens of this latter form. I could perceive no difference in the structure of the stigmas in the two forms. The pollen-grains of the short-styled form are to those of the long-styled as 100 to 82 in diameter. This plant is therefore without doubt heterostyled.

I experimented only in an imperfect manner on the relative fertility of the two forms. Short-styled flowers were dragged several times over two heads of flowers on long-styled plants, protected under a net, which were thus legitimately, though not fully, fertilised. They produced 22 seeds, or 11 per flower-head.

Three flower-heads on long-styled plants received pollen in the same manner from other long-styled plants, and were thus illegitimately fertilised. They produced 14 seeds, or only 4.66 per flower-head.

Two flower-heads on short-styled plants received pollen in like manner from long-styled flowers, and were thus legitimately fertilised. They produced 8 seeds, or 4 per flower-head.

Four heads on short-styled plants similarly received pollen from other short- styled plants, and were thus illegitimately fertilised. They produced 9 seeds, or 2.25 per flower-head.

The results from fertilising the flower-heads in the above imperfect manner cannot be fully trusted; but I may state that the four legitimately fertilised flower-heads yielded on an average 7.50 seeds per head; whereas the seven illegitimately fertilised heads yielded less than half the number, or on an average only 3.28 seeds. The legitimately crossed seeds from the long-styled flowers were finer than those from the illegitimately fertilised flowers on the same plants, in the ratio of 100 to 82, as shown by the weights of an equal number.

About a dozen plants, including both forms, were protected under nets, and early in the season they produced spontaneously hardly any seeds, though at this period the artificially fertilised flowers produced an abundance; but it is a remarkable fact that later in the season, during September, both forms became highly self-fertile. They did not, however, produce so many seeds as some neighbouring uncovered plants which were visited by insects. Therefore the flowers of neither form when left to fertilise themselves late in the season without the aid of insects, are nearly so sterile as most other heterostyled plants. A large number of insects, namely 41 kinds as observed by H. Muller, visit the flowers for the sake of the eight drops of nectar. (3/14. 'Die Befruchtung' etc. page 175 and 'Nature' January 1, 1874 page 166.) He infers from the structure of the flowers that insects would be apt to fertilise them both illegitimately as well as legitimately; but he is mistaken in supposing that the long-styled flowers cannot spontaneously fertilise themselves.

The Different Forms of Flowers on Plants of the Same Species Page 49

19th Century English Literature

Charles Darwin

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Charles Darwin

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