The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗

their filaments, but not in the size or colour of their pollen-grains; so that they thus far resembled the stamens of Lagerstroemia. I found that Cuphea purpurea was highly fertile with its own pollen when artificially aided, but sterile when insects were excluded. (4/11. Mr. Spence informs me that in several species of the genus Mollia (Tiliaceae) which he collected in South America, the stamens of the five outer cohorts have purplish filaments and green pollen, whilst the stamens of the five inner cohorts have yellow pollen. He therefore suspected that these species might prove to be heterostyled and trimorphic: but he did not notice the length of the pistils. In the allied Luhea the outer purplish stamens are destitute of anthers. I procured some specimens of Mollia lepidota and speciosa from Kew, but could not make out that their pistils differed in length in different plants; and in all those which I examined the stigma stood close beneath the uppermost anthers. The numerous stamens are graduated in length, and the pollen-grains from the longest and shortest ones did not present any marked difference in diameter. Therefore these species do not appear to be heterostyled.)]
Oxalis (Geraniaceae).
In 1863 Mr. Roland Trimen wrote to me from the Cape of Good Hope that he had there found species of Oxalis which presented three forms; and of these he enclosed drawings and dried specimens. Of one species he collected 43 flowers from distinct plants, and they consisted of 10 long-styled, 12 mid-styled, and 21 short-styled. Of another species he collected 13 flowers, consisting of 3 long-styled, 7 mid-styled, and 3 short-styled. In 1866 Professor Hildebrand proved by an examination of the specimens in several herbaria that 20 species are certainly heterostyled and trimorphic, and 51 others almost certainly so. (4/12. 'Monatsber. der Akad. der Wiss. Berlin' 1866 pages 352, 372. He gives drawings of the three forms at page 42 of his 'Geschlechter-Vertheilung' etc. 1867.) He also made some interesting observations on living plants belonging to one form alone; for at that time he did not possess the three forms of any living species. During the years 1864 to 1868 I occasionally experimented on Oxalis speciosa, but until now have never found time to publish the results. In 1871 Hildebrand published an admirable paper in which he shows in the case of two species of Oxalis, that the sexual relations of the three forms are nearly the same as in Lythrum salicaria. (4/13. 'Botanische Zeitung' 1871 pages 416 and 432.) I will now give an abstract of his observations, and afterwards of my own less complete ones. I may premise that in all the species seen by me, the stigmas of the five straight pistils of the long-styled form stand on a level with the anthers of the longest stamens in the two other forms. In the mid- styled form, the stigmas pass out between the filaments of the longest stamens (as in the short-styled form of Linum); and they stand rather nearer to the upper anthers than to the lower ones. In the short-styled form, the stigmas also pass out between the filaments nearly on a level with the tips of the sepals. The anthers in this latter form and in the mid-styled rise to the same height as the corresponding stigmas in the other two forms.
Oxalis Valdiviana.
This species, an inhabitant of the west coast of South America, bears yellow flowers. Hildebrand states that the stigmas of the three forms do not differ in any marked manner, but that the pistil of the short-styled form alone is destitute of hairs. The diameters of the pollen-grains are as follows:--
Table 4.b. Oxalis Valdiviana. Diameters of pollen-grains in divisions of the micrometer.
Column 1: Source of Pollen-grains. Column 2: Minimum diameter. Column 3: Maximum diameter.
From the: Longest stamens of short-styled form : 8 to 9. Mid-length stamens of short-styled form : 7 to 8. Longest stamens of mid-styled form : 8. Shortest stamens of mid-styled form : 8. Mid-length stamens of long-styled form : 7. Shortest stamens of long-styled form : 6.
Therefore the extreme difference in diameter is as 8.5 to 6, or as 100 to 71. The results of Hildebrand's experiments are given in Table 4.27, drawn up in accordance with my usual plan.
Table 4.27. Oxalis Valdiviana (from Hildebrand).
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of Capsules produced. Column 4: Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union : 28 : 28 : 11.9.
Long-styled by pollen of longest stamens of mid-styled. Legitimate union : 21 : 21 : 12.0.
Long-styled by pollen of own and own-form mid-length stamens. Illegitimate union : 40 : 2 : 5.5.
Long-styled by pollen of own and own-form shortest stamens. Illegitimate union : 26 : 0 : 0.
Long-styled by pollen of shortest stamens of short-styled. Illegitimate union : 16 : 1 : 1.
Long-styled by pollen of shortest stamens of mid-styled. Illegitimate union : 9 : 0 : 0.
Mid-styled by pollen of mid-length stamens of long-styled. Legitimate union : 38 : 38 : 11.3.
Mid-styled by pollen of mid-length stamens of short-styled. Legitimate union : 23 : 23 : 10.4.
Mid-styled by pollen of own and own-form longest stamens. Illegitimate union : 52 : 0 : 0.
Mid-styled by pollen of own and own-form shortest stamens. Illegitimate union : 30 : 1 : 6.
Mid-styled by pollen of shortest stamens of long-styled. Illegitimate union : 16 : 0 : 0.
Mid-styled by pollen of longest stamens of short-styled. Illegitimate union : 16 : 2 : 2.5.
Short-styled by pollen of shortest stamens of long-styled. Legitimate union: 18 : 18 : 11.0.
Short-styled by pollen of shortest stamens of mid-styled. Legitimate union: 10 : 10 : 11.3.
Short-styled by pollen of own and own-form longest stamens. Illegitimate union : 21 : 0 : 0.
Short-styled by pollen of own and own-form mid-length stamens. Illegitimate union : 22 : 0 : 0.
Short-styled by pollen of longest stamens of mid-styled. Illegitimate union: 4 : 0 : 0.
Short-styled by pollen of mid-length stamens of long-styled. Illegitimate union: 3 : 0 : 0.
We here have the remarkable result that every one of 138 legitimately fertilised flowers on the three forms yielded capsules, containing on an average 11.33 seeds. Whilst of the 255 illegitimately fertilised flowers, only 6 yielded capsules, which contained 3.83 seeds on an average. Therefore the fertility of the six legitimate to that of the twelve illegitimate unions, as judged by the proportion of flowers that yielded capsules, is as 100 to 2, and as judged by the average number of seeds per capsule as 100 to 34. It may be added that some plants which were protected by nets did not spontaneously produce any fruit; nor did one which was left uncovered by itself and was visited by bees. On the other hand, scarcely a single flower on some uncovered plants of the three forms growing near together failed to produce fruit.
Oxalis Regnelli.
This species bears white flowers and inhabits Southern Brazil. Hildebrand says that the stigma of the long-styled form is somewhat larger than that of the mid- styled, and this than that of the short-styled. The pistil of the latter is clothed with a few hairs, whilst it is very hairy in the other two forms. The diameter of the pollen-grains from both sets of the longest stamens equals 9 divisions of the micrometer,--that from the mid-length stamens of the long- styled form between 8 and 9, and of the short-styled 8,--and that from the shortest stamens of both sets 7. So that the extreme difference in diameter is as 9 to 7 or as 100 to 78. The experiments made by Hildebrand, which are not so numerous as in the last case, are given in Table 4.28 in the same manner as before.
TABLE 4.28. Oxalis Regnelli (from Hildebrand).
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of Capsules produced. Column 4: Average Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union : 6 : 6 : 10.1.
Long-styled by pollen of longest stamens of mid-styled. Legitimate union : 5 : 5 : 10.6.
Long-styled by pollen of own mid-length stamens. Illegitimate union : 4 : 0 : 0.
Long-styled by pollen of own shortest stamens. Illegitimate union : 1 : 0 : 0.
Mid-styled by pollen of mid-length stamens of short-styled. Legitimate union : 9 : 9 : 10.4.
Mid-styled by pollen of mid-length stamens of long-styled. Legitimate union : 10 : 10 : 10.1.
Mid-styled by pollen of own longest stamens. Illegitimate union : 9 : 0 : 0.
Mid-styled by pollen of own shortest stamens. Illegitimate union : 2 : 0 : 0.
Mid-styled by pollen of longest stamens of short-styled. Illegitimate union : 1 : 0 : 0.
Short-styled by pollen of shortest stamens of mid-styled. Legitimate union: 9 : 9 : 10.6.
Short-styled by pollen of shortest stamens of long-styled. Legitimate union: 2 : 2 : 9.5.
Short-styled by pollen of own mid-length stamens. Illegitimate union : 12 : 0 : 0.
Short-styled by pollen of own longest stamens. Illegitimate union : 9 : 0 : 0.
Short-styled by pollen of mid-length stamens of long-styled. Illegitimate union: 1 : 0 : 0.
The results are nearly the same as in the last case, but more striking; for 41 flowers belonging to the three forms fertilised legitimately all yielded capsules, containing on an average 10.31 seeds; whilst 39 flowers fertilised illegitimately did not yield a single capsule or seed. Therefore the fertility of the six legitimate to that of the several illegitimate unions, as judged both by the proportion of flowers which yielded capsules and by the average number of contained seeds, is as 100 to 0.
Oxalis speciosa.
This species, which bears pink flowers, was introduced from the Cape of Good Hope. A sketch of the reproductive organs of the three forms has already been given. The stigma of the long-styled form (with the papillae on its surface included) is twice as large as that of the short-styled, and that of the mid-styled intermediate in size. The pollen-grains from the stamens in the three forms are in their longer diameters as follows:--
Table 4.c. Oxalis speciosa. Diameters of pollen-grains in divisions of the micrometer.
Column 1: Source of Pollen-grains. Column 2: Minimum diameter. Column 3: Maximum diameter.
From the: Longest stamens of short-styled form : 15 to 16. Mid-length stamens of short-styled form : 12 to 13. Longest stamens of mid-styled form : 16. Shortest stamens of mid-styled form : 11 to 12. Mid-length stamens of long-styled form : 14. Shortest stamens of long-styled form : 12.
Therefore the extreme difference in diameter is as 16 to 11, or as 100 to 69; but as the measurements were taken at different times, they are probably only approximately accurate. The results of my experiments in fertilising the three forms are given in Table 4.29.
Table 4.29. Oxalis speciosa.
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of Capsules produced. Column 4: Average Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union : 19 : 15 : 57.4.
Long-styled by pollen of longest stamens of mid-styled. Legitimate union : 4 : 3 : 59.0.
Long-styled by pollen of own-form mid-length stamens. Illegitimate union : 9 : 2 : 42.5.
Long-styled by pollen of own-form shortest stamens. Illegitimate union : 11 : 0 : 0.
Long-styled by pollen of shortest stamens of mid-styled. Illegitimate union : 4 : 0 : 0.
Long-styled by pollen of mid-length stamens of short-styled. Illegitimate union : 12 : 5 : 30.0.
Mid-styled by pollen of mid-length stamens of long-styled. Legitimate union : 3 : 3 : 63.6.
Mid-styled by pollen of mid-length stamens of