The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 12

Pot 1 : 57 6/8 : 0.

Pot 1 : 65 : 0.

Pot 2 : 34 4/8 : 39.

Pot 2 : 52 4/8 : 32.

Pot 2 : 63 6/8 : 21.

Pot 3 : 57 4/8 : 53 4/8.

Pot 3 : 53 4/8 : 0.

Pot 3 : 50 6/8 : 0.

Pot 3 : 37 2/8 : 0.

Pot 4 : 64 4/8 : 34 4/8.

Pot 4 : 37 4/8 : 23 6/8.

Pot 4 : -- : 0.

Pot 5 : 53 : 0.

Pot 5 : 47 6/8 : 0.

Pot 5 : 34 6/8 : 0.

Total : 821.25 : 287.00.

The average height of the flower-stems of the sixteen crossed plants is here 51.33 inches; and that of the eight self-fertilised plants, 35.87; or as 100 to 70. But this difference in height does not give at all a fair idea of the vast superiority of the crossed plants. These latter produced altogether sixty-four flower-stems, each plant producing, on an average, exactly four flower-stems, whereas the eight self-fertilised plants produced only fifteen flower-stems, each producing an average only of 1.87 stems, and these had a less luxuriant appearance. We may put the result in another way: the number of flower-stems on the crossed plants was to those on an equal number of self-fertilised plants as 100 to 48.

Three crossed seeds in a state of germination were also planted in three separate pots; and three self-fertilised seeds in the same state in three other pots. These plants were therefore at first exposed to no compet.i.tion with one another, and when turned out of their pots into the open ground they were planted at a moderate distance apart, so that they were exposed to much less severe compet.i.tion than in the last case. The longest leaves on the three crossed plants, when turned out, exceeded those on the self-fertilised plants by a mere trifle, namely, on an average by .17 of an inch. When fully grown the three crossed plants produced twenty-six flower-stems; the two tallest of which on each plant were on an average 54.04 inches in height. The three self-fertilised plants produced twenty-three flower-stems, the two tallest of which on each plant had an average height of 46.18 inches. So that the difference between these two lots, which hardly competed together, is much less than in the last case when there was moderately severe compet.i.tion, namely, as 100 to 85, instead of as 100 to 70.

THE EFFECTS ON THE OFFSPRING OF INTERCROSSING DIFFERENT FLOWERS ON THE SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.

A fine plant growing in my garden (one of the foregoing seedlings) was covered with a net, and six flowers were crossed with pollen from another flower on the same plant, and six others were fertilised with their own pollen. All produced good capsules. The seeds from each were placed in separate watch-gla.s.ses, and no difference could be perceived by the eye between the two lots of seeds; and when they were weighed there was no difference of any significance, as the seeds from the self-fertilised capsules weighed 7.65 grains, whilst those from the crossed capsules weighed 7.7 grains. Therefore the sterility of the present species, when insects are excluded, is not due to the impotence of pollen on the stigma of the same flower. Both lots of seeds and seedlings were treated in exactly the same manner as in Table 3/23, excepting that after the pairs of germinating seeds had been planted on the opposite sides of eight pots, all the remaining seeds were thickly sown on the opposite sides of Pots 9 and 10 in Table 3/24. The young plants during the following spring were turned out of their pots, without being disturbed, and planted in the open ground in two rows, not very close together, so that they were subjected to only moderately severe compet.i.tion with one another. Very differently to what occurred in the first experiment, when the plants were subjected to somewhat severe mutual compet.i.tion, an equal number on each side either died or did not produce flower-stems. The tallest flower-stems on the surviving plants were measured, as shown in Table 3/24.

TABLE 3/24. Digitalis purpurea.

The tallest Flower-stem on each Plant measured in inches: 0 signifies that the Plant died, or did not produce a Flower-stem.

Column 1: Number (Name) of Pot.

Column 2: Plants raised from a Cross between different Flowers on the same Plant.

Column 3: Plants raised from Flowers fertilised with their own Pollen.

Pot 1 : 49 4/8 : 45 5/8.

Pot 1 : 46 7/8 : 52.

Pot 1 : 43 6/8 : 0.

Pot 2 : 38 4/8 : 54 4/8.

Pot 2 : 47 4/8 : 47 4/8.

Pot 2 : 0 : 32 5/8.

Pot 3 : 54 7/8 : 46 5/8.

Pot 4 : 32 1/8 : 41 3/8.

Pot 4 : 0 : 29 7/8.

Pot 4 : 43 7/8 : 37 1/8.

Pot 5 : 46 6/8 : 42 1/8.

Pot 5 : 40 4/8 : 42 1/8.

Pot 5 : 43 : 0.

Pot 6 : 48 2/8 : 47 7/8.

Pot 6 : 46 2/8 : 48 3/8.

Pot 7 : 48 5/8 : 25.

Pot 7 : 42 : 40 5/8.

Pot 8 : 46 7/8 : 39 1/8.

Pot 9 : 49 : 30 3/8.

Pot 9 : 50 3/8 : 15.

Pot 9 : 46 3/8 : 36 7/8.

Pot 9 : 47 6/8 : 44 1/8.

Pot 9 : 0 : 31 6/8.

Crowded Plants.

Pot 10 : 46 4/8 : 47 7/8.

Pot 10 : 35 2/8 : 0.

Pot 10 : 24 5/8 : 34 7/8.

Pot 10 : 41 4/8 : 40 7/8.

Pot 10 : 17 3/8 : 41 1/8.

Crowded Plants.

Total : 1078.00 : 995.38.

The average height of the flower-stems on the twenty-five crossed plants in all the pots taken together is 43.12 inches, and that of the twenty-five self-fertilised plants 39.82, or as 100 to 92. In order to test this result, the plants planted in pairs in Pots 1 and 8 were considered by themselves, and the average height of the sixteen crossed plants is here 44.9, and that of the sixteen self-fertilised plants 42.03, or as 100 to 94. Again, the plants raised from the thickly sown seed in Pots 9 and 10, which were subjected to very severe mutual compet.i.tion, were taken by themselves, and the average height of the nine crossed plants is 39.86, and that of the nine self-fertilised plants 35.88, or as 100 to 90. The plants in these two latter pots (9 and 10), after being measured, were cut down close to the ground and weighed: the nine crossed plants weighed 57.66 ounces, and the nine self-fertilised plants 45.25 ounces, or as 100 to 78. On the whole we may conclude, especially from the evidence of weight, that seedlings from a cross between flowers on the same plant have a decided, though not great, advantage over those from flowers fertilised with their own pollen, more especially in the case of the plants subjected to severe mutual compet.i.tion. But the advantage is much less than that exhibited by the crossed offspring of distinct plants, for these exceeded the self-fertilised plants in height as 100 to 70, and in the number of flower-stems as 100 to 48. Digitalis thus differs from Ipomoea, and almost certainly from Mimulus, as with these two species a cross between flowers on the same plant did no good.

CALCEOLARIA.

A BUSHY GREENHOUSE VARIETY, WITH YELLOW FLOWERS BLOTCHED WITH PURPLE.

The flowers in this genus are constructed so as to favour or almost ensure cross-fertilisation (3/6. Hildebrand as quoted by H. Muller 'Die Befruchtung der Blumen' 1873 page 277.); and Mr. Anderson remarks that extreme care is necessary to exclude insects in order to preserve any kind true. (3/7. 'Gardeners' Chronicle' 1853 page 534.) He adds the interesting statement, that when the corolla is cut quite away, insects, as far as he has seen, never discover or visit the flowers. This plant is, however, self-fertile if insects are excluded. So few experiments were made by me, that they are hardly worth giving. Crossed and self-fertilised seeds were sown on opposite sides of a pot, and after a time the crossed seedlings slightly exceeded the self-fertilised in height. When a little further grown, the longest leaves on the former were very nearly 3 inches in length, whilst those on the self-fertilised plants were only 2 inches. Owing to an accident, and to the pot being too small, only one plant on each side grew up and flowered; the crossed plant was 19 1/2 inches in height, and the self-fertilised one 15 inches; or as 100 to 77.

Linaria vulgaris.

It has been mentioned in the introductory chapter that two large beds of this plant were raised by me many years ago from crossed and self-fertilised seeds, and that there was a conspicuous difference in height and general appearance between the two lots. The trial was afterwards repeated with more care; but as this was one of the first plants experimented on, my usual method was not followed. Seeds were taken from wild plants growing in this neighbourhood and sown in poor soil in my garden. Five plants were covered with a net, the others being left exposed to the bees, which incessantly visit the flowers of this species, and which, according to H. Muller, are the exclusive fertilisers. This excellent observer remarks that, as the stigma lies between the anthers and is mature at the same time with them, self-fertilisation is possible. (3/8. 'Die Befruchtung' etc. page 279.) But so few seeds are produced by protected plants, that the pollen and stigma of the same flower seem to have little power of mutual interaction. The exposed plants bore numerous capsules forming solid spikes. Five of these capsules were examined and appeared to contain an equal number of seeds; and these being counted in one capsule, were found to be 166. The five protected plants produced altogether only twenty-five capsules, of which five were much finer than all the others, and these contained an average of 23.6 seeds, with a maximum in one capsule of fifty-five. So that the number of seeds in the capsules on the exposed plants to the average number in the finest capsules on the protected plants was as 100 to 14.

Some of the spontaneously self-fertilised seeds from under the net, and some seeds from the uncovered plants naturally fertilised and almost certainly intercrossed by the bees, were sown separately in two large pots of the same size; so that the two lots of seedlings were not subjected to any mutual compet.i.tion. Three of the crossed plants when in full flower were measured, but no care was taken to select the tallest plants; their heights were 7 4/8, 7 2/8, and 6 4/8 inches; averaging 7.08 in height. The three tallest of all the self-fertilised plants were then carefully selected, and their heights were 6 3/8, 5 5/8, and 5 2/8, averaging 5.75 in height. So that the naturally crossed plants were to the spontaneously self-fertilised plants in height, at least as much as 100 to 81.

Verbasc.u.m thapsus.

The flowers of this plant are frequented by various insects, chiefly by bees, for the sake of the pollen. Hermann Muller, however, has shown ('Die Befruchtung' etc. page 277) that V. nigrum secretes minute drops of nectar. The arrangement of the reproductive organs, though not at all complex, favours cross-fertilisation; and even distinct species are often crossed, for a greater number of naturally produced hybrids have been observed in this genus than in almost any other. (3/9. I have given a striking case of a large number of such hybrids between Verbasc.u.m thapsus and lychnitis found growing wild: 'Journal of Linnean Society Botany' volume 10 page 451.) Nevertheless the present species is perfectly self-fertile, if insects are excluded; for a plant protected by a net was as thickly loaded with fine capsules as the surrounding uncovered plants. Verbasc.u.m lychnitis is rather less self-fertile, for some protected plants did not yield quite so many capsules as the adjoining uncovered plants.