The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 14

Origanum vulgare.

This plant exists, according to H. Muller, under two forms; one hermaphrodite and strongly proterandrous, so that it is almost certain to be fertilised by pollen from another flower; the other form is exclusively female, has a smaller corolla, and must of course be fertilised by pollen from a distinct plant in order to yield any seeds.

The plants on which I experimented were hermaphrodites; they had been cultivated for a long period as a pot-herb in my kitchen garden, and were, like so many long-cultivated plants, extremely sterile. As I felt doubtful about the specific name I sent specimens to Kew, and was a.s.sured that the species was Origanum vulgare. My plants formed one great clump, and had evidently spread from a single root by stolons. In a strict sense, therefore, they all belonged to the same individual. My object in experimenting on them was, firstly, to ascertain whether crossing flowers borne by plants having distinct roots, but all derived as.e.xually from the same individual, would be in any respect more advantageous than self-fertilisation; and, secondly, to raise for future trial seedlings which would const.i.tute really distinct individuals.

Several plants in the above clump were covered by a net, and about two dozen seeds (many of which, however, were small and withered) were obtained from the flowers thus spontaneously self-fertilised. The remainder of the plants were left uncovered and were incessantly visited by bees, so that they were doubtless crossed by them. These exposed plants yielded rather more and finer seed (but still very few) than did the covered plants. The two lots of seeds thus obtained were sown on opposite sides of two pots; the seedlings were carefully observed from their first growth to maturity, but they did not differ at any period in height or in vigour, the importance of which latter observation we shall presently see. When fully grown, the tallest crossed plant in one pot was a very little taller than the tallest self-fertilised plant on the opposite side, and in the other pot exactly the reverse occurred. So that the two lots were in fact equal; and a cross of this kind did no more good than crossing two flowers on the same plant of Ipomoea or Mimulus.

The plants were turned out of the two pots without being disturbed and planted in the open ground, in order that they might grow more vigorously. In the following summer all the self-fertilised and some of the quasi-crossed plants were covered by a net. Many flowers on the latter were crossed by me with pollen from a distinct plant, and others were left to be crossed by the bees. These quasi-crossed plants produced rather more seed than did the original ones in the great clump when left to the action of the bees. Many flowers on the self-fertilised plants were artificially self-fertilised, and others were allowed to fertilise themselves spontaneously under the net, but they yielded altogether very few seeds. These two lots of seeds--the product of a cross between distinct seedlings, instead of as in the last case between plants multiplied by stolons, and the product of self-fertilised flowers--were allowed to germinate on bare sand, and several equal pairs were planted on opposite sides of two LARGE pots. At a very early age the crossed plants showed some superiority over the self-fertilised, which was ever afterwards retained. When the plants were fully grown, the two tallest crossed and the two tallest self-fertilised plants in each pot were measured, as shown in Table 3/28. I regret that from want of time I did not measure all the pairs; but the tallest on each side seemed fairly to represent the average difference between the two lots.

TABLE 3/28. Origanum vulgare.

Heights of Plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants (two tallest in each pot).

Column 3: Self-fertilised Plants (two tallest in each pot).

Pot 1 : 26 : 24.

Pot 1 : 21 : 21.

Pot 2 : 17 : 12.

Pot 2 : 16 : 11 4/8.

Total : 80.0 : 68.5.

The average height of the crossed plants is here 20 inches, and that of the self-fertilised 17.12; or as 100 to 86. But this excess of height by no means gives a fair idea of the vast superiority in vigour of the crossed over the self-fertilised plants. The crossed flowered first and produced thirty flower-stems, whilst the self-fertilised produced only fifteen, or half the number. The pots were then bedded out, and the roots probably came out of the holes at the bottom and thus aided their growth. Early in the following summer the superiority of the crossed plants, owing to their increase by stolons, over the self-fertilised plants was truly wonderful. In Pot 1, and it should be remembered that very large pots had been used, the oval clump of crossed plants was 10 by 4 1/2 inches across, with the tallest stem, as yet young, 5 1/2 inches in height; whilst the clump of self-fertilised plants, on the opposite side of the same pot, was only 3 1/2 by 2 1/2 inches across, with the tallest young stem 4 inches in height. In Pot 2, the clump of crossed plants was 18 by 9 inches across, with the tallest young stem 8 1/2 inches in height; whilst the clump of self-fertilised plants on the opposite side of the same pot was 12 by 4 1/2 inches across, with the tallest young stem 6 inches in height. The crossed plants during this season, as during the last, flowered first. Both the crossed and self-fertilised plants being left freely exposed to the visits of bees, manifestly produced much more seed than their grand-parents,--the plants of the original clump still growing close by in the same garden, and equally left to the action of the bees.

5. ACANTHACEAE.--Thunbergia alata.

It appears from Hildebrand's description ('Botanische Zeitung' 1867 page 285) that the conspicuous flowers of this plant are adapted for cross-fertilisation. Seedlings were twice raised from purchased seed; but during the early summer, when first experimented on, they were extremely sterile, many of the anthers containing hardly any pollen.

Nevertheless, during the autumn these same plants spontaneously produced a good many seeds. Twenty-six flowers during the two years were crossed with pollen from a distinct plant, but they yielded only eleven capsules; and these contained very few seeds! Twenty-eight flowers were fertilised with pollen from the same flower, and these yielded only ten capsules, which, however, contained rather more seed than the crossed capsules. Eight pairs of germinating seeds were planted on opposite sides of five pots; and exactly half the crossed and half the self-fertilised plants exceeded their opponents in height. Two of the self-fertilised plants died young, before they were measured, and their crossed opponents were thrown away. The six remaining pairs of these grew very unequally, some, both of the crossed and self-fertilised plants, being more than twice as tall as the others. The average height of the crossed plants was 60 inches, and that of the self-fertilised plants 65 inches, or as 100 to 108. A cross, therefore, between distinct individuals here appears to do no good; but this result deduced from so few plants in a very sterile condition and growing very unequally, obviously cannot be trusted.]

CHAPTER IV.

CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.

Bra.s.sica oleracea, crossed and self-fertilised plants.

Great effect of a cross with a fresh stock on the weight of the offspring.

Iberis umbellata.

Papaver vagum.

Eschscholtzia californica, seedlings from a cross with a fresh stock not more vigorous, but more fertile than the self-fertilised seedlings.

Reseda lutea and odorata, many individuals sterile with their own pollen.

Viola tricolor, wonderful effects of a cross.

Adonis aestivalis.

Delphinium consolida.

Viscaria oculata, crossed plants hardly taller, but more fertile than the self-fertilised.

Dianthus caryophyllus, crossed and self-fertilised plants compared for four generations.

Great effects of a cross with a fresh stock.

Uniform colour of the flowers on the self-fertilised plants.

Hibiscus africa.n.u.s.

[6. CRUCIFERAE.--Bra.s.sica oleracea.

VAR. CATTELL'S EARLY BARNES CABBAGE.

The flowers of the common cabbage are adapted, as shown by H. Muller, for cross-fertilisation, and should this fail, for self-fertilisation.

(4/1. 'Die Befruchtung' etc. page 139.) It is well known that the varieties are crossed so largely by insects, that it is impossible to raise pure kinds in the same garden, if more than one kind is in flower at the same time. Cabbages, in one respect, were not well fitted for my experiments, as, after they had formed heads, they were often difficult to measure. The flower-stems also differ much in height; and a poor plant will sometimes throw up a higher stem than that of a fine plant.

In the later experiments, the fully-grown plants were cut down and weighed, and then the immense advantage from a cross became manifest.

A single plant of the above variety was covered with a net just before flowering, and was crossed with pollen from another plant of the same variety growing close by; and the seven capsules thus produced contained on an average 16.3 seeds, with a maximum of twenty in one capsule. Some flowers were artificially self-fertilised, but their capsules did not contain so many seeds as those from flowers spontaneously self-fertilised under the net, of which a considerable number were produced. Fourteen of these latter capsules contained on an average 4.1 seeds, with a maximum in one of ten seeds; so that the seeds in the crossed capsules were in number to those in the self-fertilised capsules as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed 3.88 grains, were, however, a little finer than those from the crossed capsules, fifty-eight of which weighed 3.76 grains. When few seeds are produced, these seem often to be better nourished and to be heavier than when many are produced.

The two lots of seeds in an equal state of germination were planted, some on opposite sides of a single pot, and some in the open ground. The young crossed plants in the pot at first exceeded by a little in height the self-fertilised; then equalled them; were then beaten; and lastly were again victorious. The plants, without being disturbed, were turned out of the pot, and planted in the open ground; and after growing for some time, the crossed plants, which were all of nearly the same height, exceeded the self-fertilised ones by 2 inches. When they flowered, the flower-stems of the tallest crossed plant exceeded that of the tallest self-fertilised plant by 6 inches. The other seedlings which were planted in the open ground stood separate, so that they did not compete with one another; nevertheless the crossed plants certainly grew to a rather greater height than the self-fertilised; but no measurements were made. The crossed plants which had been raised in the pot, and those planted in the open ground, all flowered a little before the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Some flowers on the crossed plants of the last generation were again crossed with pollen from another crossed plant, and produced fine capsules. The flowers on the self-fertilised plants of the last generation were allowed to fertilise themselves spontaneously under a net, and they produced some remarkably fine capsules. The two lots of seeds thus produced germinated on sand, and eight pairs were planted on opposite sides of four pots. These plants were measured to the tips of their leaves on the 20th of October of the same year, and the eight crossed plants averaged in height 8.4 inches, whilst the self-fertilised averaged 8.53 inches, so that the crossed were a little inferior in height, as 100 to 101.5. By the 5th of June of the following year these plants had grown much bulkier, and had begun to form heads. The crossed had now acquired a marked superiority in general appearance, and averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31 inches; or as 100 to 91. The plants were then turned out of their pots and planted undisturbed in the open ground. By the 5th of August their heads were fully formed, but several had grown so crooked that their heights could hardly be measured with accuracy. The crossed plants, however, were on the whole considerably taller than the self-fertilised.

In the following year they flowered; the crossed plants flowering before the self-fertilised in three of the pots, and at the same time in Pot 2.

The flower-stems were now measured, as shown in Table 4/29.

TABLE 3/29. Bra.s.sica oleracea.

Measured in inches to tops of flower-stems: 0 signifies that a Flower-stem was not formed.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 49 2/8 : 44.

Pot 1 : 39 4/8 : 41.

Pot 2 : 37 4/8 : 38.

Pot 2 : 33 4/8 : 35 4/8.

Pot 3 : 47 : 51 1/8.

Pot 3 : 40 : 41 2/8.

Pot 3 : 42 : 46 4/8.

Pot 4 : 43 6/8 : 20 2/8.

Pot 4 : 37 2/8 : 33 3/8.

Pot 4 : 0 : 0.

Total : 369.75 : 351.00.

The nine flower-stems on the crossed plants here average 41.08 inches, and the nine on the self-fertilised plants 39 inches in height, or as 100 to 95. But this small difference, which, moreover, depended almost wholly on one of the self-fertilised plants being only 20 inches high, does not in the least show the vast superiority of the crossed over the self-fertilised plants. Both lots, including the two plants in Pot 4, which did not flower, were now cut down close to the ground and weighed, but those in Pot 2 were excluded, for they had been accidentally injured by a fall during transplantation, and one was almost killed. The eight crossed plants weighed 219 ounces, whilst the eight self-fertilised plants weighed only 82 ounces, or as 100 to 37; so that the superiority of the former over the latter in weight was great.