The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 9

In Pot 1 the tallest crossed plant was 8 1/2 inches, and the tallest self-fertilised 5 inches in height. In Pot 2, the tallest crossed plant was 6 1/2 inches, and the tallest self-fertilised plant, which consisted of the white variety, 7 inches in height; and this was the first instance in my experiments on Mimulus in which the tallest self-fertilised plant exceeded the tallest crossed. Nevertheless, the two tallest crossed plants taken together were to the two tallest self-fertilised plants in height as 100 to 80. As yet the crossed plants were superior to the self-fertilised in fertility; for twelve flowers on the crossed plants were crossed and yielded ten capsules, the seeds of which weighed 1.71 grain. Twenty flowers on the self-fertilised plants were self-fertilised, and produced fifteen capsules, all appearing poor; and the seeds from ten of them weighed only .68 grain, so that from an equal number of capsules the crossed seeds were to the self-fertilised in weight as 100 to 40.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.

Seeds from both lots of the fourth generation, fertilised in the usual manner, were sown on opposite sides of three pots. When the seedlings flowered, most of the self-fertilised plants were found to consist of the tall white variety. Several of the crossed plants in Pot 1 likewise belonged to this variety, as did a very few in Pots 2 and 3. The tallest crossed plant in Pot 1 was 7 inches, and the tallest self-fertilised plant on the opposite side 8 inches; in Pots 2 and 3 the tallest crossed were 4 1/2 and 5 1/2, and the tallest self-fertilised 7 and 6 1/2 inches in height; so that the average height of the tallest plants in the two lots was as 100 for the crossed to 126 for the self-fertilised; and thus we have a complete reversal of what occurred in the four previous generations. Nevertheless, in all three pots the crossed plants retained their habit of flowering before the self-fertilised. The plants were unhealthy from being crowded and from the extreme heat of the season, and were in consequence more or less sterile; but the crossed plants were somewhat less sterile than the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.

Seeds from plants of the fifth generation crossed and self-fertilised in the usual manner were sown on opposite sides of several pots. On the self-fertilised side every single plant belonged to the tall white variety. On the crossed side some plants belonged to this variety, but the greater number approached in character to the old and shorter kinds with smaller yellowish flowers blotched with coppery brown. When the plants on both sides were from 2 to 3 inches in height they were equal, but when fully grown the self-fertilised were decidedly the tallest and finest plants, but, from want of time, they were not actually measured.

In half the pots the first plant which flowered was a self-fertilised one, and in the other half a crossed one. And now another remarkable change was clearly perceived, namely, that the self-fertilised plants had become more self-fertile than the crossed. The pots were all put under a net to exclude insects, and the crossed plants produced spontaneously only fifty-five capsules, whilst the self-fertilised plants produced eighty-one capsules, or as 100 to 147. The seeds from nine capsules of both lots were placed in separate watch-gla.s.ses for comparison, and the self-fertilised appeared rather the more numerous.

Besides these spontaneously self-fertilised capsules, twenty flowers on the crossed plants again crossed yielded sixteen capsules; twenty-five flowers on the self-fertilised plants again self-fertilised yielded seventeen capsules, and this is a larger proportional number of capsules than was produced by the self-fertilised flowers on the self-fertilised plants in the previous generations. The contents of ten capsules of both these lots were compared in separate watch-gla.s.ses, and the seeds from the self-fertilised appeared decidedly more numerous than those from the crossed plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.

Crossed and self-fertilised seeds from the crossed and self-fertilised plants of the sixth generation were sown in the usual manner on opposite sides of three pots, and the seedlings were well and equally thinned.

Every one of the self-fertilised plants (and many were raised) in this, as well as in the eighth and ninth generations, belonged to the tall white variety. Their uniformity of character, in comparison with the seedlings first raised from the purchased seed, was quite remarkable. On the other hand, the crossed plants differed much in the tints of their flowers, but not, I think, to so great a degree as those first raised. I determined this time to measure the plants on both sides carefully. The self-fertilised seedlings came up rather before the crossed, but both lots were for a time of equal height. When first measured, the average height of the six tallest crossed plants in the three pots was 7.02, and that of the six tallest self-fertilised plants 8.97 inches, or as 100 to 128. When fully grown the same plants were again measured, with the result shown in Table 3/18.

TABLE 3/18. Mimulus luteus (Seventh Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 11 2/8 : 19 1/8.

Pot 1 : 11 7/8 : 18.

Pot 2 : 12 6/8 : 18 2/8.

Pot 2 : 11 2/8 : 14 6/8.

Pot 3 : 9 6/8 : 12 6/8.

Pot 3 : 11 6/8 : 11.

Total : 68.63 : 93.88.

The average height of the six crossed is here 11.43, and that of the six self-fertilised 15.64, or as 100 to 137.

As it is now evident that the tall white variety transmitted its characters faithfully, and as the self-fertilised plants consisted exclusively of this variety, it was manifest that they would always exceed in height the crossed plants which belonged chiefly to the original shorter varieties. This line of experiment was therefore discontinued, and I tried whether intercrossing two self-fertilised plants of the sixth generation, growing in distinct pots, would give their offspring any advantage over the offspring of flowers on one of the same plants fertilised with their own pollen. These latter seedlings formed the seventh generation of self-fertilised plants, like those in the right hand column in Table 3/18; the crossed plants were the product of six previous self-fertilised generations with an intercross in the last generation. The seeds were allowed to germinate on sand, and were planted in pairs on opposite sides of four pots, all the remaining seeds being sown crowded on opposite sides of Pot 5 in Table 3/19; the three tallest on each side in this latter pot being alone measured. All the plants were twice measured--the first time whilst young, and the average height of the crossed plants to that of the self-fertilised was then as 100 to 122. When fully grown they were again measured, as in Table 3/19.

TABLE 3/19. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Intercrossed Plants from Self-fertilised Plants of the Sixth Generation.

Column 3: Self-fertilised Plants of the Seventh Generation.

Pot 1 : 12 6/8 : 15 2/8.

Pot 1 : 10 4/8 : 11 5/8.

Pot 1 : 10 : 11.

Pot 1 : 14 5/8 : 11.

Pot 2 : 10 2/8 : 11 3/8.

Pot 2 : 7 6/8 : 11 4/8.

Pot 2 : 12 1/8 : 8 5/8.

Pot 2 : 7 : 14 3/8.

Pot 3 : 13 5/8 : 10 3/8.

Pot 3 : 12 2/8 : 11 6/8.

Pot 4 : 7 1/8 : 14 6/8.

Pot 4 : 8 2/8 : 7.

Pot 4 : 7 2/8 : 8.

Pot 5 : 8 5/8 : 10 2/8 Pot 5 : 9 : 9 3/8.

Pot 5 : 8 2/8 : 9 2/8.

Crowded.

Total : 159.38 : 175.50.

The average height of the sixteen intercrossed plants is here 9.96 inches, and that of the sixteen self-fertilised plants 10.96, or as 100 to 110; so that the intercrossed plants, the progenitors of which had been self-fertilised for the six previous generations, and had been exposed during the whole time to remarkably uniform conditions, were somewhat inferior in height to the plants of the seventh self-fertilised generation. But as we shall presently see that a similar experiment made after two additional generations of self-fertilisation gave a different result, I know not how far to trust the present one. In three of the five pots in Table 3/19 a self-fertilised plant flowered first, and in the other two a crossed plant. These self-fertilised plants were remarkably fertile, for twenty flowers fertilised with their own pollen produced no less than nineteen very fine capsules!

THE EFFECTS OF A CROSS WITH A DISTINCT STOCK.

Some flowers on the self-fertilised plants in Pot 4 in Table 3/19 were fertilised with their own pollen, and plants of the eighth self-fertilised generation were thus raised, merely to serve as parents in the following experiment. Several flowers on these plants were allowed to fertilise themselves spontaneously (insects being of course excluded), and the plants raised from these seeds formed the ninth self-fertilised generation; they consisted wholly of the tall white variety with crimson blotches. Other flowers on the same plants of the eighth self-fertilised generation were crossed with pollen taken from another plant of the same lot; so that the seedlings thus raised were the offspring of eight previous generations of self-fertilisation with an intercross in the last generation; these I will call the INTERCROSSED PLANTS. Lastly, other flowers on the same plants of the eighth self-fertilised generation were crossed with pollen taken from plants which had been raised from seed procured from a garden at Chelsea. The Chelsea plants bore yellow flowers blotched with red, but differed in no other respect. They had been grown out of doors, whilst mine had been cultivated in pots in the greenhouse for the last eight generations, and in a different kind of soil. The seedlings raised from this cross with a wholly different stock may be called the CHELSEA-CROSSED. The three lots of seeds thus obtained were allowed to germinate on bare sand; and whenever a seed in all three lots, or in only two, germinated at the same time, they were planted in pots superficially divided into three or two compartments. The remaining seeds, whether or not in a state of germination, were thickly sown in three divisions in a large pot, 10, in Table 3/20. When the plants had grown to their full height they were measured, as shown in Table 3/20; but only the three tallest plants in each of the three divisions in Pot 10 were measured.

TABLE 3/20. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Plants from Self-fertilised Plants of the Eighth Generation crossed by Chelsea Plants.

Column 3: Plants from an intercross between the Plants of the Eighth Self-fertilised Generation.

Column 4: Self-fertilised Plants of the Ninth Generation from Plants of the Eighth Self-fertilised Generation.

Pot 1 : 30 7/8 : 14 : 9 4/8.

Pot 1 : 28 3/8 : 13 6/8 : 10 5/8.

Pot 1 : -- : 13 7/8 : 10.

Pot 2 : 20 6/8 : 11 4/8 : 11 6/8.

Pot 2 : 22 2/8 : 12 : 12 3/8.