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

by Charles Darwin

PERIOD OF FLOWERING.

In some cases, as with Digitalis, Dianthus, and Reseda, a larger number of the crossed than of the self-fertilised plants threw up flower-stems; but this probably was merely the result of their greater power of growth; for in the first generation of Lobelia fulgens, in which the self-fertilised plants greatly exceeded in height the crossed plants, some of the latter failed to throw up flower-stems. With a large number of species, the crossed plants exhibited a well-marked tendency to flower before the self-fertilised ones growing in the same pots. It should however be remarked that no record was kept of the flowering of many of the species; and when a record was kept, the flowering of the first plant in each pot was alone observed, although two or more pairs grew in the same pot. I will now give three lists,—one of the species in which the first plant that flowered was a crossed one,—a second in which the first that flowered was a self-fertilised plant,—and a third of those which flowered at the same time.

[SPECIES, OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF CROSSED PARENTAGE.

Ipomoea purpurea.

I record in my notes that in all ten generations many of the crossed plants flowered before the self-fertilised; but no details were kept.

Mimulus luteus (First Generation).

Ten flowers on the crossed plants were fully expanded before one on the self-fertilised.

Mimulus luteus (Second and Third Generation).

In both these generations a crossed plant flowered before one of the self-fertilised in all three pots.

Mimulus luteus (Fifth Generation).

In all three pots a crossed plant flowered first; yet the self-fertilised plants, which belonged to the new tall variety, were in height to the crossed as 126 to 100.

Mimulus luteus.

Plants derived from a cross with a fresh stock as well as the intercrossed plants of the old stock, flowered before the self-fertilised plants in nine out of the ten pots.

Salvia coccinea.

A crossed plant flowered before any one of the self-fertilised in all three pots.

Origanum vulgare.

During two successive seasons several crossed plants flowered before the self-fertilised.

Brassica oleracea (First Generation).

All the crossed plants growing in pots and in the open ground flowered first.

Brassica oleracea (Second Generation).

A crossed plant in three out of the four pots flowered before any one of the self-fertilised.

Iberis umbellata.

In both pots a crossed plant flowered first.

Eschscholtzia californica.

Plants derived from the Brazilian stock crossed by the English stock flowered in five out of the nine pots first; in four of them a self-fertilised plant flowered first; and not in one pot did an intercrossed plant of the old stock flower first.

Viola tricolor.

A crossed plant in five out of the six pots flowered before any one of the self-fertilised.

Dianthus caryophyllus (First Generation).

In two large beds of plants, four of the crossed plants flowered before any one of the self-fertilised.

Dianthus caryophyllus (Second Generation).

In both pots a crossed plant flowered first.

Dianthus caryophyllus (Third Generation).

In three out of the four pots a crossed plant flowered first; yet the crossed were to the self-fertilised in height only as 100 to 99, but in weight as 100 to 49.

Dianthus caryophyllus.

Plants derived from a cross with a fresh stock, and the intercrossed plants of the old stock, both flowered before the self-fertilised in nine out of the ten pots.

Hibiscus africanus.

In three out of the four pots a crossed plant flowered before any one of the self-fertilised; yet the latter were to the crossed in height as 109 to 100.

Tropaeolum minus.

A crossed plant flowered before any one of the self-fertilised in three out of the four pots, and simultaneously in the fourth pot.

Limnanthes douglasii.

A crossed plant flowered before any one of the self-fertilised in four out of the five pots.

Phaseolus multiflorus.

In both pots a crossed plant flowered first.

Specularia speculum.

In all four pots a crossed plant flowered first.

Lobelia ramosa (First Generation).

In all four pots a crossed plant flowered before any one of the self-fertilised.

Lobelia ramosa (Second Generation).

In all four pots a crossed plant flowered some days before any one of the self-fertilised.

Nemophila insignis.

In four out of the five pots a crossed plant flowered first.

Borago officinalis.

In both pots a crossed plant flowered first.

Petunia violacea (Second Generation).

In all three pots a crossed plant flowered first.

Nicotiana tabacum.

A plant derived from a cross with a fresh stock flowered before any one of the self-fertilised plants of the fourth generation, in fifteen out of the sixteen pots.

Cyclamen persicum.

During two successive seasons a crossed plant flowered some weeks before any one of the self-fertilised in all four pots.

Primula veris (equal-styled var.)

In all three pots a crossed plant flowered first.

Primula sinensis.

In all four pots plants derived from an illegitimate cross between distinct plants flowered before any one of the self-fertilised plants.

Primula sinensis.

A legitimately crossed plant flowered before any one of the self-fertilised plants in seven out of the eight pots.

Fagopyrum esculentum.

A legitimately crossed plant flowered from one to two days before any one of the self-fertilised plants in all three pots.

Zea mays.

In all four pots a crossed plant flowered first.

Phalaris canariensis.

The crossed plants flowered before the self-fertilised in the open ground, but simultaneously in the pots.

SPECIES OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF SELF-FERTILISED PARENTAGE.

Eschscholtzia californica (First Generation).

The crossed plants were at first taller than the self-fertilised, but on their second growth during the following year the self-fertilised exceeded the crossed in height, and now they flowered first in three out of the four pots.

Lupinus luteus.

Although the crossed plants were to the self-fertilised in height as 100 to 82; yet in all three pots the self-fertilised plants flowered first.

Clarkia elegans.

Although the crossed plants were, as in the last case, to the self-fertilised in height as 100 to 82, yet in the two pots the self-fertilised flowered first.

Lobelia fulgens (First Generation).

The crossed plants were to the self-fertilised in height only as 100 to 127, and the latter flowered much before the crossed.

Petunia violacea (Third Generation).

The crossed plants were to the self-fertilised in height as 100 to 131, and in three out of the four pots a self-fertilised plant flowered first; in the fourth pot simultaneously.

Petunia violacea (Fourth generation).

Although the crossed plants were to the self-fertilised in height as 100 to 69, yet in three out of the five pots a self-fertilised plant flowered first; in the fourth pot simultaneously, and only in the fifth did a crossed plant flower first.

Nicotiana tabacum (First Generation).

The crossed plants were to the self-fertilised in height only as 100 to 178, and a self-fertilised plant flowered first in all four pots.

Nicotiana tabacum (Third Generation).

The crossed plants were to the self-fertilised in height as 100 to 101, and in four out of the five pots a self-fertilised plant flowered first.

Canna warscewiczi.

In the three generations taken together the crossed were to the self-fertilised in height as 100 to 101; in the first generation the self-fertilised plants showed some tendency to flower first, and in the third generation they flowered first in nine out of the twelve pots.

SPECIES IN WHICH THE CROSSED AND SELF-FERTILISED PLANTS FLOWERED ALMOST SIMULTANEOUSLY.

Mimulus luteus (Sixth Generation).

The crossed plants were inferior in height and vigour to the self-fertilised plants, which all belonged to the new white-flowered tall variety, yet in only half the pots did the self-fertilised plants flower first, and in the other half the crossed plants.

Viscaria oculata.

The crossed plants were only a little taller than the self-fertilised (namely, as 100 to 97), but considerably more fertile, yet both lots flowered almost simultaneously.

Lathyrus odoratus (Second Generation).

Although the crossed plants were to the self-fertilised in height as 100 to 88, yet there was no marked difference in their period of flowering.

Lobelia fulgens (Second Generation).

Although the crossed plants were to the self-fertilised in height as 100 to 91, yet they flowered simultaneously.

Nicotiana tabacum (Third Generation).

Although the crossed plants were to the self-fertilised in height as 100 to 83, yet in half the pots a self-fertilised plant flowered first, and in the other half a crossed plant.]

These three lists include fifty-eight cases, in which the period of flowering of the crossed and self-fertilised plants was recorded. In forty-four of them a crossed plant flowered first either in a majority of the pots or in all; in nine instances a self-fertilised plant flowered first, and in five the two lots flowered simultaneously. One of the most striking cases is that of Cyclamen, in which the crossed plants flowered some weeks before the self-fertilised in all four pots during two seasons. In the second generation of Lobelia ramosa, a crossed plant flowered in all four pots some days before any one of the self-fertilised. Plants derived from a cross with a fresh stock generally showed a very strongly marked tendency to flower before the self-fertilised and the intercrossed plants of the old stock; all three lots growing in the same pots. Thus with Mimulus and Dianthus, in only one pot out of ten, and in Nicotiana in only one pot out of sixteen, did a self-fertilised plant flower before the plants of the two crossed kinds,—these latter flowering almost simultaneously.

A consideration of the two first lists, especially of the second one, shows that a tendency to flower first is generally connected with greater power of growth, that is, with greater height. But there are some remarkable exceptions to this rule, proving that some other cause comes into play. Thus the crossed plants both of Lupinus luteus and Clarkia elegans were to the self-fertilised plants in height as 100 to 82, and yet the latter flowered first. In the third generation of Nicotiana, and in all three generations of Canna, the crossed and self-fertilised plants were of nearly equal height, yet the self-fertilised tended to flower first. On the other hand, with Primula sinensis, plants raised from a cross between two distinct individuals, whether these were legitimately or illegitimately crossed, flowered before the illegitimately self-fertilised plants, although all the plants were of nearly equal height in both cases. So it was with respect to height and flowering with Phaseolus, Specularia, and Borago. The crossed plants of Hibiscus were inferior in height to the self-fertilised, in the ratio of 100 to 109, and yet they flowered before the self-fertilised in three out of the four pots. On the whole, there can be no doubt that the crossed plants exhibit a tendency to flower before the self-fertilised, almost though not quite so strongly marked as to grow to a greater height, to weigh more, and to be more fertile.

A few other cases not included in the above three lists deserve notice. In all three pots of Viola tricolor, naturally crossed plants the offspring of crossed plants flowered before naturally crossed plants the offspring of self-fertilised plants. Flowers on two plants, both of self-fertilised parentage, of the sixth generation of Mimulus luteus were intercrossed, and other flowers on the same plants were fertilised with their own pollen; intercrossed seedlings and seedlings of the seventh self-fertilised generation were thus raised, and the latter flowered before the intercrossed in three out of the five pots. Flowers on a plant both of Mimulus luteus and of Ipomoea purpurea were crossed with pollen from other flowers on the same plant, and other flowers were fertilised with their own pollen; intercrossed seedlings of this peculiar kind, and others strictly self-fertilised being thus raised. In the case of the Mimulus the self-fertilised plants flowered first in seven out of the eight pots, and in the case of the Ipomoea in eight out of the ten pots; so that an intercross between the flowers on the same plant was very far from giving to the offspring thus raised, any advantage over the strictly self-fertilised plants in their period of flowering.