The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 27

Total : 138.50 : 158.75.

The average height of the four crossed plants is here 34.62, and that of the four self-fertilised plants 39.68, or as 100 to 115. So that the crossed plants, far from beating the self-fertilised, were completely beaten by them.

There can be no doubt that the result would have been widely different, if any two varieties out of the numberless ones which exist had been crossed. Notwithstanding that both had been self-fertilised for many previous generations, each would almost certainly have possessed its own peculiar const.i.tution; and this degree of differentiation would have been sufficient to make a cross highly beneficial. I have spoken thus confidently of the benefit which would have been derived from crossing any two varieties of the pea from the following facts: Andrew Knight in speaking of the results of crossing reciprocally very tall and short varieties, says, "I had in this experiment a striking instance of the stimulative effects of crossing the breeds; for the smallest variety, whose height rarely exceeded 2 feet, was increased to 6 feet; whilst the height of the large and luxuriant kind was very little diminished."

(5/15. 'Philosophical Transactions' 1799 page 200.) Recently Mr. Laxton has made numerous crosses, and everyone had been astonished at the vigour and luxuriance of the new varieties which he has thus raised and afterwards fixed by selection. He gave me seed-peas produced from crosses between four distinct kinds; and the plants thus raised were extraordinarily vigorous, being in each case from 1 to 2 or even 3 feet taller than the parent-forms, which were raised at the same time close alongside. But as I did not measure their actual height I cannot give the exact ratio, but it must have been at least as 100 to 75. A similar trial was subsequently made with two other peas from a different cross, and the result was nearly the same. For instance, a crossed seedling between the Maple and Purple-podded pea was planted in poor soil and grew to the extraordinary height of 116 inches; whereas the tallest plant of either parent variety, namely, a Purple-podded pea, was only 70 inches in height; or as 100 to 60.

Sarothamnus scoparius.

Bees incessantly visit the flowers of the common Broom, and these are adapted by a curious mechanism for cross-fertilisation. When a bee alights on the wing-petals of a young flower, the keel is slightly opened and the short stamens spring out, which rub their pollen against the abdomen of the bee. If a rather older flower is visited for the first time (or if the bee exerts great force on a younger flower), the keel opens along its whole length, and the longer as well as the shorter stamens, together with the much elongated curved pistil, spring forth with violence. The flattened, spoon-like extremity of the pistil rests for a time on the back of the bee, and leaves on it the load of pollen with which it is charged. As soon as the bee flies away, the pistil instantly curls round, so that the stigmatic surface is now upturned and occupies a position, in which it would be rubbed against the abdomen of another bee visiting the same flower. Thus, when the pistil first escapes from the keel, the stigma is rubbed against the back of the bee, dusted with pollen from the longer stamens, either of the same or another flower; and afterwards against the lower surface of the bee dusted with pollen from the shorter stamens, which is often shed a day or two before that from the longer stamens. (5/16. These observations have been quoted in an abbreviated form by the Reverend G. Henslow, in the 'Journal of Linnean Society Botany' volume 9 1866 page 358. Hermann Muller has since published a full and excellent account of the flower in his 'Befruchtung' etc. page 240.) By this mechanism cross-fertilisation is rendered almost inevitable, and we shall immediately see that pollen from a distinct plant is more effective than that from the same flower.

I need only add that, according to H. Muller, the flowers do not secrete nectar, and he thinks that bees insert their proboscides only in the hope of finding nectar; but they act in this manner so frequently and for so long a time that I cannot avoid the belief that they obtain something palatable within the flowers.

If the visits of bees are prevented, and if the flowers are not dashed by the wind against any object, the keel never opens, so that the stamens and pistil remain enclosed. Plants thus protected yield very few pods in comparison with those produced by neighbouring uncovered bushes, and sometimes none at all. I fertilised a few flowers on a plant growing almost in a state of nature with pollen from another plant close alongside, and the four crossed capsules contained on an average 9.2 seeds. This large number no doubt was due to the bush being covered up, and thus not exhausted by producing many pods; for fifty pods gathered from an adjoining plant, the flowers of which had been fertilised by the bees, contained an average of only 7.14 seeds. Ninety-three pods spontaneously self-fertilised on a large bush which had been covered up, but had been much agitated by the wind, contained an average of 2.93 seeds. Ten of the finest of these ninety-three capsules yielded an average of 4.30 seeds, that is less than half the average number in the four artificially crossed capsules. The ratio of 7.14 to 2.93, or as 100 to 41, is probably the fairest for the number of seeds per pod, yielded by naturally-crossed and spontaneously self-fertilised flowers. The crossed seeds compared with an equal number of the spontaneously self-fertilised seeds were heavier, in the ratio of 100 to 88. We thus see that besides the mechanical adaptations for cross-fertilisation, the flowers are much more productive with pollen from a distinct plant than with their own pollen.

Eight pairs of the above crossed and self-fertilised seeds, after they had germinated on sand, were planted (1867) on the opposite sides of two large pots. When several of the seedlings were an inch and a half in height, there was no marked difference between the two lots. But even at this early age the leaves of the self-fertilised seedlings were smaller and of not so bright a green as those of the crossed seedlings. The pots were kept in the greenhouse, and as the plants on the following spring (1868) looked unhealthy and had grown but little, they were plunged, still in their pots, into the open ground. The plants all suffered much from the sudden change, especially the self-fertilised, and two of the latter died. The remainder were measured, and I give the measurements in Table 5/58, because I have not seen in any other species so great a difference between the crossed and self-fertilised seedlings at so early an age.

TABLE 5/58. Sarothamnus scoparius (very young plants).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 4 4/8 : 2 4/8.

Pot 1 : 6 : 1 4/8.

Pot 1 : 2 : 1.

Pot 2 : 2 : 1 4/8.

Pot 2 : 2 4/8 : 1.

Pot 2 : 0 4/8 : 0 4/8.

Total : 17.5 : 8.0.

The six crossed plants here average 2.91, and the six self-fertilised 1.33 inches in height; so that the former were more than twice as high as the latter, or as 100 to 46.

In the spring of the succeeding year (1869) the three crossed plants in Pot 1 had all grown to nearly a foot in height, and they had smothered the three little self-fertilised plants so completely that two were dead; and the third, only an inch and a half in height, was dying. It should be remembered that these plants had been bedded out in their pots, so that they were subjected to very severe compet.i.tion. This pot was now thrown away.

The six plants in Pot 2 were all alive. One of the self-fertilised was an inch and a quarter taller than any one of the crossed plants; but the other two self-fertilised plants were in a very poor condition. I therefore resolved to leave these plants to struggle together for some years. By the autumn of the same year (1869) the self-fertilised plant which had been victorious was now beaten. The measurements are shown in Table 5/59.

TABLE 5/59. Pot 2.--Sarothamnus scoparius.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

: 15 6/8 : 13 1/8.

: 9 6/8 : 3.

: 8 2/8 : 2 4/8.

The same plants were again measured in the autumn of the following year, 1870.

TABLE 5/60. Pot 2.--Sarothamnus scoparius.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

: 26 2/8 : 14 2/8.

: 16 4/8 : 11 4/8.

: 14 : 9 6/8.

Total : 56.75 : 35.50.

The three crossed plants now averaged 18.91, and the three self-fertilised 11.83 inches in height; or as 100 to 63. The three crossed plants in Pot 1, as already shown, had beaten the three self-fertilised plants so completely, that any comparison between them was superfluous.

The winter of 1870-1871 was severe. In the spring the three crossed plants in Pot 2 had not even the tips of their shoots in the least injured, whereas all three self-fertilised plants were killed half-way down to the ground; and this shows how much more tender they were. In consequence not one of these latter plants bore a single flower during the ensuing summer of 1871, whilst all three crossed plants flowered.

Ononis minutissima.

This plant, of which seeds were sent me from North Italy, produces, besides the ordinary papilionaceous flowers, minute, imperfect, closed or cleistogene flowers, which can never be cross-fertilised, but are highly self-fertile. Some of the perfect flowers were crossed with pollen from a distinct plant, and six capsules thus produced yielded on an average 3.66 seeds, with a maximum of five in one. Twelve perfect flowers were marked and allowed to fertilise themselves spontaneously under a net, and they yielded eight capsules, containing on an average 2.38 seeds, with a maximum of three seeds in one. So that the crossed and self-fertilised capsules from the perfect flowers yielded seeds in the proportion of 100 to 65. Fifty-three capsules produced by the cleistogene flowers contained on an average 4.1 seeds, so that these were the most productive of all; and the seeds themselves looked finer even than those from the crossed perfect flowers.

The seeds from the crossed perfect flowers and from the self-fertilised cleistogene flowers were allowed to germinate on sand; but unfortunately only two pairs germinated at the same time. These were planted on the opposite sides of the same pot, which was kept in the greenhouse. In the summer of the same year, when the seedlings were about 4 1/2 inches in height, the two lots were equal. In the autumn of the following year (1868) the two crossed plants were of exactly the same height, namely, 11 4/8 inches, and the two self-fertilised plants 12 6/8 and 7 2/8 inches; so that one of the self-fertilised exceeded considerably in height all the others. By the autumn of 1869 the two crossed plants had acquired the supremacy; their height being 16 4/8 and 15 1/8, whilst that of the two self-fertilised plants was 14 5/8 and 11 4/8 inches.

By the autumn of 1870, the heights were as follows:--

TABLE 5/61. Ononis minutissima.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

: 20 3/8 : 17 4/8.

: 19 2/8 : 17 2/8.

Total : 39.63 : 34.75.

So that the mean height of the two crossed plants was 19.81, and that of the two self-fertilised 17.37 inches; or as 100 to 88. It should be remembered that the two lots were at first equal in height; that one of the self-fertilised plants then had the advantage, the two crossed plants being at last victorious.]

SUMMARY ON THE LEGUMINOSAE.