The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 21

In the summer of 1867, which was a very unfavourable one, I again crossed several flowers under a net with pollen from a distinct plant, and fertilised other flowers on the same plant with their own pollen.

The former yielded a much larger proportion of capsules than the latter; and many of the seeds in the self-fertilised capsules, though numerous, were so poor that an equal number of seeds from the crossed and self-fertilised capsules were in weight as 100 to 45. The two lots were allowed to germinate on sand, and pairs were planted on the opposite sides of four pots. When nearly two-thirds grown they were measured, as shown in Table 4/44.

TABLE 4/44. Delphinium consolida.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 11 : 11.

Pot 2 : 19 : 16 2/8.

Pot 2 : 16 2/8 : 11 4/8.

Pot 3 : 26 : 22.

Pot 4 : 9 4/8 : 8 2/8.

Pot 4 : 8 : 6 4/8.

Total : 89.75 : 75.50.

The six crossed plants here average 14.95, and the six self-fertilised 12.50 inches in height; or as 100 to 84. When fully grown they were again measured, but from want of time only a single plant on each side was measured; so that I have thought it best to give the earlier measurements. At the later period the three tallest crossed plants still exceeded considerably in height the three tallest self-fertilised, but not in quite so great a degree as before. The pots were left uncovered in the greenhouse, but whether the flowers were intercrossed by bees or self-fertilised I do not know. The six crossed plants produced 282 mature and immature capsules, whilst the six self-fertilised plants produced only 159; or as 100 to 56. So that the crossed plants were very much more productive than the self-fertilised.

11. CARYOPHYLLACEAE.--Viscaria oculata.

Twelve flowers were crossed with pollen from another plant, and yielded ten capsules, containing by weight 5.77 grains of seeds. Eighteen flowers were fertilised with their own pollen and yielded twelve capsules, containing by weight 2.63 grains. Therefore the seeds from an equal number of crossed and self-fertilised flowers would have been in weight as 100 to 38. I had previously selected a medium-sized capsule from each lot, and counted the seeds in both; the crossed one contained 284, and the self-fertilised one 126 seeds; or as 100 to 44. These seeds were sown on opposite sides of three pots, and several seedlings raised; but only the tallest flower-stem of one plant on each side was measured.

The three on the crossed side averaged 32.5 inches, and the three on the self-fertilised side 34 inches in height; or as 100 to 104. But this trial was on much too small a scale to be trusted; the plants also grew so unequally that one of the three flower-stems on the crossed plants was very nearly twice as tall as that on one of the others; and one of the three flower-stems on the self-fertilised plants exceeded in an equal degree one of the others.

In the following year the experiment was repeated on a larger scale: ten flowers were crossed on a new set of plants and yielded ten capsules containing by weight 6.54 grains of seed. Eighteen spontaneously self-fertilised capsules were gathered, of which two contained no seed; the other sixteen contained by weight 6.07 grains of seed. Therefore the weight of seed from an equal number of crossed and spontaneously self-fertilised flowers (instead of artificially fertilised as in the previous case) was as 100 to 58.

The seeds after germinating on sand were planted in pairs on the opposite sides of four pots, with all the remaining seeds sown crowded in the opposite sides of a fifth pot; in this latter pot only the tallest plant on each side was measured. Until the seedlings had grown about 5 inches in height no difference could be perceived in the two lots. Both lots flowered at nearly the same time. When they had almost done flowering, the tallest flower-stem on each plant was measured, as shown in Table 4/45.

TABLE 4/45. Viscaria oculata.

Tallest flower-stem on each plant measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 19 : 32 3/8.

Pot 1 : 33 : 38.

Pot 1 : 41 : 38.

Pot 1 : 41 : 28 7/8.

Pot 2 : 37 4/8 : 36.

Pot 2 : 36 4/8 : 32 3/8.

Pot 2 : 38 : 35 6/8.

Pot 3 : 44 4/8 : 36.

Pot 3 : 39 4/8 : 20 7/8.

Pot 3 : 39 : 30 5/8.

Pot 4 : 30 2/8 : 36.

Pot 4 : 31 : 39.

Pot 4 : 33 1/8 : 29.

Pot 4 : 24 : 38 4/8.

Pot 5 : 30 2/8 : 32.

Crowded.

Total : 517.63 : 503.36.

The fifteen crossed plants here average 34.5, and the fifteen self-fertilised 33.55 inches in height; or as 100 to 97. So that the excess of height of the crossed plants is quite insignificant. In productiveness, however, the difference was much more plainly marked.

All the capsules were gathered from both lots of plants (except from the crowded and unproductive ones in Pot 5), and at the close of the season the few remaining flowers were added in. The fourteen crossed plants produced 381, whilst the fourteen self-fertilised plants produced only 293 capsules and flowers; or as 100 to 77.

Dianthus caryophyllus.

The common carnation is strongly proterandrous, and therefore depends to a large extent upon insects for fertilisation. I have seen only humble-bees visiting the flowers, but I dare say other insects likewise do so. It is notorious that if pure seed is desired, the greatest care is necessary to prevent the varieties which grow in the same garden from intercrossing. (4/10. 'Gardeners' Chronicle' 1847 page 268.) The pollen is generally shed and lost before the two stigmas in the same flower diverge and are ready to be fertilised. I was therefore often forced to use for self-fertilisation pollen from the same plant instead of from the same flower. But on two occasions, when I attended to this point, I was not able to detect any marked difference in the number of seeds produced by these two forms of self-fertilisation.

Several single-flowered carnations were planted in good soil, and were all covered with a net. Eight flowers were crossed with pollen from a distinct plant and yielded six capsules, containing on an average 88.6 seeds, with a maximum in one of 112 seeds. Eight other flowers were self-fertilised in the manner above described, and yielded seven capsules containing on an average 82 seeds, with a maximum in one of 112 seeds. So that there was very little difference in the number of seeds produced by cross-fertilisation and self-fertilisation, namely, as 100 to 92. As these plants were covered by a net, they produced spontaneously only a few capsules containing any seeds, and these few may perhaps be attributed to the action of Thrips and other minute insects which haunt the flowers. A large majority of the spontaneously self-fertilised capsules produced by several plants contained no seeds, or only a single one. Excluding these latter capsules, I counted the seeds in eighteen of the finest ones, and these contained on an average 18 seeds. One of the plants was spontaneously self-fertile in a higher degree than any of the others. On another occasion a single covered-up plant produced spontaneously eighteen capsules, but only two of these contained any seed, namely 10 and 15.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.

The many seeds obtained from the above crossed and artificially self-fertilised flowers were sown out of doors, and two large beds of seedlings, closely adjoining one another, thus raised. This was the first plant on which I experimented, and I had not then formed any regular scheme of operation. When the two lots were in full flower, I measured roughly a large number of plants but record only that the crossed were on an average fully 4 inches taller than the self-fertilised. Judging from subsequent measurements, we may a.s.sume that the crossed plants were about 28 inches, and the self-fertilised about 24 inches in height; and this will give us a ratio of 100 to 86.

Out of a large number of plants, four of the crossed ones flowered before any one of the self-fertilised plants.

Thirty flowers on these crossed plants of the first generation were again crossed with pollen from a distinct plant of the same lot, and yielded twenty-nine capsules, containing on an average 55.62 seeds, with a maximum in one of 110 seeds.

Thirty flowers on the self-fertilised plants were again self-fertilised; eight of them with pollen from the same flower, and the remainder with pollen from another flower on the same plant; and these produced twenty-two capsules, containing on an average 35.95 seeds, with a maximum in one of sixty-one seeds. We thus see, judging by the number of seeds per capsule, that the crossed plants again crossed were more productive than the self-fertilised again self-fertilised, in the ratio of 100 to 65. Both the crossed and self-fertilised plants, from having grown much crowded in the two beds, produced less fine capsules and fewer seeds than did their parents.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The crossed and self-fertilised seeds from the crossed and self-fertilised plants of the last generation were sown on opposite sides of two pots; but the seedlings were not thinned enough, so that both lots grew very irregularly, and most of the self-fertilised plants after a time died from being smothered. My measurements were, therefore, very incomplete. From the first the crossed seedlings appeared the finest, and when they were on an average, by estimation, 5 inches high, the self-fertilised plants were only 4 inches. In both pots the crossed plants flowered first. The two tallest flower-stems on the crossed plants in the two pots were 17 and 16 1/2 inches in height; and the two tallest flower-stems on the self-fertilised plants 10 1/2 and 9 inches; so that their heights were as 100 to 58. But this ratio, deduced from only two pairs, obviously is not in the least trustworthy, and would not have been given had it not been otherwise supported. I state in my notes that the crossed plants were very much more luxuriant than their opponents, and seemed to be twice as bulky. This latter estimate may be believed from the ascertained weights of the two lots in the next generation. Some flowers on these crossed plants were again crossed with pollen from another plant of the same lot, and some flowers on the self-fertilised plants again self-fertilised; and from the seeds thus obtained the plants of the next generation were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

The seeds just alluded to were allowed to germinate on bare sand, and were planted in pairs on the opposite sides of four pots. When the seedlings were in full flower, the tallest stem on each plant was measured to the base of the calyx. The measurements are given in Table 4/46. In Pot 1 the crossed and self-fertilised plants flowered at the same time; but in the other three pots the crossed flowered first. These latter plants also continued flowering much later in the autumn than the self-fertilised.

TABLE 4/46. Dianthus caryophyllus (third generation).

Tallest flower-stem on each plant measured in inches.

Column 1: Number (Name) of Pot.