CHAPTER IV. CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
Brassica oleracea, crossed and self-fertilised plants.
Great effect of a cross with a fresh stock on the weight of the
offspring.
Iberis umbellata.
Papaver vagum.
Eschscholtzia californica, seedlings from a cross with a fresh stock not
more vigorous, but more fertile than the self-fertilised seedlings.
Reseda lutea and odorata, many individuals sterile with their own pollen.
Viola tricolor, wonderful effects of a cross.
Adonis aestivalis.
Delphinium consolida.
Viscaria oculata, crossed plants hardly taller, but more fertile than
the self-fertilised.
Dianthus caryophyllus, crossed and self-fertilised plants compared for
four generations.
Great effects of a cross with a fresh stock.
Uniform colour of the flowers on the self-fertilised plants.
Hibiscus africanus.
[6. CRUCIFERAE.—Brassica oleracea.
VAR. CATTELL’S EARLY BARNES CABBAGE.
The flowers of the common cabbage are adapted, as shown by H. Muller, for cross-fertilisation, and should this fail, for self-fertilisation. (4/1. ‘Die Befruchtung’ etc. page 139.) It is well known that the varieties are crossed so largely by insects, that it is impossible to raise pure kinds in the same garden, if more than one kind is in flower at the same time. Cabbages, in one respect, were not well fitted for my experiments, as, after they had formed heads, they were often difficult to measure. The flower-stems also differ much in height; and a poor plant will sometimes throw up a higher stem than that of a fine plant. In the later experiments, the fully-grown plants were cut down and weighed, and then the immense advantage from a cross became manifest.
A single plant of the above variety was covered with a net just before flowering, and was crossed with pollen from another plant of the same variety growing close by; and the seven capsules thus produced contained on an average 16.3 seeds, with a maximum of twenty in one capsule. Some flowers were artificially self-fertilised, but their capsules did not contain so many seeds as those from flowers spontaneously self-fertilised under the net, of which a considerable number were produced. Fourteen of these latter capsules contained on an average 4.1 seeds, with a maximum in one of ten seeds; so that the seeds in the crossed capsules were in number to those in the self-fertilised capsules as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed 3.88 grains, were, however, a little finer than those from the crossed capsules, fifty-eight of which weighed 3.76 grains. When few seeds are produced, these seem often to be better nourished and to be heavier than when many are produced.
The two lots of seeds in an equal state of germination were planted, some on opposite sides of a single pot, and some in the open ground. The young crossed plants in the pot at first exceeded by a little in height the self-fertilised; then equalled them; were then beaten; and lastly were again victorious. The plants, without being disturbed, were turned out of the pot, and planted in the open ground; and after growing for some time, the crossed plants, which were all of nearly the same height, exceeded the self-fertilised ones by 2 inches. When they flowered, the flower-stems of the tallest crossed plant exceeded that of the tallest self-fertilised plant by 6 inches. The other seedlings which were planted in the open ground stood separate, so that they did not compete with one another; nevertheless the crossed plants certainly grew to a rather greater height than the self-fertilised; but no measurements were made. The crossed plants which had been raised in the pot, and those planted in the open ground, all flowered a little before the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Some flowers on the crossed plants of the last generation were again crossed with pollen from another crossed plant, and produced fine capsules. The flowers on the self-fertilised plants of the last generation were allowed to fertilise themselves spontaneously under a net, and they produced some remarkably fine capsules. The two lots of seeds thus produced germinated on sand, and eight pairs were planted on opposite sides of four pots. These plants were measured to the tips of their leaves on the 20th of October of the same year, and the eight crossed plants averaged in height 8.4 inches, whilst the self-fertilised averaged 8.53 inches, so that the crossed were a little inferior in height, as 100 to 101.5. By the 5th of June of the following year these plants had grown much bulkier, and had begun to form heads. The crossed had now acquired a marked superiority in general appearance, and averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31 inches; or as 100 to 91. The plants were then turned out of their pots and planted undisturbed in the open ground. By the 5th of August their heads were fully formed, but several had grown so crooked that their heights could hardly be measured with accuracy. The crossed plants, however, were on the whole considerably taller than the self-fertilised. In the following year they flowered; the crossed plants flowering before the self-fertilised in three of the pots, and at the same time in Pot 2. The flower-stems were now measured, as shown in Table 4/29.