The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 49

Sometimes crossed and self-fertilised seeds were sown in separate rows in the open ground, which was kept clear of weeds; so that the seedlings were not subjected to any compet.i.tion with other kinds of plants. Those however in each row had to struggle with the adjoining ones in the same row. When fully grown, several of the tallest plants in each row were selected, measured, and compared. The result was in several cases (but not so invariably as might have been expected) that the crossed plants did not exceed in height the self-fertilised in nearly so great a degree as when grown in pairs in the pots. Thus with the plants of Digitalis, which competed together in pots, the crossed were to the self-fertilised in height as 100 to 70; whilst those which were grown separately were only as 100 to 85. Nearly the same result was observed with Bra.s.sica.

With Nicotiana the crossed were to the self-fertilised plants in height, when grown extremely crowded together in pots, as 100 to 54; when grown much less crowded in pots as 100 to 66, and when grow in the open ground, so as to be subjected to but little compet.i.tion, as 100 to 72.

On the other hand with Zea, there was a greater difference in height between the crossed and self-fertilised plants growing out of doors, than between the pairs which grew in pots in the hothouse; but this may be attributed to the self-fertilised plants being more tender, so that they suffered more than the crossed, when both lots were exposed to a cold and wet summer. Lastly, with one out of two series of Reseda odorata, grown out of doors in rows, as well as with Beta vulgaris, the crossed plants did not at all exceed the self-fertilised in height, or exceeded them by a mere trifle.

The innate power of the crossed plants to resist unfavourable conditions far better than did the self-fertilised plants, was shown on two occasions in a curious manner, namely, with Iberis and in the third generation of Petunia, by the great superiority in height of the crossed over the self-fertilised seedlings, when both sets were grown under extremely unfavourable conditions; whereas owing to special circ.u.mstances exactly the reverse occurred with the plants raised from the same seeds and grown in pairs in pots. A nearly a.n.a.logous case was observed on two other occasions with plants of the first generation of Nicotiana.

The crossed plants always withstood the injurious effects of being suddenly removed into the open air after having been kept in the greenhouse better than did the self-fertilised. On several occasions they also resisted much better cold and intemperate weather. This was manifestly the case with some crossed and self-fertilised plants of Ipomoea, which were suddenly moved from the hothouse to the coldest part of a cool greenhouse. The offspring of plants of the eighth self-fertilised generation of Mimulus crossed by a fresh stock, survived a frost which killed every single self-fertilised and intercrossed plant of the same old stock. Nearly the same result followed with some crossed and self-fertilised plants of Viola tricolor. Even the tips of the shoots of the crossed plants of Sarothamnus scoparius were not touched by a very severe winter; whereas all the self-fertilised plants were killed halfway down to the ground, so that they were not able to flower during the next summer. Young crossed seedlings of Nicotiana withstood a cold and wet summer much better than the self-fertilised seedlings. I have met with only one exception to the rule of crossed plants being hardier than the self-fertilised: three long rows of Eschscholtzia plants, consisting of crossed seedlings from a fresh stock, of intercrossed seedlings of the same stock, and of self-fertilised ones, were left unprotected during a severe winter, and all perished except two of the self-fertilised. But this case is not so anomalous as it at first appears, for it should be remembered that the self-fertilised plants of Eschscholtzia always grow taller and are heavier than the crossed; the whole benefit of a cross with this species being confined to increased fertility.

Independently of any external cause which could be detected, the self-fertilised plants were more liable to premature death than were the crossed; and this seems to me a curious fact. Whilst the seedlings were very young, if one died its antagonist was pulled up and thrown away, and I believe that many more of the self-fertilised died at this early age than of the crossed; but I neglected to keep any record. With Beta vulgaris, however, it is certain that a large number of the self-fertilised seeds perished after germinating beneath the ground, whereas the crossed seeds sown at the same time did not thus suffer.

When a plant died at a somewhat more advanced age the fact was recorded; and I find in my notes that out of several hundred plants, only seven of the crossed died, whilst of the self-fertilised at least twenty-nine were thus lost, that is more than four times as many. Mr. Galton, after examining some of my tables, remarks: "It is very evident that the columns with the self-fertilised plants include the larger number of exceptionally small plants;" and the frequent presence of such puny plants no doubt stands in close relation with their liability to premature death. The self-fertilised plants of Petunia completed their growth and began to wither sooner than did the intercrossed plants; and these latter considerably before the offspring from a cross with a fresh stock.

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.

Bra.s.sica oleracea (First Generation).

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

Bra.s.sica 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 africa.n.u.s.

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.