The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 39

In Tables 6/88, 6/89 and 6/90, we have the measurements of fifty-six plants derived from two plants of the third self-fertilised generation crossed with pollen from a fresh stock, and of fifty-six plants of the fourth self-fertilised generation derived from the same two plants.

These crossed and self-fertilised plants were treated in three different ways, having been put, firstly, into moderately close compet.i.tion with one another in pots; secondly, having been subjected to unfavourable conditions and to very severe compet.i.tion from being greatly crowded in two large pots; and thirdly, having been sown separately in open and good ground, so as not to suffer from any mutual compet.i.tion. In all these cases the crossed plants in each lot were greatly superior to the self-fertilised. This was shown in several ways,--by the earlier germination of the crossed seeds, by the more rapid growth of the seedlings whilst quite young, by the earlier flowering of the mature plants, as well as by the greater height which they ultimately attained.

The superiority of the crossed plants was shown still more plainly when the two lots were weighed; the weight of the crossed plants to that of the self-fertilised in the two crowded pots being as 100 to 37. Better evidence could hardly be desired of the immense advantage derived from a cross with a fresh stock.

26. PRIMULACEAE.--Cyclamen persic.u.m. (6/5. Cyclamen repandum according to Lecoq 'Geographie Botanique de l'Europe' tome 8 1858 page 150, is proterandrous, and this I believe to be the case with Cyclamen persic.u.m.)

Ten flowers crossed with pollen from plants known to be distinct seedlings, yielded nine capsules, containing on an average 34.2 seeds, with a maximum of seventy-seven in one. Ten flowers self-fertilised yielded eight capsules, containing on an average only 13.1 seeds, with a maximum of twenty-five in one. This gives a ratio of 100 to 38 for the average number of seeds per capsule for the crossed and self-fertilised flowers. The flowers hang downwards, and as the stigmas stand close beneath the anthers, it might have been expected that pollen would have fallen on them, and that they would have been spontaneously self-fertilised; but these covered-up plants did not produce a single capsule. On some other occasions uncovered plants in the same greenhouse produced plenty of capsules, and I suppose that the flowers had been visited by bees, which could hardly fail to carry pollen from plant to plant.

The seeds obtained in the manner just described were placed on sand, and after germinating were planted in pairs,--three crossed and three self-fertilised plants on the opposite sides of four pots. When the leaves were 2 or 3 inches in length, including the foot-stalks, the seedlings on both sides were equal. In the course of a month or two the crossed plants began to show a slight superiority over the self-fertilised, which steadily increased; and the crossed flowered in all four pots some weeks before, and much more profusely than the self-fertilised. The two tallest flower-stems on the crossed plants in each pot were now measured, and the average height of the eight stems was 9.49 inches. After a considerable interval of time the self-fertilised plants flowered, and several of their flower-stems (but I forgot to record how many) were roughly measured, and their average height was a little under 7.5 inches; so that the flower-stems on the crossed plants to those on the self-fertilised were at least as 100 to 79. The reason why I did not make more careful measurements of the self-fertilised plants was, that they looked such poor specimens that I determined to there them re-potted in larger pots and in the following year to measure them carefully; but we shall see that this was partly frustrated by so few flower-stems being then produced.

These plants were left uncovered in the greenhouse; and the twelve crossed plants produced forty capsules, whilst the twelve self-fertilised plants produced only five; or as 100 to 12. But this difference does not give a just idea of the relative fertility of the two lots. I counted the seeds in one of the finest capsules on the crossed plants, and it contained seventy-three; whilst the finest of the five capsules produced by the self-fertilised plants contained only thirty-five good seeds. In the other four capsules most of the seeds were barely half as large as those in the crossed capsules.

TABLE 6/91. Cyclamen persic.u.m: 0 implies that no flower-stem was produced.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 10 : 0.

Pot 1 : 9 2/8 : 0.

Pot 1 : 10 2/8 : 0.

Pot 2 : 9 2/8 : 0.

Pot 2 : 10 : 0.

Pot 2 : 10 2/8 : 0.

Pot 3 : 9 1/8 : 8.

Pot 3 : 9 5/8 : 6 7/8.

Pot 3 : 9 5/8 : 6 6/8.

Pot 4 : 11 1/8 : 0.

Pot 4 : 10 5/8 : 7 7/8.

Pot 4 : 10 6/8 : 0.

Total : 119.88 : 29.50.

In the following year the crossed plants again bore many flowers before the self-fertilised bore a single one. The three tallest flower-stems on the crossed plants in each of the pots were measured, as shown in Table 6/91. In Pots 1 and 2 the self-fertilised plants did not produce a single flower-stem; in Pot 4 only one; and in Pot 3 six, of which the three tallest were measured.

The average height of the twelve flower-stems on the crossed plants is 9.99, and that of the four flower-stems on the self-fertilised plants 7.37 inches; or as 100 to 74. The self-fertilised plants were miserable specimens, whilst the crossed ones looked very vigorous.

ANAGALLIS.

Anagallis collina, var. grandiflora (pale red and blue-flowered sub-varieties).

Firstly, twenty-five flowers on some plants of the red variety were crossed with pollen from a distinct plant of the same variety, and produced ten capsules; thirty-one flowers were fertilised with their own pollen, and produced eighteen capsules. These plants, which were grown in pots in the greenhouse, were evidently in a very sterile condition, and the seeds in both sets of capsules, especially in the self-fertilised, although numerous, were of so poor a quality that it was very difficult to determine which were good and which bad. But as far as I could judge, the crossed capsules contained on an average 6.3 good seeds, with a maximum in one of thirteen; whilst the self-fertilised contained 6.05 such seeds, with a maximum in one of fourteen.

Secondly, eleven flowers on the red variety were castrated whilst young and fertilised with pollen from the blue variety, and this cross evidently much increased their fertility; for the eleven flowers yielded seven capsules, which contained on an average twice as many good seeds as before, namely, 12.7; with a maximum in two of the capsules of seventeen seeds. Therefore these crossed capsules yielded seeds compared with those in the foregoing self-fertilised capsules, as 100 to 48.

These seeds were also conspicuously larger than those from the cross between two individuals of the same red variety, and germinated much more freely. The flowers on most of the plants produced by the cross between the two-coloured varieties (of which several were raised), took after their mother, and were red-coloured. But on two of the plants the flowers were plainly stained with blue, and to such a degree in one case as to be almost intermediate in tint.

The crossed seeds of the two foregoing kinds and the self-fertilised were sown on the opposite sides of two large pots, and the seedlings were measured when fully grown, as shown in Tables 6/92a and 6/92b.

TABLE 6/92a. Anagallis collina: Red variety crossed by a distinct plant of the red variety, and red variety self-fertilised.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 23 4/8 : 15 4/8.

Pot 1 : 21 : 15 4/8.

Pot 1 : 17 2/8 : 14.

Total : 61.75 : 45.00.

TABLE 6/92b. Anagallis collina: Red variety crossed by blue variety, and red variety self-fertilised.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 2 : 30 4/8 : 24 4/8.

Pot 2 : 27 3/8 : 18 4/8.

Pot 2 : 25 : 11 6/8.

Total : 82.88 : 54.75.

Total of both lots: : 144.63 : 99.75.

As the plants of the two lots are few in number, they may be run together for the general average; but I may first state that the height of the seedlings from the cross between two individuals of the red variety is to that of the self-fertilised plants of the red variety as 100 to 73; whereas the height of the crossed offspring from the two varieties to the self-fertilised plants of the red variety is as 100 to 66. So that the cross between the two varieties is here seen to be the most advantageous. The average height of all six crossed plants in the two lots taken together is 48.20, and that of the six self-fertilised plants 33.25; or as 100 to 69.

These six crossed plants produced spontaneously twenty-six capsules, whilst the six self-fertilised plants produced only two, or as 100 to 8.

There is therefore the same extraordinary difference in fertility between the crossed and self-fertilised plants as in the last genus, Cyclamen, which belongs to the same family of the Primulaceae.

Primula veris. British flora. (var. officinalis, Linn.).

THE COWSLIP.

Most of the species in this genus are heterostyled or dimorphic; that is, they present two forms,--one long-styled with short stamens, and the other short-styled with long stamens. (6/6. See my paper 'On the Two Forms or Dimorphic Condition in the Species of Primula' in 'Journal of the Proceedings of the Linnean Society' volume 6 1862 page 77. A second paper, to which I presently refer 'On the Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic and Trimorphic Plants' was published in volume 10 1867 page 393 of the same journal.) For complete fertilisation it is necessary that pollen from the one form should be applied to the stigma of the other form; and this is effected under nature by insects. Such unions, and the seedlings raised from them, I have called legitimate. If one form is fertilised with pollen from the same form, the full complement of seed is not produced; and in the case of some heterostyled genera no seed at all is produced. Such unions, and the seedlings raised from them, I have called illegitimate.