The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 36

34 2/8 : 38 : 27 3/8.

36 2/8 : 36 2/8 : 23.

35 2/8 : 39 5/8 : 25.

32 4/8 : 37 : 24 1/8.

37 : 36 : 22 4/8.

36 4/8 : 41 3/8 : 23 3/8.

40 7/8 : 37 2/8 : 21 5/8.

37 2/8 : 40 : 23 4/8.

38 2/8 : 41 2/8 : 21 3/8.

38 5/8 : 36 : 21 2/8.

366.76 : 382.76 : 233.13.

The ten Westerham-crossed plants here average 36.67 inches in height; the ten intercrossed plants, 38.27 inches; and the ten self-fertilised, 23.31 inches. These three lots of plants were also weighed; the Westerham-crossed plants weighed 28 ounces; the intercrossed plants, 41 ounces; and the self-fertilised, 14.75 ounces. We thus get the following ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to 63.

The Westerham-crossed plants in weight to the self-fertilised as 100 to 53.

The Westerham-crossed plants in height to the intercrossed as 100 to 104.

The Westerham-crossed plants in weight to the intercrossed as 100 to 146.

The intercrossed plants in height to the self-fertilised as 100 to 61.

The intercrossed plants in weight to the self-fertilised as 100 to 36.

Here the relative heights of the three lots are nearly the same (within three or four per cent) as with the plants in the pots. In weight there is a much greater difference: the Westerham-crossed exceed the self-fertilised by much less than they did before; but the self-fertilised plants in the pots had become slightly withered, as before stated, and were in consequence unfairly light. The Westerham-crossed plants are here inferior in weight to the intercrossed plants in a much higher degree than in the pots; and this appeared due to their being much less branched, owing to their having germinated in greater numbers and consequently being much crowded. Their leaves were of a brighter green than those of the intercrossed and self-fertilised plants.

RELATIVE FERTILITY OF THE THREE LOTS OF PLANTS.

None of the plants in pots in the greenhouse ever produced a capsule; and this may be attributed in chief part to the exclusion of moths.

Therefore the fertility of the three lots could be judged of only by that of the plants growing out of doors, which from being left uncovered were probably cross-fertilised. The plants in the three rows were exactly of the same age and had been subjected to closely similar conditions, so that any difference in their fertility must be attributed to their different origin; namely, to the one lot being derived from plants self-fertilised for four generations and then crossed with a fresh stock; to the second lot being derived from plants of the same old stock intercrossed for five generations; and to the third lot being derived from plants self-fertilised for five generations. All the capsules, some nearly mature and some only half-grown, were gathered, counted, and weighed from the ten finest plants in each of the three rows, of which the measurements and weights have already been given. The intercrossed plants, as we have seen, were taller and considerably heavier than the plants of the other two lots, and they produced a greater number of capsules than did even the Westerham-crossed plants; and this may be attributed to the latter having grown more crowded and being in consequence less branched. Therefore the average weight of an equal number of capsules from each lot of plants seems to be the fairest standard of comparison, as their weights will have been determined chiefly by the number of the included seeds. As the intercrossed plants were taller and heavier than the plants of the other two lots, it might have been expected that they would have produced the finest or heaviest capsules; but this was very far from being the case.

The ten tallest Westerham-crossed plants produced 111 ripe and unripe capsules, weighing 121.2 grains. Therefore 100 of such capsules would have weighed 109.18 grains.

The ten tallest intercrossed plants produced 129 capsules, weighing 76.45 grains. Therefore 100 of these capsules would have weighed 59.26 grains.

The ten tallest self-fertilised plants produced only 44 capsules, weighing 22.35 grains. Therefore 100 of these capsules would have weighed 50.79 grains.

From these data we get the following ratios for the fertility of the three lots, as deduced from the relative weights of an equal number of capsules from the finest plants in each lot:--

Westerham-crossed plants to self-fertilised plants as 100 to 46.

Westerham-crossed plants to intercrossed plants as 100 to 54.

Intercrossed plants to self-fertilised plants as 100 to 86.

We here see how potent the influence of a cross with pollen from a fresh stock has been on the fertility of plants self-fertilised for four generations, in comparison with plants of the old stock when either intercrossed or self-fertilised for five generations; the flowers on all these plants having been left to be freely crossed by insects or to fertilise themselves. The Westerham-crossed plants were also much taller and heavier plants than the self-fertilised, both in the pots and open ground; but they were less tall and heavy than the intercrossed plants.

This latter result, however, would almost certainly have been reversed, if the plants had been allowed to grow for another month, as the Westerham-crossed were still growing vigorously, whilst the intercrossed had almost ceased to grow. This case reminds us of the somewhat a.n.a.logous one of Eschscholtzia, in which plants raised from a cross with a fresh stock did not grow higher than the self-fertilised or intercrossed plants, but produced a greater number of seed-capsules, which contained a far larger average number of seeds.

COLOUR OF THE FLOWERS ON THE ABOVE THREE LOTS OF PLANTS.

The original mother-plant, from which the five successive self-fertilised generations were raised, bore dingy purple flowers. At no time was any selection practised, and the plants were subjected in each generation to extremely uniform conditions. The result was, as in some previous cases, that the flowers on all the self-fertilised plants, both in the pots and open ground, were absolutely uniform in tint; this being a dull, rather peculiar flesh colour. This uniformity was very striking in the long row of plants growing in the open ground, and these first attracted my attention. I did not notice in which generation the original colour began to change and to become uniform, but I have every reason to believe that the change was gradual. The flowers on the intercrossed plants were mostly of the same tint, but not nearly so uniform as those on the self-fertilised plants, and many of them were pale, approaching almost to white. The flowers on the plants from the cross with the purple-flowered Westerham stock were, as might have been expected, much more purple and not nearly so uniform in tint. The self-fertilised plants were also remarkably uniform in height, as judged by the eye; the intercrossed less so, whilst the Westerham-crossed plants varied much in height.

Nicotiana tabac.u.m.

This plant offers a curious case. Out of six trials with crossed and self-fertilised plants, belonging to three successive generations, in one alone did the crossed show any marked superiority in height over the self-fertilised; in four of the trials they were approximately equal; and in one (i.e., in the first generation) the self-fertilised plants were greatly superior to the crossed. In no case did the capsules from flowers fertilised with pollen from a distinct plant yield many more, and sometimes they yielded much fewer seeds than the capsules from self-fertilised flowers. But when the flowers of one variety were crossed with pollen from a slightly different variety, which had grown under somewhat different conditions,--that is, by a fresh stock,--the seedlings derived from this cross exceeded in height and weight those from the self-fertilised flowers in an extraordinary degree.

Twelve flowers on some plants of the common tobacco, raised from purchased seeds, were crossed with pollen from a distinct plant of the same lot, and these produced ten capsules. Twelve flowers on the same plants were fertilised with their own pollen, and produced eleven capsules. The seeds in the ten crossed capsules weighed 31.7 grains, whilst those in ten of the self-fertilised capsules weighed 47.67 grains; or as 100 to 150. The much greater productiveness of the self-fertilised than of the crossed capsules can hardly be attributed to chance, as all the capsules of both lots were very fine and healthy ones.

The seeds were placed on sand, and several pairs in an equal state of germination were planted on the opposite sides of three pots. The remaining seeds were thickly sown on the two sides of Pot 4, so that the plants in this pot were much crowded. The tallest plant on each side of each pot was measured. Whilst the plants were quite young the four tallest crossed plants averaged 7.87 inches, and the four tallest self-fertilised 14.87 inches in height; or as 100 to 189. The heights at this age are given in the two left columns of Table 6/84.

When in full flower the tallest plants on each side were again measured, see the two right hand columns in Table 6/84. But I should state that the pots were not large enough, and the plants never grew to their proper height. The four tallest crossed plants now averaged 18.5, and the four tallest self-fertilised plants 32.75 inches in height; or as 100 to 178. In all four pots a self-fertilised plant flowered before any one of the crossed.

In Pot 4, in which the plants were extremely crowded, the two lots were at first equal; and ultimately the tallest crossed plant exceeded by a trifle the tallest self-fertilised plant. This recalled to my mind an a.n.a.logous case in the one generation of Petunia, in which the self-fertilised plants were throughout their growth taller than the crossed in all the pots except in the crowded one. Accordingly another trial was made, and some of the same crossed and self-fertilised seeds of tobacco were sown thickly on opposite sides of two additional pots; the plants being left to grow up much crowded. When they were between 13 and 14 inches in height there was no difference between the two sides, nor was there any marked difference when the plants had grown as tall as they could; for in one pot the tallest crossed plant was 26 1/2 inches in height, and exceeded by 2 inches the tallest self-fertilised plant, whilst in the other pot, the tallest crossed plant was shorter by 3 1/2 inches than the tallest self-fertilised plant, which was 22 inches in height.

TABLE 6/84. Nicotiana tabac.u.m (first generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants, May 20, 1868.

Column 3: self-fertilised Plants, May 20, 1868.

Column 4: Crossed Plants, December 6, 1868.

Column 5: self-fertilised Plants, December 6, 1868.

Pot 1 : 15 4/8 : 26 : 40 : 44.

Pot 2 : 3 : 15 : 6 4/8 : 43.

Pot 3 : 8 : 13 4/8 : 16 : 33.

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

Total : 31.5 : 59.5 : 74.0 : 131.0.

As the plants did not grow to their proper height in the above small pots in Table 6/84, four crossed and four self-fertilised plants were raised from the same seed, and were planted in pairs on the opposite sides of four very large pots containing rich soil; so that they were not exposed to at all severe mutual compet.i.tion. When these plants were in flower I neglected to measure them, but record in my notes that all four self-fertilised plants exceeded in height the four crossed plants by 2 or 3 inches. We have seen that the flowers on the original or parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than those fertilised with their own pollen; and the trial just given, as well as that in Table 6/84, show us clearly that the plants raised from the crossed seeds were inferior in height to those from the self-fertilised seeds; but only when not greatly crowded.

When crowded and thus subjected to very severe compet.i.tion, the crossed and self-fertilised plants were nearly equal in height.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Twelve flowers on the crossed plants of the last generation growing in the four large pots just mentioned, were crossed with pollen from a crossed plant growing in one of the other pots; and twelve flowers on the self-fertilised plants were fertilised with their own pollen. All these flowers of both lots produced fine capsules. Ten of the crossed capsules contained by weight 38.92 grains of seeds, and ten of the self-fertilised capsules 37.74 grains; or as 100 to 97. Some of these seeds in an equal state of germination were planted in pairs on the opposite sides of five large pots. A good many of the crossed seeds germinated before the self-fertilised, and were of course rejected. The plants thus raised were measured when several of them were in full flower.

TABLE 6/85. Nicotiana tabac.u.m (second generation).