The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 17

Pot 1 : 33 4/8 : 25.

Pot 2 : 34 2/8 : 35.

Pot 3 : 29 : 27 2/8.

Pot 4 : 22 : 15.

Total : 118.75 : 102.25.

The four crossed plants here average 29.68 inches, and the four self-fertilised 25.56 in height; or as 100 to 86. The remaining seeds were sown in a large pot in which a Cineraria had long been growing; and in this case again the two crossed plants on the one side greatly exceeded in height the two self-fertilised plants on the opposite side.

The plants in the above four pots from having been kept in the greenhouse did not produce on this or any other similar occasion many capsules; but the flowers on the crossed plants when again crossed were much more productive than the flowers on the self-fertilised plants when again self-fertilised. These plants after seeding were cut down and kept in the greenhouse; and in the following year, when grown again, their relative heights were reversed, as the self-fertilised plants in three out of the four pots were now taller than and flowered before the crossed plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The fact just given with respect to the growth of the cut-down plants made me doubtful about my first trial, so I determined to make another on a larger scale with crossed and self-fertilised seedlings raised from the crossed and self-fertilised plants of the last generation. Eleven pairs were raised and grown in compet.i.tion in the usual manner; and now the result was different, for the two lots were nearly equal during their whole growth. It would therefore be superfluous to give a table of their heights. When fully grown and measured, the crossed averaged 32.47, and the self-fertilised 32.81 inches in height; or as 100 to 101.

There was no great difference in the number of flowers and capsules produced by the two lots when both were left freely exposed to the visits of insects.

PLANTS RAISED FROM BRAZILIAN SEED.

Fritz Muller sent me from South Brazil seeds of plants which were there absolutely sterile when fertilised with pollen from the same plant, but were perfectly fertile when fertilised with pollen from any other plant.

The plants raised by me in England from these seeds were examined by Professor Asa Gray, and p.r.o.nounced to belong to E. Californica, with which they were identical in general appearance. Two of these plants were covered by a net, and were found not to be so completely self-sterile as in Brazil. But I shall recur to this subject in another part of this work. Here it will suffice to state that eight flowers on these two plants, fertilised with pollen from another plant under the net, produced eight fine capsules, each containing on an average about eighty seeds. Eight flowers on these same plants, fertilised with their own pollen, produced seven capsules, which contained on an average only twelve seeds, with a maximum in one of sixteen seeds. Therefore the cross-fertilised capsules, compared with the self-fertilised, yielded seeds in the ratio of about 100 to 15. These plants of Brazilian parentage differed also in a marked manner from the English plants in producing extremely few spontaneously self-fertilised capsules under a net.

Crossed and self-fertilised seeds from the above plants, after germinating on bare sand, were planted in pairs on the opposite sides of five large pots. The seedlings thus raised were the grandchildren of the plants which grew in Brazil; the parents having been grown in England.

As the grandparents in Brazil absolutely require cross-fertilisation in order to yield any seeds, I expected that self-fertilisation would have proved very injurious to these seedlings, and that the crossed ones would have been greatly superior in height and vigour to those raised from self-fertilised flowers. But the result showed that my antic.i.p.ation was erroneous; for as in the last experiment with plants of the English stock, so in the present one, the self-fertilised plants exceeded the crossed by a little in height. It will be sufficient to state that the fourteen crossed plants averaged 44.64, and the fourteen self-fertilised 45.12 inches in height; or as 100 to 101.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

I now tried a different experiment. Eight flowers on the self-fertilised plants of the last experiment (i.e., grandchildren of the plants which grew in Brazil) were again fertilised with pollen from the same plant, and produced five capsules, containing on an average 27.4 seeds, with a maximum in one of forty-two seeds. The seedlings raised from these seeds formed the second SELF-FERTILISED generation of the Brazilian stock.

Eight flowers on one of the crossed plants of the last experiment were crossed with pollen from another grandchild, and produced five capsules.

These contained on an average 31.6 seeds, with a maximum in one of forty-nine seeds. The seedlings raised from these seeds may be called the INTERCROSSED.

Lastly, eight other flowers on the crossed plants of the last experiment were fertilised with pollen from a plant of the English stock, growing in my garden, and which must have been exposed during many previous generations to very different conditions from those to which the Brazilian progenitors of the mother-plant had been subjected. These eight flowers produced only four capsules, containing on an average 63.2 seeds, with a maximum in one of ninety. The plants raised from these seeds may be called the ENGLISH-CROSSED. As far as the above averages can be trusted from so few capsules, the English-crossed capsules contained twice as many seeds as the intercrossed, and rather more than twice as many as the self-fertilised capsules. The plants which yielded these capsules were grown in pots in the greenhouse, so that their absolute productiveness must not be compared with that of plants growing out of doors.

The above three lots of seeds, namely, the self-fertilised, intercrossed, and English-crossed, were planted in an equal state of germination (having been as usual sown on bare sand) in nine large pots, each divided into three parts by superficial part.i.tions. Many of the self-fertilised seeds germinated before those of the two crossed lots, and these were of course rejected. The seedlings thus raised are the great-grandchildren of the plants which grew in Brazil. When they were from 2 to 4 inches in height, the three lots were equal. They were measured when four-fifths grown, and again when fully grown, and as their relative heights were almost exactly the same at these two ages, I will give only the last measurements. The average height of the nineteen English-crossed plants was 45.92 inches; that of the eighteen intercrossed plants (for one died), 43.38; and that of the nineteen self-fertilised plants, 50.3 inches. So that we have the following ratios in height:--

The English-crossed to the self-fertilised plants, as 100 to 109.

The English-crossed to the intercrossed plants, as 100 to 94.

The intercrossed to the self-fertilised plants, as 100 to 116.

After the seed-capsules had been gathered, all these plants were cut down close to the ground and weighed. The nineteen English crossed plants weighed 18.25 ounces; the intercrossed plants (with their weight calculated as if there had been nineteen) weighed 18.2 ounces; and the nineteen self-fertilised plants, 21.5 ounces. We have therefore for the weights of the three lots of plants the following ratios:--

The English-crossed to the self-fertilised plants, as 100 to 118.

The English-crossed to the intercrossed plants, as 100 to 100.

The intercrossed to the self-fertilised plants, as 100 to 118.

We thus see that in weight, as in height, the self-fertilised plants had a decided advantage over the English-crossed and intercrossed plants.

The remaining seeds of the three kinds, whether or not in a state of germination, were sown in three long parallel rows in the open ground; and here again the self-fertilised seedlings exceeded in height by between 2 and 3 inches the seedlings in the two other rows, which were of nearly equal heights. The three rows were left unprotected throughout the winter, and all the plants were killed, with the exception of two of the self-fertilised; so that as far as this little bit of evidence goes, some of the self-fertilised plants were more hardy than any of the crossed plants of either lot.

We thus see that the self-fertilised plants which were grown in the nine pots were superior in height (as 116 to 100), and in weight (as 118 to 100), and apparently in hardiness, to the intercrossed plants derived from a cross between the grandchildren of the Brazilian stock. The superiority is here much more strongly marked than in the second trial with the plants of the English stock, in which the self-fertilised were to the crossed in height as 101 to 100. It is a far more remarkable fact--if we bear in mind the effects of crossing plants with pollen from a fresh stock in the cases of Ipomoea, Mimulus, Bra.s.sica, and Iberis--that the self-fertilised plants exceeded in height (as 109 to 100), and in weight (as 118 to 100), the offspring of the Brazilian stock crossed by the English stock; the two stocks having been long subjected to widely different conditions.

If we now turn to the fertility of the three lots of plants we find a very different result. I may premise that in five out of the nine pots the first plant which flowered was one of the English-crossed; in four of the pots it was a self-fertilised plant; and in not one did an intercrossed plant flower first; so that these latter plants were beaten in this respect, as in so many other ways. The three closely adjoining rows of plants growing in the open ground flowered profusely, and the flowers were incessantly visited by bees, and certainly thus intercrossed. The manner in which several plants in the previous experiments continued to be almost sterile as long as they were covered by a net, but set a mult.i.tude of capsules immediately that they were uncovered, proves how effectually the bees carry pollen from plant to plant. My gardener gathered, at three successive times, an equal number of ripe capsules from the plants of the three lots, until he had collected forty-five from each lot. It is not possible to judge from external appearance whether or not a capsule contains any good seeds; so that I opened all the capsules. Of the forty-five from the English-crossed plants, four were empty; of those from the intercrossed, five were empty; and of those from the self-fertilised, nine were empty.

The seeds were counted in twenty-one capsules taken by chance out of each lot, and the average number of seeds in the capsules from the English-crossed plants was 67; from the intercrossed, 56; and from the self-fertilised, 48.52. It therefore follows that:--

The forty-five capsules (the four empty ones included) from the English-crossed plants contained 2747 seeds.

The forty-five capsules (the five empty ones included) from the intercrossed plants contained 2240 seeds.

The forty-five capsules (the nine empty ones included) from the self-fertilised plants contained 1746.7 seeds.

The reader should remember that these capsules are the product of cross-fertilisation, effected by the bees; and that the difference in the number of the contained seeds must depend on the const.i.tution of the plants;--that is, on whether they were derived from a cross with a distinct stock, or from a cross between plants of the same stock, or from self-fertilisation. From the above facts we obtain the following ratios:--

Number of seeds contained in an equal number of naturally fertilised capsules produced:--

By the English-crossed and self-fertilised plants, as 100 to 63.

By the English-crossed and intercrossed plants, as 100 to 81.

By the intercrossed and self-fertilised plants, as 100 to 78.

But to have ascertained the productiveness of the three lots of plants, it would have been necessary to know how many capsules were produced by the same number of plants. The three long rows, however, were not of quite equal lengths, and the plants were much crowded, so that it would have been extremely difficult to have ascertained how many capsules were produced by them, even if I had been willing to undertake so laborious a task as to collect and count all the capsules. But this was feasible with the plants grown in pots in the greenhouse; and although these were much less fertile than those growing out of doors, their relative fertility appeared, after carefully observing them, to be the same. The nineteen plants of the English-crossed stock in the pots produced altogether 240 capsules; the intercrossed plants (calculated as nineteen) produced 137.22 capsules; and the nineteen self-fertilised plants, 152 capsules. Now, knowing the number of seeds contained in forty-five capsules of each lot, it is easy to calculate the relative numbers of seeds produced by an equal number of the plants of the three lots.

Number of seeds produced by an equal number of naturally-fertilised plants:--

Plants of English-crossed and self-fertilised parentage, as 100 to 40 seeds.

Plants of English-crossed and intercrossed parentage, as 100 to 45 seeds.

Plants of intercrossed and self-fertilised parentage, as 100 to 89 seeds.

The superiority in productiveness of the intercrossed plants (that is, the product of a cross between the grandchildren of the plants which grew in Brazil) over the self-fertilised, small as it is, is wholly due to the larger average number of seeds contained in the capsules; for the intercrossed plants produced fewer capsules in the greenhouse than did the self-fertilised plants. The great superiority in productiveness of the English-crossed over the self-fertilised plants is shown by the larger number of capsules produced, the larger average number of contained seeds, and the smaller number of empty capsules. As the English-crossed and intercrossed plants were the offspring of crosses in every previous generation (as must have been the case from the flowers being sterile with their own pollen), we may conclude that the great superiority in productiveness of the English-crossed over the intercrossed plants is due to the two parents of the former having been long subjected to different conditions.

The English-crossed plants, though so superior in productiveness, were, as we have seen, decidedly inferior in height and weight to the self-fertilised, and only equal to, or hardly superior to, the intercrossed plants. Therefore, the whole advantage of a cross with a distinct stock is here confined to productiveness, and I have met with no similar case.

8. RESEDACEAE.--Reseda lutea.

Seeds collected from wild plants growing in this neighbourhood were sown in the kitchen-garden; and several of the seedlings thus raised were covered with a net. Of these, some were found (as will hereafter be more fully described) to be absolutely sterile when left to fertilise themselves spontaneously, although plenty of pollen fell on their stigmas; and they were equally sterile when artificially and repeatedly fertilised with their own pollen; whilst other plants produced a few spontaneously self-fertilised capsules. The remaining plants were left uncovered, and as pollen was carried from plant to plant by the hive and humble-bees which incessantly visit the flowers, they produced an abundance of capsules. Of the necessity of pollen being carried from one plant to another, I had ample evidence in the case of this species and of R. odorata; for those plants, which set no seeds or very few as long as they were protected from insects, became loaded with capsules immediately that they were uncovered.

Seeds from the flowers spontaneously self-fertilised under the net, and from flowers naturally crossed by the bees, were sown on opposite sides of five large pots. The seedlings were thinned as soon as they appeared above ground, so that an equal number were left on the two sides. After a time the pots were plunged into the open ground. The same number of plants of crossed and self-fertilised parentage were measured up to the summits of their flower-stems, with the result given in Table 4/35.

Those which did not produce flower-stems were not measured.

TABLE 4/35. Reseda lutea, in pots.

Heights of plants to the summits of the flower-stems measured in inches.