These were raised as heretofore with the following result:--

TABLE 2/8. Ipomoea purpurea (Seventh Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 84 4/8 : 74 6/8. Pot 1 : 84 6/8 : 84. Pot 1 : 76 2/8 : 55 4/8.

Pot 2 : 84 4/8 : 65. Pot 2 : 90 : 51 2/8. Pot 2 : 82 2/8 : 80 4/8.

Pot 3 : 83 : 67 6/8. Pot 3 : 86 : 60 2/8.

Pot 4 : 84 2/8 : 75 2/8.

Total : 755.50 : 614.25.

Each of these nine crossed plants is higher than its opponent, though in one case only by three-quarters of an inch. Their average height is 83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to 81. These plants, after growing to their full height, became very unhealthy and infested with aphides, just when the seeds were setting, so that many of the capsules failed, and nothing can be said on their relative fertility.

CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.

As just stated, the plants of the last generation, from which the present ones were raised, were very unhealthy and their seeds of unusually small size; and this probably accounts for the two lots behaving differently to what they did in any of the previous or succeeding generations. Many of the self-fertilised seeds germinated before the crossed ones, and these were of course rejected. When the crossed seedlings in Table 2/9 had grown to a height of between 1 and 2 feet, they were all, or almost all, shorter than their self-fertilised opponents, but were not then measured. When they had acquired an average height of 32.28 inches, that of the self-fertilised plants was 40.68, or as 100 to 122. Moreover, every one of the self-fertilised plants, with a single exception, exceeded its crossed opponent. When, however, the crossed plants had grown to an average height of 77.56 inches, they just exceeded (namely, by .7 of an inch) the average height of the self-fertilised plants; but two of the latter were still taller than their crossed opponents. I was so much astonished at this whole case, that I tied string to the summits of the rods; the plants being thus allowed to continue climbing upwards. When their growth was complete they were untwined, stretched straight, and measured. The crossed plants had now almost regained their accustomed superiority, as may be seen in Table 2/9.

The average height of the eight crossed plants is here 113.25 inches, and that of the self-fertilised plants 96.65, or as 100 to 85. Nevertheless two of the self-fertilised plants, as may be seen in Table 2/9, were still higher than their crossed opponents. The latter manifestly had much thicker stems and many more lateral branches, and looked altogether more vigorous than the self-fertilised plants, and generally flowered before them. The earlier flowers produced by these self-fertilised plants did not set any capsules, and their anthers contained only a small amount of pollen; but to this subject I shall return. Nevertheless capsules produced by two other self-fertilised plants of the same lot, not included in Table 2/9, which had been highly favoured by being grown in separate pots, contained the large average number of 5.1 seeds per capsule.

TABLE 2/9. Ipomoea purpurea (Eighth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 111 6/8 : 96. Pot 1 : 127 : 54. Pot 1 : 130 6/8 : 93 4/8.

Pot 2 : 97 2/8 : 94. Pot 2 : 89 4/8 : 125 6/8.

Pot 3 : 103 6/8 : 115 4/8. Pot 3 : 100 6/8 : 84 6/8. Pot 3 : 147 4/8 : 109 6/8.

Total : 908.25 : 773.25.

CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.

The plants of this generation were raised in the same manner as before, with the result shown in Table 2/10.

The fourteen crossed plants average in height 81.39 inches and the fourteen self-fertilised plants 64.07, or as 100 to 79.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 21

19th Century English Literature

Charles Darwin

Free Books in the public domain from the Classic Literature Library ©

Charles Darwin

All Pages of This Book