The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 8

Combining these data, the productiveness of the crossed to the self-fertilised plants, both spontaneously self-fertilised, was as : 100 to 35.

FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Capsules from flowers on the crossed plants fertilised by pollen from another plant, and capsules from flowers on the self-fertilised plants fertilised with their own pollen, contained seeds in the proportion of : 100 to 94.

FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

The crossed plants produced spontaneously a vast number more pods (not actually counted) than the self-fertilised, and these contained seeds in the proportion of : 100 to 89.

NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Fourteen crossed plants, spontaneously self-fertilised, and fourteen self-fertilised plants spontaneously self-fertilised, yielded capsules (the average number of seeds per capsule not having been ascertained) in the proportion of : 100 to 26.

PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH INTERCROSSED PLANTS.

The offspring of intercrossed plants of the ninth generation, crossed by a fresh stock, compared with plants of the same stock intercrossed during ten generations, both sets of plants left uncovered and naturally fertilised, produced capsules by weight as : 100 to 51.

We see in this table that the crossed plants are always in some degree more productive than the self-fertilised plants, by whatever standard they are compared. The degree differs greatly; but this depends chiefly on whether an average was taken of the seeds alone, or of the capsules alone, or of both combined. The relative superiority of the crossed plants is chiefly due to their producing a much greater number of capsules, and not to each capsule containing a larger average number of seeds. For instance, in the third generation the crossed and self-fertilised plants produced capsules in the ratio of 100 to 38, whilst the seeds in the capsules on the crossed plants were to those on the self-fertilised plants only as 100 to 94. In the eighth generation the capsules on two self-fertilised plants (not included in table 2/18), grown in separate pots and thus not subjected to any compet.i.tion, yielded the large average of 5.1 seeds. The smaller number of capsules produced by the self-fertilised plants may be in part, but not altogether, attributed to their lessened size or height; this being chiefly due to their lessened const.i.tutional vigour, so that they were not able to compete with the crossed plants growing in the same pots.

The seeds produced by the crossed flowers on the crossed plants were not always heavier than the self-fertilised seeds on the self-fertilised plants. The lighter seeds, whether produced from crossed or self-fertilised flowers, generally germinated before the heavier seeds.

I may add that the crossed plants, with very few exceptions, flowered before their self-fertilised opponents, as might have been expected from their greater height and vigour.

The impaired fertility of the self-fertilised plants was shown in another way, namely, by their anthers being smaller than those in the flowers on the crossed plants. This was first observed in the seventh generation, but may have occurred earlier. Several anthers from flowers on the crossed and self-fertilised plants of the eighth generation were compared under the microscope; and those from the former were generally longer and plainly broader than the anthers of the self-fertilised plants. The quant.i.ty of pollen contained in one of the latter was, as far as could be judged by the eye, about half of that contained in one from a crossed plant. The impaired fertility of the self-fertilised plants of the eighth generation was also shown in another manner, which may often be observed in hybrids--namely, by the first-formed flowers being sterile. For instance, the fifteen first flowers on a self-fertilised plant of one of the later generations were carefully fertilised with their own pollen, and eight of them dropped off; at the same time fifteen flowers on a crossed plant growing in the same pot were self-fertilised, and only one dropped off. On two other crossed plants of the same generation, several of the earliest flowers were observed to fertilise themselves and to produce capsules. In the plants of the ninth, and I believe of some previous generations, very many of the flowers, as already stated, were slightly monstrous; and this probably was connected with their lessened fertility.

All the self-fertilised plants of the seventh generation, and I believe of one or two previous generations, produced flowers of exactly the same tint, namely, of a rich dark purple. So did all the plants, without any exception, in the three succeeding generations of self-fertilised plants; and very many were raised on account of other experiments in progress not here recorded. My attention was first called to this fact by my gardener remarking that there was no occasion to label the self-fertilised plants, as they could always be known by their colour.

The flowers were as uniform in tint as those of a wild species growing in a state of nature; whether the same tint occurred, as is probable, in the earlier generations, neither my gardener nor self could recollect.

The flowers on the plants which were first raised from purchased seed, as well as during the first few generations, varied much in the depth of the purple tint; many were more or less pink, and occasionally a white variety appeared. The crossed plants continued to the tenth generation to vary in the same manner as before, but to a much less degree, owing, probably, to their having become more or less closely inter-related. We must therefore attribute the extraordinary uniformity of colour in the flowers on the plants of the seventh and succeeding self-fertilised generations, to inheritance not having been interfered with by crosses during several preceding generations, in combination with the conditions of life having been very uniform.

A plant appeared in the sixth self-fertilised generation, named the Hero, which exceeded by a little in height its crossed antagonist, and which transmitted its powers of growth and increased self-fertility to its children and grandchildren. A cross between the children of Hero did not give to the grandchildren any advantage over the self-fertilised grandchildren raised from the self-fertilised children. And as far as my observations can be trusted, which were made on very unhealthy plants, the great-grandchildren raised from intercrossing the grandchildren had no advantage over the seedlings from the grandchildren the product of continued self-fertilisation; and what is far more remarkable, the great-grandchildren raised by crossing the grandchildren with a fresh stock, had no advantage over either the intercrossed or self-fertilised great-grandchildren. It thus appears that Hero and its descendants differed in const.i.tution in an extraordinary manner from ordinary plants of the present species.

Although the plants raised during ten successive generations from crosses between distinct yet inter-related plants almost invariably exceeded in height, const.i.tutional vigour, and fertility their self-fertilised opponents, it has been proved that seedlings raised by intercrossing flowers on the same plant are by no means superior, on the contrary are somewhat inferior in height and weight, to seedlings raised from flowers fertilised with their own pollen. This is a remarkable fact, which seems to indicate that self-fertilisation is in some manner more advantageous than crossing, unless the cross brings with it, as is generally the case, some decided and preponderant advantage; but to this subject I shall recur in a future chapter.

The benefits which so generally follow from a cross between two plants apparently depend on the two differing somewhat in const.i.tution or character. This is shown by the seedlings from the intercrossed plants of the ninth generation, when crossed with pollen from a fresh stock, being as superior in height and almost as superior in fertility to the again intercrossed plants, as these latter were to seedlings from self-fertilised plants of the corresponding generation. We thus learn the important fact that the mere act of crossing two distinct plants, which are in some degree inter-related and which have been long subjected to nearly the same conditions, does little good as compared with that from a cross between plants belonging to different stocks or families, and which have been subjected to somewhat different conditions. We may attribute the good derived from the crossing of the intercrossed plants during the ten successive generations to their still differing somewhat in const.i.tution or character, as was indeed proved by their flowers still differing somewhat in colour. But the several conclusions which may be deduced from the experiments on Ipomoea will be more fully considered in the final chapters, after all my other observations have been given.

CHAPTER III.

SCROPHULARIACEAE, GESNERIACEAE, l.a.b.i.aTAE, ETC.

Mimulus luteus; height, vigour, and fertility of the crossed and self-fertilised plants of the first four generations.

Appearance of a new, tall, and highly self-fertile variety.

Offspring from a cross between self-fertilised plants.

Effects of a cross with a fresh stock.

Effects of crossing flowers on the same plant.

Summary on Mimulus luteus.

Digitalis purpurea, superiority of the crossed plants.

Effects of crossing flowers on the same plant.

Calceolaria.

Linaria vulgaris.

Verbasc.u.m thapsus.

Vandellia nummularifolia.

Cleistogene flowers.

Gesneria pendulina.

Salvia coccinea.

Origanum vulgare, great increase of the crossed plants by stolons.

Thunbergia alata.

In the family of the Scrophulariaceae I experimented on species in the six following genera: Mimulus, Digitalis, Calceolaria, Linaria, Verbasc.u.m, and Vandellia.

[3/2. SCROPHULARIACEAE.--Mimulus luteus.

The plants which I raised from purchased seed varied greatly in the colour of their flowers, so that hardly two individuals were quite alike; the corolla being of all shades of yellow, with the most diversified blotches of purple, crimson, orange, and coppery brown. But these plants differed in no other respect. (3/1. I sent several specimens with variously coloured flowers to Kew, and Dr. Hooker informs me that they all consisted of Mimulus luteus. The flowers with much red have been named by horticulturists as var. Youngiana.) The flowers are evidently well adapted for fertilisation by the agency of insects; and in the case of a closely allied species, Mimulus rosea, I have watched bees entering the flowers, thus getting their backs well dusted with pollen; and when they entered another flower the pollen was licked off their backs by the two-lipped stigma, the lips of which are irritable and close like a forceps on the pollen-grains. If no pollen is enclosed between the lips, these open again after a time. Mr. Kitchener has ingeniously explained the use of these movements, namely, to prevent the self-fertilisation of the flower. (3/2. 'A Year's Botany' 1874 page 118.) If a bee with no pollen on its back enters a flower it touches the stigma, which quickly closes, and when the bee retires dusted with pollen, it can leave none on the stigma of the same flower. But as soon as it enters any other flower, plenty of pollen is left on the stigma, which will be thus cross-fertilised. Nevertheless, if insects are excluded, the flowers fertilise themselves perfectly and produce plenty of seed; but I did not ascertain whether this is effected by the stamens increasing in length with advancing age, or by the bending down of the pistil. The chief interest in my experiments on the present species, lies in the appearance in the fourth self-fertilised generation of a variety which bore large peculiarly-coloured flowers, and grew to a greater height than the other varieties; it likewise became more highly self-fertile, so that this variety resembles the plant named Hero, which appeared in the sixth self-fertilised generation of Ipomoea.

Some flowers on one of the plants raised from the purchased seeds were fertilised with their own pollen; and others on the same plant were crossed with pollen from a distinct plant. The seeds from twelve capsules thus produced were placed in separate watch-gla.s.ses for comparison; and those from the six crossed capsules appeared to the eye hardly more numerous than those from the six self-fertilised capsules.

But when the seeds were weighed, those from the crossed capsules amounted to 1.02 grain, whilst those from the self-fertilised capsules were only .81 grain; so that the former were either heavier or more numerous than the latter, in the ratio of 100 to 79.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.

Having ascertained, by leaving crossed and self-fertilised seed on damp sand, that they germinated simultaneously, both kinds were thickly sown on opposite sides of a broad and rather shallow pan; so that the two sets of seedlings, which came up at the same time, were subjected to the same unfavourable conditions. This was a bad method of treatment, but this species was one of the first on which I experimented. When the crossed seedlings were on an average half an inch high, the self-fertilised ones were only a quarter of an inch high. When grown to their full height under the above unfavourable conditions, the four tallest crossed plants averaged 7.62, and the four tallest self-fertilised 5.87 inches in height; or as 100 to 77. Ten flowers on the crossed plants were fully expanded before one on the self-fertilised plants. A few of these plants of both lots were transplanted into a large pot with plenty of good earth, and the self-fertilised plants, not now being subjected to severe compet.i.tion, grew during the following year as tall as the crossed plants; but from a case which follows it is doubtful whether they would have long continued equal. Some flowers on the crossed plants were crossed with pollen from another plant, and the capsules thus produced contained a rather greater weight of seed than those on the self-fertilised plants again self-fertilised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Seeds from the foregoing plants, fertilised in the manner just stated, were sown on the opposite sides of a small pot (1) and came up crowded.

The four tallest crossed seedlings, at the time of flowering, averaged 8 inches in height, whilst the four tallest self-fertilised plants averaged only 4 inches. Crossed seeds were sown by themselves in a second small pot, and self-fertilised seeds were sown by themselves in a third small pot so that there was no compet.i.tion whatever between these two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher on an average than the self-fertilised. Both lots looked equally vigorous, but the crossed plants flowered earlier and more profusely than the self-fertilised. In Pot 1, in which the two lots competed with each other, the crossed plants flowered first and produced a large number of capsules, whilst the self-fertilised produced only nineteen.

The contents of twelve capsules from the crossed flowers on the crossed plants, and of twelve capsules from self-fertilised flowers on the self-fertilised plants, were placed in separate watch-gla.s.ses for comparison; and the crossed seeds seemed more numerous by half than the self-fertilised.

The plants on both sides of Pot 1, after they had seeded, were cut down and transplanted into a large pot with plenty of good earth, and on the following spring, when they had grown to a height of between 5 and 6 inches, the two lots were equal, as occurred in a similar experiment in the last generation. But after some weeks the crossed plants exceeded the self-fertilised ones on the opposite side of the same pot, though not nearly to so great a degree as before, when they were subjected to very severe compet.i.tion.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

Crossed seeds from the crossed plants, and self-fertilised seeds from the self-fertilised plants of the last generation, were sown thickly on opposite sides of a small pot, Number 1. The two tallest plants on each side were measured after they had flowered, and the two crossed ones were 12 and 7 1/2 inches, and the two self-fertilised ones 8 and 5 1/2 inches in height; that is, in the ratio of 100 to 69. Twenty flowers on the crossed plants were again crossed and produced twenty capsules; ten of which contained 1.33 grain weight of seeds. Thirty flowers on the self-fertilised plants were again self-fertilised and produced twenty-six capsules; ten of the best of which (many being very poor) contained only .87 grain weight of seeds; that is, in the ratio of 100 to 65 by weight.

The superiority of the crossed over the self-fertilised plants was proved in various ways. Self-fertilised seeds were sown on one side of a pot, and two days afterwards crossed seeds on the opposite side. The two lots of seedlings were equal until they were above half an inch high; but when fully grown the two tallest crossed plants attained a height of 12 1/2 and 8 3/4 inches, whilst the two tallest self-fertilised plants were only 8 and 5 1/2 inches high.

In a third pot, crossed seeds were sown four days after the self-fertilised, and the seedlings from the latter had at first, as might have been expected, an advantage; but when the two lots were between 5 and 6 inches in height, they were equal, and ultimately the three tallest crossed plants were 11, 10, and 8 inches, whilst the three tallest self-fertilised were 12, 8 1/2, and 7 1/2 inches in height. So that there was not much difference between them, the crossed plants having an average advantage of only the third of an inch. The plants were cut down, and without being disturbed were transplanted into a larger pot. Thus the two lots started fair on the following spring, and now the crossed plants showed their inherent superiority, for the two tallest were 13 inches, whilst the two tallest self-fertilised plants were only 11 and 8 1/2 inches in height; or as 100 to 75. The two lots were allowed to fertilise themselves spontaneously: the crossed plants produced a large number of capsules, whilst the self-fertilised produced very few and poor ones. The seeds from eight of the capsules on the crossed plants weighed .65 grain, whilst those from eight of the capsules on the self-fertilised plants weighed only .22 grain; or as 100 to 34.

The crossed plants in the above three pots, as in almost all the previous experiments, flowered before the self-fertilised. This occurred even in the third pot in which the crossed seeds were sown four days after the self-fertilised seeds.

Lastly, seeds of both lots were sown on opposite sides of a large pot in which a Fuchsia had long been growing, so that the earth was full of roots. Both lots grew miserably; but the crossed seedlings had an advantage at all times, and ultimately attained to a height of 3 1/2 inches, whilst the self-fertilised seedlings never exceeded 1 inch. The several foregoing experiments prove in a decisive manner the superiority in const.i.tutional vigour of the crossed over the self-fertilised plants.

In the three generations now described and taken together, the average height of the ten tallest crossed plants was 8.19 inches, and that of the ten tallest self-fertilised plants 5.29 inches (the plants having been grown in small pots), or as 100 to 65.

In the next or fourth self-fertilised generation, several plants of a new and tall variety appeared, which increased in the later self-fertilised generations, owing to its great self-fertility, to the complete exclusion of the original kinds. The same variety also appeared amongst the crossed plants, but as it was not at first regarded with any particular attention, I know not how far it was used for raising the intercrossed plants; and in the later crossed generations it was rarely present. Owing to the appearance of this tall variety, the comparison of the crossed and self-fertilised plants of the fifth and succeeding generations was rendered unfair, as all the self-fertilised and only a few or none of the crossed plants consisted of it. Nevertheless, the results of the later experiments are in some respects well worth giving.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.

Seeds of the two kinds, produced in the usual way from the two sets of plants of the third generation, were sown on opposite sides of two pots (1 and 2); but the seedlings were not thinned enough and did not grow well. Many of the self-fertilised plants, especially in one of the pots, consisted of the new and tall variety above referred to, which bore large and almost white flowers marked with crimson blotches. I will call it the WHITE VARIETY. I believe that it first appeared amongst both the crossed and self-fertilised plants of the last generation; but neither my gardener nor myself could remember any such variety in the seedlings raised from the purchased seed. It must therefore have arisen either through ordinary variation, or, judging from its appearance amongst both the crossed and self-fertilised plants, more probably through reversion to a formerly existing variety.