Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 80 : 64 4/8.
Pot 2 : 78 4/8 : 63.
Total : 158.5 : 127.5.
The two crossed plants here average 79.25, and the two self-fertilised 63.75 inches in height, or as 100 to 80. Six flowers on these two crossed plants were reciprocally crossed with pollen from the other plant, and the six pods thus produced contained on an average six peas, with a maximum in one of seven. Eighteen spontaneously self-fertilised pods from the Painted Lady, which, as already stated, had no doubt been self-fertilised for many previous generations, contained on an average only 3.93 peas, with a maximum in one of five peas; so that the number of peas in the crossed and self-fertilised pods was as 100 to 65. The self-fertilised peas were, however, quite as heavy as those from the crossed pods. From these two lots of seeds, the plants of the next generation were raised.
PLANTS OF THE SECOND GENERATION.
Many of the self-fertilised peas just referred to germinated on sand before any of the crossed ones, and were rejected. As soon as I got equal pairs, they were planted on the opposite sides of two large pots, which were kept in the greenhouse. The seedlings thus raised were the grandchildren of the Painted Lady, which was first crossed by the Purple variety. When the two lots were from 4 to 6 inches in height there was no difference between them. Nor was there any marked difference in the period of their flowering. When fully grown they were measured, as follows:--
TABLE 5/55. Lathyrus odoratus (Second Generation).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Seedlings from Plants Crossed during the two previous Generations.
Column 3: Seedlings from Plants Self-fertilised during many previous Generations.
Pot 1 : 72 4/8 : 57 4/8.
Pot 1 : 71 : 67.
Pot 1 : 52 2/8 : 56 2/8.
Pot 2 : 81 4/8 : 66 2/8.
Pot 2 : 45 2/8 : 38 7/8.
Pot 2 : 55 : 46.
Total : 377.50 : 331.86.
The average height of the six crossed plants is here 62.91, and that of the six self-fertilised 55.31 inches; or as 100 to 88. There was not much difference in the fertility of the two lots; the crossed plants having produced in the greenhouse thirty-five pods, and the self-fertilised thirty-two pods.
Seeds were saved from the self-fertilised flowers on these two lots of plants, for the sake of ascertaining whether the seedlings thus raised would inherit any difference in growth or vigour. It must therefore be understood that both lots in the following trial are plants of self-fertilised parentage; but that in the one lot the plants were the children of plants which had been crossed during two previous generations, having been before that self-fertilised for many generations; and that in the other lot they were the children of plants which had not been crossed for very many previous generations. The seeds germinated on sand and were planted in pairs on the opposite sides of four pots. They were measured, when fully grown, with the following result:--
TABLE 5/56. Lathyrus odoratus.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants from Crossed Plants.
Column 3: Self-fertilised Plants from Self-fertilised Plants.
Pot 1 : 72 : 65.
Pot 1 : 72 : 61 4/8.
Pot 2 : 58 : 64.
Pot 2 : 68 : 68 2/8.
Pot 2 : 72 4/8 : 56 4/8.
Pot 3 : 81 : 60 2/8.
Pot 4 : 77 4/8 : 76 4/8.
Total : 501 : 452.
The average height of the seven self-fertilised plants, the offspring of crossed plants, is 71.57, and that of the seven self-fertilised plants, the offspring of self-fertilised plants, is 64.57; or as 100 to 90. The self-fertilised plants from the self-fertilised produced rather more pods--namely, thirty-six--than the self-fertilised plants from the crossed, for these produced only thirty-one pods.
A few seeds of the same two lots were sown in the opposite corners of a large box in which a Brugmansia had long been growing, and in which the soil was so exhausted that seeds of Ipomoea purpurea would hardly vegetate; yet the two plants of the sweet-pea which were raised flourished well. For a long time the self-fertilised plant from the self-fertilised beat the self-fertilised plant from the crossed plant; the former flowered first, and was at one time 77 1/2 inches, whilst the latter was only 68 1/2 in height; but ultimately the plant from the previous cross showed its superiority and attained a height of 108 1/2 inches, whilst the other was only 95 inches. I also sowed some of the same two lots of seeds in poor soil in a shady place in a shrubbery.
Here again the self-fertilised plants from the self-fertilised for a long time exceeded considerably in height those from the previously crossed plants; and this may probably be attributed, in the present as in the last case, to these seeds having germinated rather sooner than those from the crossed plants; but at the close of the season the tallest of the self-fertilised plants from the crossed plants was 30 inches, whilst the tallest of the self-fertilised from the self-fertilised was 29 3/8 inches in height.
From the various facts now given we see that plants derived from a cross between two varieties of the sweet-pea, which differ in no respect except in the colour of their flowers, exceed considerably in height the offspring from self-fertilised plants, both in the first and second generations. The crossed plants also transmit their superiority in height and vigour to their self-fertilised offspring.
Pisum sativum.
The common pea is perfectly fertile when its flowers are protected from the visits of insects; I ascertained this with two or three different varieties, as did Dr. Ogle with another. But the flowers are likewise adapted for cross-fertilisation; Mr. Farrer specifies the following points, namely: "The open blossom displaying itself in the most attractive and convenient position for insects; the conspicuous vexillum; the wings forming an alighting place; the attachment of the wings to the keel, by which any body pressing on the former must press down the latter; the staminal tube enclosing nectar, and affording by means of its partially free stamen with apertures on each side of its base an open pa.s.sage to an insect seeking the nectar; the moist and sticky pollen placed just where it will be swept out of the apex of the keel against the entering insect; the stiff elastic style so placed that on a pressure being applied to the keel it will be pushed upwards out of the keel; the hairs on the style placed on that side of the style only on which there is s.p.a.ce for the pollen, and in such a direction as to sweep it out; and the stigma so placed as to meet an entering insect,--all these become correlated parts of one elaborate mechanism, if we suppose that the fertilisation of these flowers is effected by the carriage of pollen from one to the other." (5/12. "Nature" October 10, 1872 page 479. Hermann Muller gives an elaborate description of the flowers "Befruchtung" etc. page 247.) Notwithstanding these manifest provisions for cross-fertilisation, varieties which have been cultivated for very many successive generations in close proximity, although flowering at the same time, remain pure. I have elsewhere given evidence on this head, and if required could give more. (5/13. "Variation of Animals and Plants under Domestication" chapter 9 2nd edition volume 1 page 348.) There can hardly be a doubt that some of Knight"s varieties, which were originally produced by an artificial cross and were very vigorous, lasted for at least sixty years, and during all these years were self-fertilised; for had it been otherwise, they would not have kept true, as the several varieties are generally grown near together.
Most of the varieties, however, endure for a shorter period; and this may be in part due to their weakness of const.i.tution from long-continued self-fertilisation.
It is remarkable, considering that the flowers secrete much nectar and afford much pollen, how seldom they are visited by insects either in England, or, as H. Muller remarks, in North Germany. I have observed the flowers for the last thirty years, and in all this time have only thrice seen bees of the proper kind at work (one of them being Bombus muscorum), such as were sufficiently powerful to depress the keel, so as to get the undersides of their bodies dusted with pollen. These bees visited several flowers, and could hardly have failed to cross-fertilise them. Hive-bees and other small kinds sometimes collect pollen from old and already fertilised flowers, but this is of no account. The rarity of the visits of efficient bees to this exotic plant is, I believe, the chief cause of the varieties so seldom intercrossing. That a cross does occasionally take place, as might be expected from what has just been stated, is certain, from the recorded cases of the direct action of the pollen of one variety on the seed-coats of another. (5/14. "Variation of Animals and Plants under Domestication" chapter 11 2nd edition volume 1 page 428.) The late Mr. Masters, who particularly attended to the raising of new varieties of peas, was convinced that some of them had originated from accidental crosses. But as such crosses are rare, the old varieties would not often be thus deteriorated, more especially as plants departing from the proper type are generally rejected by those who collect seed for sale. There is another cause which probably tends to render cross-fertilisation rare, namely, the early age at which the pollen-tubes are exserted; eight flowers not fully expanded were examined, and in seven of these the pollen-tubes were in this state; but they had not as yet penetrated the stigma. Although so few insects visit the flowers of the pea in this country or in North Germany, and although the anthers seem here to open abnormally soon, it does not follow that the species in its native country would be thus circ.u.mstanced.
Owing to the varieties having been self-fertilised for many generations, and to their having been subjected in each generation to nearly the same conditions (as will be explained in a future chapter) I did not expect that a cross between two such plants would benefit the offspring; and so it proved on trial. In 1867 I covered up several plants of the Early Emperor pea, which was not then a very new variety, so that it must already have been propagated by self-fertilisation for at least a dozen generations. Some flowers were crossed with pollen from a distinct plant growing in the same row, and others were allowed to fertilise themselves under a net. The two lots of seeds thus obtained were sown on opposite sides of two large pots, but only four pairs came up at the same time.
The pots were kept in the greenhouse. The seedlings of both lots when between 6 and 7 inches in height were equal. When nearly full-grown they were measured, as in Table 5/57.
TABLE 5/57. Pisum sativum.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 35 : 29 6/8.
Pot 2 : 31 4/8 : 51.
Pot 2 : 35 : 45.
Pot 2 : 37 : 33.