Greater constitutional vigour of crossed plants. The effects of great crowding. Competition with other kinds of plants. Self-fertilised plants more liable to premature death. Crossed plants generally flower before the self-fertilised. Negative effects of intercrossing flowers on the same plant. Cases described. Transmission of the good effects of a cross to later generations. Effects of crossing plants of closely related parentage. Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions.
GREATER CONSTITUTIONAL VIGOUR OF CROSSED PLANTS.
As in almost all my experiments an equal number of crossed and self-fertilised seeds, or more commonly seedlings just beginning to sprout, were planted on the opposite sides of the same pots, they had to compete with one another; and the greater height, weight, and fertility of the crossed plants may be attributed to their possessing greater innate constitutional vigour. Generally the plants of the two lots whilst very young were of equal height; but afterwards the crossed gained insensibly on their opponents, and this shows that they possessed some inherent superiority, though not displayed at a very early period in life. There were, however, some conspicuous exceptions to the rule of the two lots being at first equal in height; thus the crossed seedlings of the broom (Sarothamnus scoparius) when under three inches in height were more than twice as tall as the self-fertilised plants.
After the crossed or the self-fertilised plants had once grown decidedly taller than their opponents, a still increasing advantage would tend to follow from the stronger plants robbing the weaker ones of nourishment and overshadowing them. This was evidently the case with the crossed plants of Viola tricolor, which ultimately quite overwhelmed the self-fertilised. But that the crossed plants have an inherent superiority, independently of competition, was sometimes well shown when both lots were planted separately, not far distant from one another, in good soil in the open ground. This was likewise shown in several cases, even with plants growing in close competition with one another, by one of the self-fertilised plants exceeding for a time its crossed opponent, which had been injured by some accident or was at first sickly, but being ultimately conquered by it. The plants of the eighth generation of Ipomoea were raised from small seeds produced by unhealthy parents, and the self-fertilised plants grew at first very rapidly, so that when the plants of both lots were about three feet in height, the mean height of the crossed to that of the self-fertilised was as 100 to 122; when they were about six feet high the two lots were very nearly equal, but ultimately when between eight and nine feet in height, the crossed plants asserted their usually superiority, and were to the self-fertilised in height as 100 to 85.
The constitutional superiority of the crossed over the self-fertilised plants was proved in another way in the third generation of Mimulus, by self-fertilised seeds being sown on one side of a pot, and after a certain interval of time crossed seeds on the opposite side. The self-fertilised seedlings thus had (for I ascertained that the seeds germinated simultaneously) a clear advantage over the crossed in the start for the race. Nevertheless they were easily beaten (as may be seen under the head of Mimulus) when the crossed seeds were sown two whole days after the self-fertilised. But when the interval was four days, the two lots were nearly equal throughout life. Even in this latter case the crossed plants still possessed an inherent advantage, for after both lots had grown to their full height they were cut down, and without being disturbed were transferred to a larger pot, and when in the ensuing year they had again grown to their full height they were measured; and now the tallest crossed plants were to the tallest self-fertilised plants in height as 100 to 75, and in fertility (i.e., by weight of seeds produced by an equal number of capsules from both lots) as 100 to 34.