Chapter VI · SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC. (continued 2)

As pollen is deposited at a very early age on the pistil, generally in contact with the stigma, some flowers whilst still in bud were castrated for my first experiment, and were afterwards fertilised with pollen from a distinct plant. Other flowers were fertilised with their own pollen. From the seeds thus obtained, I succeeded in rearing only three pairs of plants of equal age. The three crossed plants averaged 32.79 inches, and the three self-fertilised 32.08 inches in height; so that they were nearly equal, the crossed having a slight advantage. As the same result followed in all three generations, it would be superfluous to give the heights of all the plants, and I will give only the averages.

In order to raise crossed and self-fertilised plants of the second generation, some flowers on the above crossed plants were crossed within twenty-four hours after they had expanded with pollen from a distinct plant; and this interval would probably not be too great to allow of cross-fertilisation being effectual. Some flowers on the self-fertilised plants of the last generation were also self-fertilised. From these two lots of seeds, ten crossed and twelve self-fertilised plants of equal ages were raised; and these were measured when fully grown. The crossed averaged 36.98, and the self-fertilised averaged 37.42 inches in height; so that here again the two lots were nearly equal; but the self-fertilised had a slight advantage.

In order to raise plants of the third generation, a better plan was followed, and flowers on the crossed plants of the second generation were selected in which the stamens were too short to reach the stigmas, so that they could not possibly have been self-fertilised. These flowers were crossed with pollen from a distinct plant. Flowers on the self-fertilised plants of the second generation were again self-fertilised. From the two lots of seeds thus obtained, twenty-one crossed and nineteen self-fertilised plants of equal age, and forming the third generation, were raised in fourteen large pots. They were measured when fully grown, and by an odd chance the average height of the two lots was exactly the same, namely, 35.96 inches; so that neither side had the least advantage over the other. To test this result, all the plants on both sides in ten out of the above fourteen pots were cut down after they had flowered, and in the ensuing year the stems were again measured; and now the crossed plants exceeded by a little (namely, 1.7 inches) the self-fertilised. They were again cut down, and on their flowering for the third time, the self-fertilised plants had a slight advantage (namely, 1.54 inches) over the crossed. Hence the result arrived at with these plants during the previous trials was confirmed, namely, that neither lot had any decided advantage over the other. It may, however, be worth mentioning that the self-fertilised plants showed some tendency to flower before the crossed plants: this occurred with all three pairs of the first generation; and with the cut down plants of the third generation, a self-fertilised plant flowered first in nine out of the twelve pots, whilst in the remaining three pots a crossed plant flowered first.

If we consider all the plants of the three generations taken together, the thirty-four crossed plants average 35.98, and the thirty-four self-fertilised plants 36.39 inches in height; or as 100 to 101. We may therefore conclude that the two lots possessed equal powers of growth; and this I believe to be the result of long-continued self-fertilisation, together with exposure to similar conditions in each generation, so that all the individuals had acquired a closely similar constitution.

30. GRAMINACEAE.—Zea mays.

This plant is monoecious, and was selected for trial on this account, no other such plant having been experimented on. (6/8. Hildebrand remarks that this species seems at first sight adapted to be fertilised by pollen from the same plant, owing to the male flowers standing above the female flowers; but practically it must generally be fertilised by pollen from another plant, as the male flowers usually shed their pollen before the female flowers are mature: ‘Monatsbericht der K. Akad.’ Berlin October 1872 page 743.) It is also anemophilous, or is fertilised by the wind; and of such plants only the common beet had been tried. Some plants were raised in the greenhouse, and were crossed with pollen taken from a distinct plant; and a single plant, growing quite separately in a different part of the house, was allowed to fertilise itself spontaneously. The seeds thus obtained were placed on damp sand, and as they germinated in pairs of equal age were planted on the opposite sides of four very large pots; nevertheless they were considerably crowded. The pots were kept in the hothouse. The plants were first measured to the tips of their leaves when only between 1 and 2 feet in height, as shown in Table 6/97.

TABLE 6/97. Zea mays.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 23 4/8 : 17 3/8. Pot 1 : 12 : 20 3/8. Pot 1 : 21 : 20.

Pot 2 : 22 : 20. Pot 2 : 19 1/8 : 18 3/8. Pot 2 : 21 4/8 : 18 5/8.

Pot 3 : 22 1/8 : 18 5/8. Pot 3 : 20 3/8 : 15 2/8. Pot 3 : 18 2/8 : 16 4/8. Pot 3 : 21 5/8 : 18. Pot 3 : 23 2/8 : 16 2/8.

Pot 4 : 21 : 18. Pot 4 : 22 1/8 : 12 6/8. Pot 4 : 23 : 15 4/8. Pot 4 : 12 : 18.

Total : 302.88 : 263.63.

The fifteen crossed plants here average 20.19, and the fifteen self-fertilised plants 17.57 inches in height; or as 100 to 87. Mr. Galton made a graphical representation, in accordance with the method described in the introductory chapter, of the above measurements, and adds the words “very good” to the curves thus formed.

Shortly afterwards one of the crossed plants in Pot 1 died; another became much diseased and stunted; and the third never grew to its full height. They seemed to have been all injured, probably by some larva gnawing their roots. Therefore all the plants on both sides of this pot were rejected in the subsequent measurements. When the plants were fully grown they were again measured to the tips of the highest leaves, and the eleven crossed plants now averaged 68.1, and the eleven self-fertilised plants 62.34 inches in height; or as 100 to 91. In all four pots a crossed plant flowered before any one of the self-fertilised; but three of the plants did not flower at all. Those that flowered were also measured to the summits of the male flowers: the ten crossed plants averaged 66.51, and the nine self-fertilised plants 61.59 inches in height; or as 100 to 93.

A large number of the same crossed and self-fertilised seeds were sown in the middle of the summer in the open ground in two long rows. Very much fewer of the self-fertilised than of the crossed plants produced flowers; but those that did flower, flowered almost simultaneously. When fully grown the ten tallest plants in each row were selected and measured to the tips of their highest leaves, as well as to the summits of their male flowers. The crossed averaged to the tips of their leaves 54 inches in height, and the self-fertilised 44.65, or as 100 to 83; and to the summits of their male flowers, 53.96 and 43.45 inches; or as 100 to 80.

Phalaris canariensis.

Hildebrand has shown in the paper referred to under the last species, that this hermaphrodite grass is better adapted for cross-fertilisation than for self-fertilisation. Several plants were raised in the greenhouse close together, and their flowers were mutually intercrossed. Pollen from a single plant growing quite separately was collected and placed on the stigmas of the same plant. The seeds thus produced were self-fertilised, for they were fertilised with pollen from the same plant, but it will have been a mere chance whether with pollen from the same flowers. Both lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of four pots, which were kept in the greenhouse. When the plants were a little over a foot in height they were measured, and the crossed plants averaged 13.38, and the self-fertilised 12.29 inches in height; or as 100 to 92.

When in full flower they were again measured to the extremities of their culms, as shown in Table 6/98.

TABLE 6/98. Phalaris canariensis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 42 2/8 : 41 2/8. Pot 1 : 39 6/8 : 45 4/8.

Pot 2 : 37 : 31 6/8. Pot 2 : 49 4/8 : 37 2/8. Pot 4 : 29 : 42 3/8. Pot 2 : 37 : 34 7/8.

Pot 3 : 37 6/8 : 28. Pot 3 : 35 4/8 : 28. Pot 3 : 43 : 34.

Pot 4 : 40 2/8 : 35 1/8. Pot 4 : 37 : 34 4/8.

Total : 428.00 : 392.63.

The eleven crossed plants now averaged 38.9, and the eleven self-fertilised plants 35.69 inches in height; or as 100 to 92, which is the same ratio as before. Differently to what occurred with the maize, the crossed plants did not flower before the self-fertilised; and though both lots flowered very poorly from having been kept in pots in the greenhouse, yet the self-fertilised plants produced twenty-eight flower-heads, whilst the crossed produced only twenty!

Two long rows of the same seeds were sown out of doors, and care was taken that they were sown in nearly equal number; but a far greater number of the crossed than of the self-fertilised seeds yielded plants. The self-fertilised plants were in consequence not so much crowded as the crossed, and thus had an advantage over them. When in full flower, the twelve tallest plants were carefully selected from both rows and measured, as shown in Table 6/99.

TABLE 6/99. Phalaris canariensis (growing in the open ground).

Heights of plants measured in inches.

Column 1: Crossed Plants, twelve tallest.

Column 2: Self-fertilised Plants, twelve tallest.

Total : 429.5 : 402.0.

The twelve crossed plants here average 35.78, and the twelve self-fertilised 33.5 inches in height; or as 100 to 93. In this case the crossed plants flowered rather before the self-fertilised, and thus differed from those growing in the pots.]