But this does not appear to me nearly a sufficient cause, although his plants were slightly less productive than the wild ones growing on the Siebengbirge. My plants exhibited no tendency to become equal-styled, so as to lose their proper long-styled character, as not rarely happens under cultivation with several heterostyled species of Primula; but it would appear that they had been greatly affected in function, either by long-continued cultivation or by some other cause. We shall see in a future chapter that heterostyled plants illegitimately fertilised during several successive generations sometimes become more self-fertile; and this may have been the case with my stock of the present species of Pulmonaria; but in this case we must assume that the long-styled plants were at first sufficiently fertile to yield some seed, instead of being absolutely self-sterile like the German plants.

Pulmonaria angustifolia.

(FIGURE 3.6. Pulmonaria angustifolia. Left: Long-styled form. Right: Short-styled form.)

Seedlings of this plant, raised from plants growing wild in the Isle of Wight, were named for me by Dr. Hooker. It is so closely allied to the last species, differing chiefly in the shape and spotting of the leaves, that the two have been considered by several eminent botanists--for instance, Bentham--as mere varieties. But, as we shall presently see, good evidence can be assigned for ranking them as distinct. Owing to the doubts on this head, I tried whether the two would mutually fertilise one another. Twelve short-styled flowers of P. angustifolia were legitimately fertilised with pollen from long-styled plants of P. officinalis (which, as we have just seen, are moderately self-fertile), but they did not produce a single fruit. Thirty-six long-styled flowers of P. angustifolia were also illegitimately fertilised during two seasons with pollen from the long-styled P. officinalis, but all these flowers dropped off unimpregnated. Had the plants been mere varieties of the same species these illegitimate crosses would probably have yielded some seeds, judging from my success in illegitimately fertilising the long-styled flowers of P. officinalis; and the twelve legitimate crosses, instead of yielding no fruit, would almost certainly have yielded a considerable number, namely, about nine, judging from the results given in Table 3.20. Therefore P. officinalis and angustifolia appear to be good and distinct species, in conformity with other important functional differences between them, immediately to be described.

TABLE 3.20. Pulmonaria angustifolia.

Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of Fruits produced. Column 4: Average Number of Seeds per Fruit.

Long-styled by pollen of short-styled. Legitimate union : 18 : 9 : 2.11.

Long-styled by own-form pollen. Illegitimate union : 18 : 0 : 0.

Short-styled by pollen of long-styled. Legitimate union: 18 : 15 : 2.60.

Short-styled by own-form pollen. Illegitimate union : 12 : 7 : 1.86.

The long-styled and short-styled flowers of P. angustifolia differ from one another in structure in nearly the same manner as those of P. officinalis. But in Figure 3.6 a slight bulging of the corolla in the long-styled form, where the anthers are seated, has been overlooked. My son William, who examined a large number of wild plants in the Isle of Wight, observed that the corolla, though variable in size, was generally larger in the long-styled flowers than in the short-styled; and certainly the largest corollas of all were found on the long- styled plants, and the smallest on the short-styled. Exactly the reverse occurs, according to Hildebrand, with P. officinalis. Both the pistils and stamens of P. angustifolia vary much in length; so that in the short-styled form the distance between the stigma and the anthers varied from 119 to 65 divisions of the micrometer, and in the long-styled from 115 to 112. From an average of seven measurements of each form the distance between these organs in the long-styled is to the same distance in the short-styled form as 100 to 69; so that the stigma in the one form does not stand on a level with the anthers in the other. The long-styled pistil is sometimes thrice as long as that of the short-styled; but from an average of ten measurements of both, its length to that of the short-styled was as 100 to 56.

The Different Forms of Flowers on Plants of the Same Species Page 46

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Charles Darwin

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