The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 11

Pot 6 : 18 3/8 : 1 4/8.

Pot 6 : 11 : 2.

Pot 7 : 21 : 15 1/8.

Pot 7 : 11 6/8 : 11.

Pot 7 : 12 1/8 : 11 2/8.

Crowded.

Total : 210.88 : 140.75.

The fifteen crossed plants here average 14.05, and the fifteen self-fertilised plants 9.38 in height, or as 100 to 67. But if all the plants under ten inches in height are struck out, the ratio of the eleven crossed plants to the eight self-fertilised plants is as 100 to 82.

On the following spring, some remaining seeds of the two lots were treated in exactly the same manner; and the measurements of the seedlings are given in Table 3/22.

TABLE 3/22. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Plants raised from a Cross between different Flowers on the same Plant.

Column 3: Plants raised from Flowers fertilised with their own Pollen.

Pot 1 : 15 1/8 : 19 1/8.

Pot 1 : 12 : 20 5/8.

Pot 1 : 10 1/8 : 12 6/8.

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

Pot 2 : 13 5/8 : 19 3/8.

Pot 2 : 20 1/8 : 17 4/8.

Pot 3 : 18 7/8 : 12 6/8.

Pot 3 : 15 : 15 6/8.

Pot 3 : 13 7/8 : 17.

Pot 4 : 19 2/8 : 16 2/8.

Pot 4 : 19 6/8 : 21 5/8.

Pot 5 : 25 3/8 : 22 5/8.

Pot 6 : 15 : 19 5/8.

Pot 6 : 20 2/8 : 16 2/8.

Pot 6 : 27 2/8 : 19 5/8.

Pot 7 : 7 6/8 : 7 6/8.

Pot 7 : 14 : 8.

Pot 7 : 13 4/8 : 7.

Pot 8 : 18 2/8 : 20 3/8.

Pot 8 : 18 6/8 : 17 6/8.

Pot 8 : 18 3/8 : 15 4/8.

Pot 8 : 18 3/8 : 15 1/8.

Crowded.

Total : 370.88 : 353.63.

Here the average height of the twenty-two crossed plants is 16.85, and that of the twenty-two self-fertilised plants 16.07; or as 100 to 95.

But if four of the plants in Pot 7, which are much shorter than any of the others, are struck out (and this would be the fairest plan), the twenty-one crossed are to the nineteen self-fertilised plants in height as 100 to 100.6--that is, are equal. All the plants, except the crowded ones in Pot 8, after being measured were cut down, and the eighteen crossed plants weighed 10 ounces, whilst the same number of self-fertilised plants weighed 10 1/4 ounces, or as 100 to 102.5; but if the dwarfed plants in Pot 7 had been excluded, the self-fertilised would have exceeded the crossed in weight in a higher ratio. In all the previous experiments in which seedlings were raised from a cross between distinct plants, and were put into compet.i.tion with self-fertilised plants, the former generally flowered first; but in the present case, in seven out of the eight pots a self-fertilised plant flowered before a crossed one on the opposite side. Considering all the evidence with respect to the plants in Table3/ 22, a cross between two flowers on the same plant seems to give no advantage to the offspring thus produced, the self-fertilised plants being in weight superior. But this conclusion cannot be absolutely trusted, owing to the measurements given in Table 3/21, though these latter, from the cause already a.s.signed, are very much less trustworthy than the present ones.]

SUMMARY OF OBSERVATIONS ON Mimulus luteus.

In the three first generations of crossed and self-fertilised plants, the tallest plants alone on each side of the several pots were measured; and the average height of the ten crossed to that of the ten self-fertilised plants was as 100 to 64. The crossed were also much more fertile than the self-fertilised, and so much more vigorous that they exceeded them in height, even when sown on the opposite side of the same pot after an interval of four days. The same superiority was likewise shown in a remarkable manner when both kinds of seeds were sown on the opposite sides of a pot with very poor earth full of the roots of another plant. In one instance crossed and self-fertilised seedlings, grown in rich soil and not put into compet.i.tion with each other, attained to an equal height. When we come to the fourth generation the two tallest crossed plants taken together exceeded by only a little the two tallest self-fertilised plants, and one of the latter beat its crossed opponent,--a circ.u.mstance which had not occurred in the previous generations. This victorious self-fertilised plant consisted of a new white-flowered variety, which grew taller than the old yellowish varieties. From the first it seemed to be rather more fertile, when self-fertilised, than the old varieties, and in the succeeding self-fertilised generations became more and more self-fertile. In the sixth generation the self-fertilised plants of this variety compared with the crossed plants produced capsules in the proportion of 147 to 100, both lots being allowed to fertilise themselves spontaneously. In the seventh generation twenty flowers on one of these plants artificially self-fertilised yielded no less than nineteen very fine capsules!

This variety transmitted its characters so faithfully to all the succeeding self-fertilised generations, up to the last or ninth, that all the many plants which were raised presented a complete uniformity of character; thus offering a remarkable contrast with the seedlings raised from the purchased seeds. Yet this variety retained to the last a latent tendency to produce yellow flowers; for when a plant of the eighth self-fertilised generation was crossed with pollen from a yellow-flowered plant of the Chelsea stock, every single seedling bore yellow flowers. A similar variety, at least in the colour of its flowers, also appeared amongst the crossed plants of the third generation. No attention was at first paid to it, and I know not how far it was at first used either for crossing or self-fertilisation. In the fifth generation most of the self-fertilised plants, and in the sixth and all the succeeding generations every single plant consisted of this variety; and this no doubt was partly due to its great and increasing self-fertility. On the other hand, it disappeared from amongst the crossed plants in the later generations; and this was probably due to the continued intercrossing of the several plants. From the tallness of this variety, the self-fertilised plants exceeded the crossed plants in height in all the generations from the fifth to the seventh inclusive; and no doubt would have done so in the later generations, had they been grown in compet.i.tion with one another. In the fifth generation the crossed plants were in height to the self-fertilised, as 100 to 126; in the sixth, as 100 to 147; and in the seventh generation, as 100 to 137.

This excess of height may be attributed not only to this variety naturally growing taller than the other plants, but to its possessing a peculiar const.i.tution, so that it did not suffer from continued self-fertilisation.

This variety presents a strikingly a.n.a.logous case to that of the plant called the Hero, which appeared in the sixth self-fertilised generation of Ipomoea. If the seeds produced by Hero had been as greatly in excess of those produced by the other plants, as was the case with Mimulus, and if all the seeds had been mingled together, the offspring of Hero would have increased to the entire exclusion of the ordinary plants in the later self-fertilised generations, and from naturally growing taller would have exceeded the crossed plants in height in each succeeding generation.

Some of the self-fertilised plants of the sixth generation were intercrossed, as were some in the eighth generation; and the seedlings from these crosses were grown in compet.i.tion with self-fertilised plants of the two corresponding generations. In the first trial the intercrossed plants were less fertile than the self-fertilised, and less tall in the ratio of 100 to 110. In the second trial, the intercrossed plants were more fertile than the self-fertilised in the ratio of 100 to 73, and taller in the ratio of 100 to 92. Notwithstanding that the self-fertilised plants in the second trial were the product of two additional generations of self-fertilisation, I cannot understand this discordance in the results of the two a.n.a.logous experiments.

The most important of all the experiments on Mimulus are those in which flowers on plants of the eighth self-fertilised generation were again self-fertilised; other flowers on distinct plants of the same lot were intercrossed; and others were crossed with a new stock of plants from Chelsea. The Chelsea-crossed seedlings were to the intercrossed in height as 100 to 56, and in fertility as 100 to 4; and they were to the self-fertilised plants, in height as 100 to 52, and in fertility as 100 to 3. These Chelsea-crossed plants were also much more hardy than the plants of the other two lots; so that altogether the gain from the cross with a fresh stock was wonderfully great.

Lastly, seedlings raised from a cross between flowers on the same plant were not superior to those from flowers fertilised with their own pollen; but this result cannot be absolutely trusted, owing to some previous observations, which, however, were made under very unfavourable circ.u.mstances.

[Digitalis purpurea.

The flowers of the common Foxglove are proterandrous; that is, the pollen is mature and mostly shed before the stigma of the same flower is ready for fertilisation. This is effected by the larger humble-bees, which, whilst in search of nectar, carry pollen from flower to flower.

The two upper and longer stamens shed their pollen before the two lower and shorter ones. The meaning of this fact probably is, as Dr. Ogle remarks, that the anthers of the longer stamens stand near to the stigma, so that they would be the most likely to fertilise it (3/3.

'Popular Science Review' January 1870 page 50.); and as it is an advantage to avoid self-fertilisation, they shed their pollen first, thus lessening the chance. There is, however, but little danger of self-fertilisation until the bifid stigma opens; for Hildebrand found that pollen placed on the stigma before it had opened produced no effect. (3/4. 'Geschlechter-Vertheilung bei den Pflanzen' 1867 page 20.) The anthers, which are large, stand at first transversely with respect to the tubular corolla, and if they were to dehisce in this position they would, as Dr. Ogle also remarks, smear with pollen the whole back and sides of an entering humble-bee in a useless manner; but the anthers twist round and place themselves longitudinally before they dehisce. The lower and inner side of the mouth of the corolla is thickly clothed with hairs, and these collect so much of the fallen pollen that I have seen the under surface of a humble-bee thickly dusted with it; but this can never be applied to the stigma, as the bees in retreating do not turn their under surfaces upwards. I was therefore puzzled whether these hairs were of any use; but Mr. Belt has, I think, explained their use: the smaller kinds of bees are not fitted to fertilise the flowers, and if they were allowed to enter easily they would steal much nectar, and fewer large bees would haunt the flowers. Humble-bees can crawl into the dependent flowers with the greatest ease, using the "hairs as footholds while sucking the honey; but the smaller bees are impeded by them, and when, having at length struggled through them, they reach the slippery precipice above, they are completely baffled." Mr. Belt says that he watched many flowers during a whole season in North Wales, and "only once saw a small bee reach the nectary, though many were seen trying in vain to do so." (3/5. 'The Naturalist in Nicaragua' 1874 page 132. But it appears from H. Muller 'Die Befruchtung der Blumen' 1873 page 285, that small insects sometimes succeed in entering the flowers.)

I covered a plant growing in its native soil in North Wales with a net, and fertilised six flowers each with its own pollen, and six others with pollen from a distinct plant growing within the distance of a few feet.

The covered plant was occasionally shaken with violence, so as to imitate the effects of a gale of wind, and thus to facilitate as far as possible self-fertilisation. It bore ninety-two flowers (besides the dozen artificially fertilised), and of these only twenty-four produced capsules; whereas almost all the flowers on the surrounding uncovered plants were fruitful. Of the twenty-four spontaneously self-fertilised capsules, only two contained their full complement of seed; six contained a moderate supply; and the remaining sixteen extremely few seeds. A little pollen adhering to the anthers after they had dehisced, and accidentally falling on the stigma when mature, must have been the means by which the above twenty-four flowers were partially self-fertilised; for the margins of the corolla in withering do not curl inwards, nor do the flowers in dropping off turn round on their axes, so as to bring the pollen-covered hairs, with which the lower surface is clothed, into contact with the stigma--by either of which means self-fertilisation might be effected.

Seeds from the above crossed and self-fertilised capsules, after germinating on bare sand, were planted in pairs on the opposite sides of five moderately-sized pots, which were kept in the greenhouse. The plants after a time appeared starved, and were therefore, without being disturbed, turned out of their pots, and planted in the open ground in two close parallel rows. They were thus subjected to tolerably severe compet.i.tion with one another; but not nearly so severe as if they had been left in the pots. At the time when they were turned out, their leaves were between 5 and 8 inches in length, and the longest leaf on the finest plant on each side of each pot was measured, with the result that the leaves of the crossed plants exceeded, on an average, those of the self-fertilised plants by .4 of an inch.

In the following summer the tallest flower-stem on each plant, when fully grown, was measured. There were seventeen crossed plants; but one did not produce a flower-stem. There were also, originally, seventeen self-fertilised plants, but these had such poor const.i.tutions that no less than nine died in the course of the winter and spring, leaving only eight to be measured, as in Table 3/23.

TABLE 3/23. Digitalis purpurea.

The tallest Flower-stem on each Plant measured in inches: 0 means that the Plant died before a Flower-stem was produced.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 53 6/8 : 27 4/8.

Pot 1 : 57 4/8 : 55 6/8.