The Different Forms of Flowers on Plants of the Same Species Part 2

Column 1: Nature of the Union.

Column 2: Number of Flowers fertilised.

Column 3: Number of Capsules.

Column 4: Number of good Capsules.

Column 5: Weight of Seed in grains.

Column 6: Number of Capsules.

Column 7: Weight of Seed in grains.

Column 8: Number of good Capsules.

Column 9: Weight of Seed in grains.

The two legitimate unions : 100 : 77 : 71 : 39 :: 100 : 50 :: 100 : 54.

The two illegitimate unions : 100 : 45 : 31 : 11 :: 100 : 24 :: 100 : 35.

We here see that the long-styled flowers fertilised with pollen from the short- styled yield more capsules, especially good ones (i.e. containing more than one or two seeds), and that these capsules contain a greater proportional weight of seeds than do the flowers of the long-styled when fertilised with pollen from a distinct plant of the same form. So it is with the short-styled flowers, if treated in an a.n.a.logous manner. Therefore I have called the former method of fertilisation a legitimate union, and the latter, as it fails to yield the full complement of capsules and seeds, an illegitimate union. These two kinds of union are graphically represented in Figure 1.2.

(FIGURE 1.2. Primula veris.

Graphic representation of two kinds of union between: Left: Long-styled form.

Right: Short-styled form.)

If we consider the results of the two legitimate unions taken together and the two illegitimate ones, as shown in Table 1.7, we see that the former compared with the latter yielded capsules, whether containing many seeds or only a few, in the proportion of 77 to 45, or as 100 to 58. But the inferiority of the illegitimate unions is here perhaps too great, for on a subsequent occasion 100 long-styled and short-styled flowers were illegitimately fertilised, and they together yielded 53 capsules: therefore the rate of 77 to 53, or as 100 to 69, is a fairer one than that of 100 to 58. Returning to Table 1.7, if we consider only the good capsules, those from the two legitimate unions were to those from the two illegitimate in number as 71 to 31, or as 100 to 44. Again, if we take an equal number of capsules, whether good or bad, from the legitimately and illegitimately fertilised flowers, we find that the former contained seeds by weight compared with the latter as 50 to 24, or as 100 to 48; but if all the poor capsules are rejected, of which many were produced by the illegitimately fertilised flowers, the proportion is 54 to 35, or as 100 to 65. In this and all other cases, the relative fertility of the two kinds of union can, I think, be judged of more truly by the average number of seeds per capsule than by the proportion of flowers which yield capsules. The two methods might have been combined by giving the average number of seeds produced by all the flowers which were fertilised, whether they yielded capsules or not; but I have thought that it would be more instructive always to show separately the proportion of flowers which produced capsules, and the average number of apparently good seeds which the capsules contained.

Flowers legitimately fertilised set seeds under conditions which cause the almost complete failure of illegitimately fertilised flowers. Thus in the spring of 1862 forty flowers were fertilised at the same time in both ways. The plants were accidentally exposed in the greenhouse to too hot a sun, and a large number of umbels perished. Some, however, remained in moderately good health, and on these there were twelve flowers which had been fertilised legitimately, and eleven which had been fertilised illegitimately. The twelve legitimate unions yielded seven fine capsules, containing on an average each 57.3 good seeds; whilst the eleven illegitimate unions yielded only two capsules, of which one contained 39 seeds, but so poor, that I do not suppose one would have germinated, and the other contained 17 fairly good seeds.

From the facts now given the superiority of a legitimate over an illegitimate union admits of not the least doubt; and we have here a case to which no parallel exists in the vegetable or, indeed, in the animal kingdom. The individual plants of the present species, and as we shall see of several other species of Primula, are divided into two sets or bodies, which cannot be called distinct s.e.xes, for both are hermaphrodites; yet they are to a certain extent s.e.xually distinct, for they require reciprocal union for perfect fertility. As quadrupeds are divided into two nearly equal bodies of different s.e.xes, so here we have two bodies, approximately equal in number, differing in their s.e.xual powers and related to each other like males and females. There are many hermaphrodite animals which cannot fertilise themselves, but most unite with another hermaphrodite. So it is with numerous plants; for the pollen is often mature and shed, or is mechanically protruded, before the flower's own stigma is ready; and such flowers absolutely require the presence of another hermaphrodite for s.e.xual union. But with the cowslip and various other species of Primula there is this wide difference, that one individual, though it can fertilise itself imperfectly, must unite with another individual for full fertility; it cannot, however, unite with any other individual in the same manner as an hermaphrodite plant can unite with any other one of the same species; or as one snail or earth-worm can unite with any other hermaphrodite individual. On the contrary, an individual belonging to one form of the cowslip in order to be perfectly fertile must unite with one of the other form, just as a male quadruped must and can unite only with the female.

I have spoken of the legitimate unions as being fully fertile; and I am fully justified in doing so, for flowers artificially fertilised in this manner yielded rather more seeds than plants naturally fertilised in a state of nature.

The excess may be attributed to the plants having been grown separately in good soil. With respect to the illegitimate unions, we shall best appreciate their degree of lessened fertility by the following facts. Gartner estimated the sterility of the unions between distinct species, in a manner which allows of a strict comparison with the results of the legitimate and illegitimate unions of Primula. (1/6. 'Versuche uber die b.a.s.t.a.r.derzeugung' 1849 page 216.) With P.

veris, for every 100 seeds yielded by the two legitimate unions, only 64 were yielded by an equal number of good capsules from the two illegitimate unions.

With P. Sinensis, as we shall hereafter see, the proportion was nearly the same- -namely, as 100 to 62. Now Gartner has shown that, on the calculation of Verbasc.u.m lychnitis yielding with its own pollen 100 seeds, it yielded when fertilised by the pollen of Verbasc.u.m Phoeniceum 90 seeds; by the pollen of Verbasc.u.m nigrum, 63 seeds; by that of Verbasc.u.m blattaria, 62 seeds. So again, Dianthus barbatus fertilised by the pollen of D. superbus yielded 81 seeds, and by the pollen of D. j.a.ponicus 66 seeds, relatively to the 100 seeds produced by its own pollen. We thus see--and the fact is highly remarkable--that with Primula the illegitimate unions relatively to the legitimate are more sterile than crosses between distinct species of other genera relatively to their pure unions. Mr. Scott has given a still more striking ill.u.s.tration of the same fact: he crossed Primula auricula with pollen of four other species (P. palinuri, viscosa, hirsuta, and verticillata), and these hybrid unions yielded a larger average number of seeds than did P. auricula when fertilised illegitimately with its own-form pollen. (1/7. 'Journal of the Linnean Society Botany' volume 8 1864 page 93.)

The benefit which heterostyled dimorphic plants derive from the existence of the two forms is sufficiently obvious, namely, the intercrossing of distinct plants being thus ensured. (1/8. I have shown in my work on the 'Effects of Cross and Self-fertilisation' how greatly the offspring from intercrossed plants profit in height, vigour, and fertility.) Nothing can be better adapted for this end than the relative positions of the anthers and stigmas in the two forms, as shown in Figure 1.2; but to this whole subject I shall recur. No doubt pollen will occasionally be placed by insects or fall on the stigma of the same flower; and if cross-fertilisation fails, such self-fertilisation will be advantageous to the plant, as it will thus be saved from complete barrenness. But the advantage is not so great as might at first be thought, for the seedlings from illegitimate unions do not generally consist of both forms, but all belong to the parent form; they are, moreover, in some degree weakly in const.i.tution, as will be shown in a future chapter. If, however, a flower's own pollen should first be placed by insects or fall on the stigma, it by no means follows that cross-fertilisation will be thus prevented. It is well known that if pollen from a distinct species be placed on the stigma of a plant, and some hours afterwards its own pollen be placed on it, the latter will be prepotent and will quite obliterate any effect from the foreign pollen; and there can hardly be a doubt that with heterostyled dimorphic plants, pollen from the other form will obliterate the effects of pollen from the same form, even when this has been placed on the stigma a considerable time before. To test this belief, I placed on several stigmas of a long-styled cowslip plenty of pollen from the same plant, and after twenty-four hours added some from a short-styled dark-red Polyanthus, which is a variety of the cowslip. From the flowers thus treated 30 seedlings were raised, and all these, without exception, bore reddish flowers; so that the effect of pollen from the same form, though placed on the stigmas twenty-four hours previously, was quite destroyed by that of pollen from a plant belonging to the other form.

Finally, I may remark that of the four kinds of unions, that of the short-styled illegitimately fertilised with its own-form pollen seems to be the most sterile of all, as judged by the average number of seeds, which the capsules contained.

A smaller proportion, also, of these seeds than of the others germinated, and they germinated more slowly. The sterility of this union is the more remarkable, as it has already been shown that the short-styled plants yield a larger number of seeds than the long-styled, when both forms are fertilised, either naturally or artificially, in a legitimate manner.

In a future chapter, when I treat of the offspring from heterostyled dimorphic and trimorphic plants illegitimately fertilised with their own-form pollen, I shall have occasion to show that with the present species and several others, equal-styled varieties sometimes appear.

Primula elatior, Jacq.

Bardfield oxlip of English authors.

This plant, as well as the last or cowslip (P. veris, vel officinalis), and the primrose (P. vulgaris, vel acaulis) have been considered by some botanists as varieties of the same species. But they are all three undoubtedly distinct, as will be shown in the next chapter. The present species resembles to a certain extent in general appearance the common oxlip, which is a hybrid between the cowslip and primrose. Primula elatior is found in England only in two or three of the eastern counties; and I was supplied with living plants by Mr. Doubleday, who, as I believe, first called attention to its existence in England. It is common in some parts of the Continent; and H. Muller has seen several kinds of humble-bees and other bees, and Bombylius, visiting the flowers in North Germany. (1/9. 'Die Befruchtung der Blumen' page 347.)

The results of my trials on the relative fertility of the two forms, when legitimately and illegitimately fertilised, are given in Table 1.8.

TABLE 1.8. Primula elatior.

Column 1: Nature of the Union.

Column 2: Number of Flowers fertilised.

Column 3: Number of good Capsules produced.

Column 4: Maximum Number of Seeds in any one Capsule.

Column 5: Minimum Number of Seeds in any one Capsule.

Column 6: Average Number of Seeds per Capsule.

Long-styled by pollen of short-styled. Legitimate union : 10 : 6 : 62 : 34 : 46.5.

Long-styled by own-form pollen. Illegitimate union : 20 : 4 : 49* : 2 : 27.7.

(*These seeds were so poor and small that they could hardly have germinated.)

Short-styled by pollen of long-styled. Legitimate union: 10 : 8 : 61 : 37 : 47.7.

Short-styled by own-form pollen. Illegitimate union : 17 : 3 : 19 : 9 : 12.1.

SUMMARY:

The two legitimate unions together : 20 : 14 : 62 : 37 : 47.1.

The two illegitimate unions together : 37 : 7 : 49* : 2 : 35.5.

(*These seeds were so poor and small that they could hardly have germinated.)

If we compare the fertility of the two legitimate unions taken together with that of the two illegitimate unions together, as judged by the proportional number of flowers which when fertilised in the two methods yielded capsules, the ratio is as 100 to 27; so that by this standard the present species is much more sterile than P. veris, when both species are illegitimately fertilised. If we judge of the relative fertility of the two kinds of unions by the average number of seeds per capsule, the ratio is as 100 to 75. But this latter number is probably much too high, as many of the seeds produced by the illegitimately fertilised long-styled flowers were so small that they probably would not have germinated, and ought not to have been counted. Several long-styled and short- styled plants were protected from the access of insects, and must have been spontaneously self-fertilised. They yielded altogether only six capsules, containing any seeds; and their average number was only 7.8 per capsule. Some, moreover, of these seeds were so small that they could hardly have germinated.

Herr W. Breitenbach informs me that he examined, in two sites near the Lippe (a tributary of the Rhine), 894 flowers produced by 198 plants of this species; and he found 467 of these flowers to be long-styled, 411 short-styled, and 16 equal- styled. I have heard of no other instance with heterostyled plants of equal- styled flowers appearing in a state of nature, though far from rare with plants which have been long cultivated. It is still more remarkable that in eighteen cases the same plant produced both long-styled and short-styled, or long-styled and equal-styled flowers; and in two out of the eighteen cases, long-styled, short-styled, and equal-styled flowers. The long-styled flowers greatly preponderated on these eighteen plants,--61 consisting of this form, 15 of equal-styled, and 9 of the short-styled form.

Primula vulgaris (var. acaulis, Linn.) The primrose of English Writers.

(FIGURE 1.3. Outlines of pollen-grains of Primula vulgaris, distended with water, much magnified and drawn under the camera lucida. The upper and smaller grains from the long-styled form; the lower and larger grains from the short- styled.)

Mr. J. Scott examined 100 plants growing near Edinburgh, and found 44 to be long-styled, and 56 short-styled; and I took by chance 79 plants in Kent, of which 39 were long-styled and 40 short-styled; so that the two lots together consisted of 83 long-styled and 96 short-styled plants. In the long-styled form the pistil is to that of the short-styled in length, from an average of five measurements, as 100 to 51. The stigma in the long-styled form is conspicuously more globose and much more papillose than in the short-styled, in which latter it is depressed on the summit; it is equally broad in the two forms. In both it stands nearly, but not exactly, on a level with the anthers of the opposite form; for it was found, from an average of 15 measurements, that the distance between the middle of the stigma and the middle of the anthers in the short- styled form is to that in the long-styled as 100 to 93. The anthers do not differ in size in the two forms. The pollen-grains from the short-styled flowers before they were soaked in water were decidedly broader, in proportion to their length, than those from the long-styled; after being soaked they were relatively to those from the long-styled as 100 to 71 in diameter, and more transparent. A large number of flowers from the two forms were compared, and 12 of the finest flowers from each lot were measured, but there was no sensible difference between them in size. Nine long-styled and eight short-styled plants growing together in a state of nature were marked, and their capsules collected after they had been naturally fertilised; and the seeds from the short-styled weighed exactly twice as much as those from an equal number of long-styled plants. So that the primrose resembles the cowslip in the short-styled plants, being the more productive of the two forms. The results of my trials on the fertility of the two forms, when legitimately and illegitimately fertilised, are given in Table 1.9.

TABLE 1.9. Primula vulgaris.

Column 1: Nature of the Union.

Column 2: Number of Flowers fertilised.

Column 3: Number of good Capsules produced.

Column 4: Maximum Number of Seeds in any one Capsule.

Column 5: Minimum Number of Seeds in any one Capsule.

Column 6: Average Number of Seeds per Capsule.

Long-styled by pollen of short-styled. Legitimate union : 12 : 11 : 77 : 47 : 66.9.

Long-styled by own-form pollen. Illegitimate union : 21 : 14 : 66 : 30 : 52.2.

Short-styled by pollen of long-styled. Legitimate union: 8 : 7 : 75 : 48 : 65.0.

Short-styled by own-form pollen. Illegitimate union : 18 : 7 : 43 : 5 : 18.8 (This average is perhaps rather too low).

SUMMARY: