The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 46

Iberis umbellata--offspring from English variety crossed by slightly different Algerine variety, compared with the self-fertilised offspring of the English variety, in fertility: .. : .. : .. : .. : 75.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second self-fertilised generation: 19 : 45.92 : 19 : 50.30 : 109.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second self-fertilised generation, in weight: .. : .. : .. : .. : 118.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second self-fertilised generation, in fertility: .. : .. : .. : .. : 40.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second intercrossed generation, in height: 19 : 45.92 : 18 : 43.38 : 94.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second intercrossed generation, in weight: .. : .. : .. : .. : 100.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second intercrossed generation, in fertility: .. : .. : .. : .. : 45.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the fourth self-fertilised generation: 16 : 32.82 : 10 : 26.55 : 81.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the fourth self-fertilised generation, in fertility: .. : .. : .. : .. : 33.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with the offspring of plants self-fertilised for three generations and then crossed by plants of the third intercrossed generation: 16 : 32.82 : 15 : 28.00 : 85.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with the offspring of plants self-fertilised for three generations and then crossed by plants of the third intercrossed generation, in fertility: .. : .. : .. : .. : 45.

Pisum sativum--offspring from a cross between two closely allied varieties, compared with the self-fertilised offspring of one of the varieties, or with intercrossed plants of the same stock: ? : : ? : : 60 to 75.

Lathyrus odoratus--offspring from two varieties, differing only in colour of their flowers, compared with the self-fertilised offspring of one of the varieties: in first generation: 2 : 79.25 : 2 : 63.75 : 80.

Lathyrus odoratus--offspring from two varieties, differing only in colour of their flowers, compared with the self-fertilised offspring of one of the varieties: in second generation: 6 : 62.91 : 6 : 55.31 : 88.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, in height: 21 : 50.05 : 21 : 33.23 : 66.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, in weight: .. : .. : .. : .. : 23.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, grown in open ground, in height: 10 : 36.67 : 10 : 23.31 : 63.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, grown in open ground, in weight: .. : .. : .. : .. : 53.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, grown in open ground, in fertility: .. : .. : .. : .. : 46.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, in height: 21 : 50.05 : 22 : 54.11 : 108.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, in weight: .. : .. : .. : .. : 101.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, grown in open ground, in height: 10 : 36.67 : 10 : 38.27 : 104.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, grown in open ground, in weight: .. : .. : .. : .. : 146.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, grown in open ground, in fertility: .. : .. : .. : .. : 54.

Nicotiana tabac.u.m--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown not much crowded in pots, in height: 26 : 63.29 : 26 : 41.67 : 66.

Nicotiana tabac.u.m--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown much crowded in pots, in height: 12 : 31.53 : 12 : 17.21 : 54.

Nicotiana tabac.u.m--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown much crowded in pots, in weight: .. : .. : .. : .. : 37.

Nicotiana tabac.u.m--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown in open ground, in height: 20 : 48.74 : 20 : 35.20 : 72.

Nicotiana tabac.u.m--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown in open ground, in weight: .. : .. : .. : .. : 63.

Anagallis collina--offspring from a red variety crossed by a blue variety, compared with the self-fertilised offspring of the red variety: 3 : 27.62 : 3 : 18.21 : 66.

Anagallis collina--offspring from a red variety crossed by a blue variety, compared with the self-fertilised offspring of the red variety, in fertility: .. : .. : .. : .. : 6.

Primula veris--offspring from long-styled plants of the third illegitimate generation, crossed by a fresh stock, compared with plants of the fourth illegitimate and self-fertilised generation: 8 : 7.03 : 8 : 3.21 : 46.

Primula veris--offspring from long-styled plants of the third illegitimate generation, crossed by a fresh stock, compared with plants of the fourth illegitimate and self-fertilised generation, in fertility: .. : .. : .. : .. : 5.

Primula veris--offspring from long-styled plants of the third illegitimate generation, crossed by a fresh stock, compared with plants of the fourth illegitimate and self-fertilised generation, in fertility in following year: .. : .. : .. : .. : 3.5.

Primula veris--(equal-styled, red-flowered variety)--offspring from plants self-fertilised for two generations and then crossed by a different variety, compared with plants of the third self-fertilised generation: 3 : 8.66 : 3 : 7.33 : 85.

Primula veris--(equal-styled, red-flowered variety)--offspring from plants self-fertilised for two generations and then crossed by a different variety, compared with plants of the third self-fertilised generation, in fertility: .. : .. : .. : .. : 11.

In these three tables the measurements of fifty-seven species, belonging to fifty-two genera and to thirty great natural families, are given. The species are natives of various parts of the world. The number of crossed plants, including those derived from a cross between plants of the same stock and of two different stocks, amounts to 1,101; and the number of self-fertilised plants (including a few in Table 7/C derived from a cross between plants of the same old stock) is 1,076. Their growth was observed from the germination of the seeds to maturity; and most of them were measured twice and some thrice. The various precautions taken to prevent either lot being unduly favoured, have been described in the introductory chapter. Bearing all these circ.u.mstances in mind, it may be admitted that we have a fair basis for judging of the comparative effects of cross-fertilisation and of self-fertilisation on the growth of the offspring.

It will be the most convenient plan first to consider the results given in Table 7/C, as an opportunity will thus be afforded of incidentally discussing some important points. If the reader will look down the right hand column of this table, he will see at a glance what an extraordinary advantage in height, weight, and fertility the plants derived from a cross with a fresh stock or with another sub-variety have over the self-fertilised plants, as well as over the intercrossed plants of the same old stock. There are only two exceptions to this rule, and these are hardly real ones. In the case of Eschscholtzia, the advantage is confined to fertility. In that of Petunia, though the plants derived from a cross with a fresh stock had an immense superiority in height, weight, and fertility over the self-fertilised plants, they were conquered by the intercrossed plants of the same old stock in height and weight, but not in fertility. It has, however, been shown that the superiority of these intercrossed plants in height and weight was in all probability not real; for if the two sets had been allowed to grow for another month, it is almost certain that those from a cross with the fresh stock would have been victorious in every way over the intercrossed plants.

Before we consider in detail the several cases given in Table 7/C, some preliminary remarks must be made. There is the clearest evidence, as we shall presently see, that the advantage of a cross depends wholly on the plants differing somewhat in const.i.tution; and that the disadvantages of self-fertilisation depend on the two parents, which are combined in the same hermaphrodite flower, having a closely similar const.i.tution. A certain amount of differentiation in the s.e.xual elements seems indispensable for the full fertility of the parents, and for the full vigour of the offspring. All the individuals of the same species, even those produced in a state of nature, differ somewhat, though often very slightly, from one another in external characters and probably in const.i.tution. This obviously holds good between the varieties of the same species, as far as external characters are concerned; and much evidence could be advanced with respect to their generally differing somewhat in const.i.tution. There can hardly be a doubt that the differences of all kinds between the individuals and varieties of the same species depend largely, and as I believe exclusively, on their progenitors having been subjected to different conditions; though the conditions to which the individuals of the same species are exposed in a state of nature often falsely appear to us the same. For instance, the individuals growing together are necessarily exposed to the same climate, and they seem to us at first sight to be subjected to identically the same conditions; but this can hardly be the case, except under the unusual contingency of each individual being surrounded by other kinds of plants in exactly the same proportional numbers. For the surrounding plants absorb different amounts of various substances from the soil, and thus greatly affect the nourishment and even the life of the individuals of any particular species. These will also be shaded and otherwise affected by the nature of the surrounding plants. Moreover, seeds often lie dormant in the ground, and those which germinate during any one year will often have been matured during very different seasons.

Seeds are widely dispersed by various means, and some will occasionally be brought from distant stations, where their parents have grown under somewhat different conditions, and the plants produced from such seeds will intercross with the old residents, thus mingling their const.i.tutional peculiarities in all sorts of proportions.

Plants when first subjected to culture, even in their native country, cannot fail to be exposed to greatly changed conditions of life, more especially from growing in cleared ground, and from not having to compete with many or any surrounding plants. They are thus enabled to absorb whatever they require which the soil may contain. Fresh seeds are often brought from distant gardens, where the parent-plants have been subjected to different conditions. Cultivated plants like those in a state of nature frequently intercross, and will thus mingle their const.i.tutional peculiarities. On the other hand, as long as the individuals of any species are cultivated in the same garden, they will apparently be subjected to more uniform conditions than plants in a state of nature, as the individuals have not to compete with various surrounding species. The seeds sown at the same time in a garden have generally been matured during the same season and in the same place; and in this respect they differ much from the seeds sown by the hand of nature. Some exotic plants are not frequented by the native insects in their new home, and therefore are not intercrossed; and this appears to be a highly important factor in the individuals acquiring uniformity of const.i.tution.

In my experiments the greatest care was taken that in each generation all the crossed and self-fertilised plants should be subjected to the same conditions. Not that the conditions were absolutely the same, for the more vigorous individuals will have robbed the weaker ones of nutriment, and likewise of water when the soil in the pots was becoming dry; and both lots at one end of the pot will have received a little more light than those at the other end. In the successive generations, the plants were subjected to somewhat different conditions, for the seasons necessarily varied, and they were sometimes raised at different periods of the year. But as they were all kept under gla.s.s, they were exposed to far less abrupt and great changes of temperature and moisture than are plants growing out of doors. With respect to the intercrossed plants, their first parents, which were not related, would almost certainly have differed somewhat in const.i.tution; and such const.i.tutional peculiarities would be variously mingled in each succeeding intercrossed generation, being sometimes augmented, but more commonly neutralised in a greater or less degree, and sometimes revived through reversion; just as we know to be the case with the external characters of crossed species and varieties. With the plants which were self-fertilised during the successive generations, this latter important source of some diversity of const.i.tution will have been wholly eliminated; and the s.e.xual elements produced by the same flower must have been developed under as nearly the same conditions as it is possible to conceive.

In Table 7/C the crossed plants are the offspring of a cross with a fresh stock, or with a distinct variety; and they were put into compet.i.tion either with self-fertilised plants, or with intercrossed plants of the same old stock. By the term fresh stock I mean a non-related plant, the progenitors of which have been raised during some generations in another garden, and have consequently been exposed to somewhat different conditions. In the case of Nicotiana, Iberis, the red variety of Primula, the common Pea, and perhaps Anagallis, the plants which were crossed may be ranked as distinct varieties or sub-varieties of the same species; but with Ipomoea, Mimulus, Dianthus, and Petunia, the plants which were crossed differed exclusively in the tint of their flowers; and as a large proportion of the plants raised from the same lot of purchased seeds thus varied, the differences may be estimated as merely individual. Having made these preliminary remarks, we will now consider in detail the several cases given in Table 7/C, and they are well worthy of full consideration.

1. Ipomoea purpurea.

Plants growing in the same pots, and subjected in each generation to the same conditions, were intercrossed for nine consecutive generations.

These intercrossed plants thus became in the later generations more or less closely inter-related. Flowers on the plants of the ninth intercrossed generation were fertilised with pollen taken from a fresh stock, and seedlings thus raised. Other flowers on the same intercrossed plants were fertilised with pollen from another intercrossed plant, producing seedlings of the tenth intercrossed generation. These two sets of seedlings were grown in compet.i.tion with one another, and differed greatly in height and fertility. For the offspring from the cross with a fresh stock exceeded in height the intercrossed plants in the ratio of 100 to 78; and this is nearly the same excess which the intercrossed had over the self-fertilised plants in all ten generations taken together, namely, as 100 to 77. The plants raised from the cross with a fresh stock were also greatly superior in fertility to the intercrossed, namely, in the ratio of 100 to 51, as judged by the relative weight of the seed-capsules produced by an equal number of plants of the two sets, both having been left to be naturally fertilised. It should be especially observed that none of the plants of either lot were the product of self-fertilisation. On the contrary, the intercrossed plants had certainly been crossed for the last ten generations, and probably, during all previous generations, as we may infer from the structure of the flowers and from the frequency of the visits of humble-bees. And so it will have been with the parent-plants of the fresh stock. The whole great difference in height and fertility between the two lots must be attributed to the one being the product of a cross with pollen from a fresh stock, and the other of a cross between plants of the same old stock.

This species offers another interesting case. In the five first generations in which intercrossed and self-fertilised plants were put into compet.i.tion with one another, every single intercrossed plant beat its self-fertilised antagonist, except in one instance, in which they were equal in height. But in the sixth generation a plant appeared, named by me the Hero, remarkable for its tallness and increased self-fertility, and which transmitted its characters to the next three generations. The children of Hero were again self-fertilised, forming the eighth self-fertilised generation, and were likewise intercrossed one with another; but this cross between plants which had been subjected to the same conditions and had been self-fertilised during the seven previous generations, did not effect the least good; for the intercrossed grandchildren were actually shorter than the self-fertilised grandchildren, in the ratio of 100 to 107. We here see that the mere act of crossing two distinct plants does not by itself benefit the offspring. This case is almost the converse of that in the last paragraph, on which the offspring profited so greatly by a cross with a fresh stock. A similar trial was made with the descendants of Hero in the following generation, and with the same result. But the trial cannot be fully trusted, owing to the extremely unhealthy condition of the plants. Subject to this same serious cause of doubt, even a cross with a fresh stock did not benefit the great-grandchildren of Hero; and if this were really the case, it is the greatest anomaly observed by me in all my experiments.

2. Mimulus luteus.

During the three first generations the intercrossed plants taken together exceeded in height the self-fertilised taken together, in the ratio of 100 to 65, and in fertility in a still higher degree. In the fourth generation a new variety, which grew taller and had whiter and larger flowers than the old varieties, began to prevail, especially amongst the self-fertilised plants. This variety transmitted its characters with remarkable fidelity, so that all the plants in the later self-fertilised generations belonged to it. These consequently exceeded the intercrossed plants considerably in height. Thus in the seventh generation the intercrossed plants were to the self-fertilised in height as 100 to 137. It is a more remarkable fact that the self-fertilised plants of the sixth generation had become much more fertile than the intercrossed plants, judging by the number of capsules spontaneously produced, in the ratio of 147 to 100. This variety, which as we have seen appeared amongst the plants of the fourth self-fertilised generation, resembles in almost all its const.i.tutional peculiarities the variety called Hero which appeared in the sixth self-fertilised generation of Ipomoea. No other such case, with the partial exception of that of Nicotiana, occurred in my experiments, carried on during eleven years.

Two plants of this variety of Mimulus, belonging to the sixth self-fertilised generation, and growing in separate pots, were intercrossed; and some flowers on the same plants were again self-fertilised. From the seeds thus obtained, plants derived from a cross between the self-fertilised plants, and others of the seventh self-fertilised generation, were raised. But this cross did not do the least good, the intercrossed plants being inferior in height to the self-fertilised, in the ratio of 100 to 110. This case is exactly parallel with that given under Ipomoea, of the grandchildren of Hero, and apparently of its great-grandchildren; for the seedlings raised by intercrossing these plants were not in any way superior to those of the corresponding generation raised from the self-fertilised flowers.

Therefore in these several cases the crossing of plants, which had been self-fertilised for several generations and which had been cultivated all the time under as nearly as possible the same conditions, was not in the least beneficial.