The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
CHAPTER IX.
THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS. Fertility of plants of crossed and self-fertilised parentage, both lots being fertilised in the same manner. Fertility of the parent-plants when first crossed and self-fertilised, and of their crossed and self-fertilised offspring when again crossed and self-fertilised. Comparison of the fertility of flowers fertilised with their own pollen and with that from other flowers on the same plant. Self-sterile plants. Causes of self-sterility. The appearance of highly self-fertile varieties. Self-fertilisation apparently in some respects beneficial, independently of the assured production of seeds. Relative weights and rates of germination of seeds from crossed and self-fertilised flowers. The present chapter is devoted to the Fertility of plants, as influenced by cross-fertilisation and self-fertilisation. The subject consists of two distinct branches; firstly, the relative productiveness or fertility of flowers crossed with pollen from a distinct plant and with their own pollen, as shown by the proportional number of capsules which they produce, together with the number of the contained seeds. Secondly, the degree of innate fertility or sterility of the seedlings raised from crossed and self-fertilised seeds; such seedlings being of the same age, grown under the same conditions, and fertilised in the same manner. These two branches of the subject correspond with the two which have to be considered by any one treating of hybrid plants; namely, in the first place the comparative productiveness of a species when fertilised with pollen from a distinct species and with its own pollen; and in the second place, the fertility of its hybrid offspring. These two classes of cases do not always run parallel; thus some plants, as Gartner has shown, can be crossed with great ease, but yield excessively sterile hybrids; while others are crossed with extreme difficulty, but yield fairly fertile hybrids.
The natural order to follow in this chapter would have been first to consider the effects on the fertility of the parent-plants of crossing them, and of fertilising them with their own pollen; but as we have discussed in the two last chapters the relative height, weight, and constitutional vigour of crossed and self-fertilised plants--that is, of plants raised from crossed and self-fertilised seeds--it will be convenient here first to consider their relative fertility. The cases observed by me are given in Table 9/D, in which plants of crossed and self-fertilised parentage were left to fertilise themselves, being either crossed by insects or spontaneously self-fertilised. It should be observed that the results cannot be considered as fully trustworthy, for the fertility of a plant is a most variable element, depending on its age, health, nature of the soil, amount of water given, and temperature to which it is exposed. The number of the capsules produced and the number of the contained seeds, ought to have been ascertained on a large number of crossed and self-fertilised plants of the same age and treated in every respect alike. In these two latter respects my observations may be trusted, but a sufficient number of capsules were counted only in a few instances.
The fertility, or as it may perhaps better be called the productiveness, of a plant depends on the number of capsules produced, and on the number of seeds which these contain. But from various causes, chiefly from the want of time, I was often compelled to rely on the number of the capsules alone. Nevertheless, in the more interesting cases, the seeds were also counted or weighed. The average number of seeds per capsule is a more valuable criterion of fertility than the number of capsules produced. This latter circumstance depends partly on the size of the plant; and we know that crossed plants are generally taller and heavier than the self-fertilised; but the difference in this respect is rarely sufficient to account for the difference in the number of the capsules produced. It need hardly be added that in Table 9/D the same number of crossed and self-fertilised plants are always compared. Subject to the foregoing sources of doubt I will now give the table, in which the parentage of the plants experimented on, and the manner of determining their fertility are explained. Fuller details may be found in the previous part of this work, under the head of each species. TABLE 9/D.--RELATIVE FERTILITY OF PLANTS OF CROSSED AND SELF-FERTILISED PARENTAGE, BOTH SETS BEING FERTILISED IN THE SAME MANNER. FERTILITY JUDGED OF BY VARIOUS STANDARDS. THAT OF THE CROSSED PLANTS TAKEN AS 100.
Column 1: Name of plant and feature observed. Column 2: x, in the expression, as 100 to x. Ipomoea purpurea--first generation: seeds per capsule on crossed and self-fertilised plants, not growing much crowded, spontaneously self-fertilised under a net, in number: 99. Ipomoea purpurea--seeds per capsule on crossed and self-fertilised plants from the same parents as in the last case, but growing much crowded, spontaneously self-fertilised under a net, in number: 93. Ipomoea purpurea--productiveness of the same plants, as judged by the number of capsules produced, and average number of seeds per capsule: 45. Ipomoea purpurea--third generation: seeds per capsule on crossed and self-fertilised plants, spontaneously self-fertilised under a net, in number: 94. Ipomoea purpurea--productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 35. Ipomoea purpurea--fifth generation: seeds per capsule on crossed and self-fertilised plants, left uncovered in the hothouse, and spontaneously fertilised: 89. Ipomoea purpurea--ninth generation: number of capsules on crossed plants to those on self-fertilised plants, spontaneously self-fertilised under a net: 26. Mimulus luteus--an equal number of capsules on plants descended from self-fertilised plants of the 8th generation crossed by a fresh stock, and on plants of the 9th self-fertilised generation, both sets having been left uncovered and spontaneously fertilised, contained seeds, by weight: 30. Mimulus luteus--productiveness of the same plants, as judged by the number of capsules produced, and the average weight of seeds per capsule: 3.
Vandellia nummularifolia--seeds per capsule from cleistogene flowers on the crossed and self-fertilised plants, in number: 106. Salvia coccinea--crossed plants, compared with self-fertilised plants, produced flowers, in number: 57. Iberis umbellata--plants left uncovered in greenhouse; intercrossed plants of the 3rd generation, compared with self-fertilised plants of the 3rd generation, yielded seeds, in number: 75. Iberis umbellata--plants from a cross between two varieties, compared with self-fertilised plants of the 3rd generation, yielded seeds, by weight : 75. Papaver vagum--crossed and self-fertilised plants, left uncovered, produced capsules, in number: 99. Eschscholtzia californica--Brazilian stock; plants left uncovered and cross-fertilised by bees; capsules on intercrossed plants of the 2nd generation, compared with capsules on self-fertilised plants of 2nd generation, contained seeds, in number: 78. Eschscholtzia californica--productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 89. Eschscholtzia californica--plants left uncovered and cross-fertilised by bees; capsules on plants derived from intercrossed plants of the 2nd generation of the Brazilian stock crossed by English stock, compared with capsules on self-fertilised plants of 2nd generation, contained seeds, in number: 63. Eschscholtzia californica--productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 40. Reseda odorata--crossed and self-fertilised plants, left uncovered and cross-fertilised by bees; produced capsules in number (about): 100.
Viola tricolor--crossed and self-fertilised plants, left uncovered and cross-fertilised by bees, produced capsules in number: 10. Delphinium consolida--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 56. Viscaria oculata--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 77. Dianthus caryophyllus--plants spontaneously self-fertilised under a net; capsules on intercrossed and self-fertilised plants of the 3rd generation contained seeds in number: 125. Dianthus caryophyllus--plants left uncovered and cross-fertilised by insects: offspring from plants self-fertilised for three generations and then crossed by an intercrossed plant of the same stock, compared with plants of the 4th self-fertilised generation, produced seeds by weight: 73. Dianthus caryophyllus--plants left uncovered and cross-fertilised by insects: offspring from plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the 4th self-fertilised generation, produced seeds by weight: 33. Tropaeolum minus--crossed and self-fertilised plants, left uncovered in the greenhouse, produced seeds in number: 64. Limnanthes douglasii--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number (about): 100. Lupinus luteus--crossed and self-fertilised plants of the 2nd generation, left uncovered in the greenhouse, produced seeds in number (judged from only a few pods): 88. Phaseolus multiflorus--crossed and self-fertilised plants, left uncovered in the greenhouse, produced seeds in number (about): 100. Lathyrus odoratus--crossed and self-fertilised plants of the 2nd generation, left uncovered in the greenhouse, but certainly self-fertilised, produced pods in number: 91.
Clarkia elegans--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 60. Nemophila insignis--crossed and self-fertilised plants, covered by a net and spontaneously self-fertilised in the greenhouse, produced capsules in number: 29. Petunia violacea--left uncovered and cross-fertilised by insects: plants of the 5th intercrossed and self-fertilised generations produced seeds, as judged by the weight of an equal number of capsules: 86. Petunia violacea--left uncovered as above: offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the 5th self-fertilised generation, produced seeds, as judged by the weight of an equal number of capsules: 46. Cyclamen persicum--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 12. Anagallis collina--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 8. Primula veris--left uncovered in open ground and cross-fertilised by insects: offspring from plants of the 3rd illegitimate generation crossed by a fresh stock, compared with plants of the 4th illegitimate and self-fertilised generation, produced capsules in number: 5. Same plants in the following year: 3.5. Primula veris--(equal-styled variety): left uncovered in open ground and cross-fertilised by insects: offspring from plants self-fertilised for two generations and then crossed by another variety, compared with plants of the 3rd self-fertilised generation, produced capsules in number: 15. Primula veris--(equal-styled variety) same plants; average number of seeds per capsule: 71.
Primula veris--(equal-styled variety) productiveness of the same plants, as judged by the number of capsules produced and the average number of seeds per capsule: 11. This table includes thirty-three cases relating to twenty-three species, and shows the degree of innate fertility of plants of crossed parentage in comparison with those of self-fertilised parentage; both lots being fertilised in the same manner. With several of the species, as with Eschscholtzia, Reseda, Viola, Dianthus, Petunia, and Primula, both lots were certainly cross-fertilised by insects, and so it probably was with several of the others; but in some of the species, as with Nemophila, and in some of the trials with Ipomoea and Dianthus, the plants were covered up, and both lots were spontaneously self-fertilised. This also was necessarily the case with the capsules produced by the cleistogene flowers of Vandellia.
The fertility of the crossed plants is represented in Table 9/D by 100, and that of the self-fertilised by the other figures. There are five cases in which the fertility of the self-fertilised plants is approximately equal to that of the crossed; nevertheless, in four of these cases the crossed plants were plainly taller, and in the fifth somewhat taller than the self-fertilised. But I should state that in some of these five cases the fertility of the two lots was not strictly ascertained, as the capsules were not actually counted, from appearing equal in number and from all apparently containing a full complement of seeds. In only two instances in the table, namely, with Vandellia and in the third generation of Dianthus, the capsules on the self-fertilised plants contained more seed than those on the crossed plants. With Dianthus the ratio between the number of seeds contained in the self-fertilised and crossed capsules was as 125 to 100; both sets of plants were left to fertilise themselves under a net; and it is almost certain that the greater fertility of the self-fertilised plants was here due merely to their having varied and become less strictly dichogamous, so as to mature their anthers and stigmas more nearly at the same time than is proper to the species.
Excluding the seven cases now referred to, there remain twenty-six in which the crossed plants were manifestly much more fertile, sometimes to an extraordinary degree, than the self-fertilised with which they grew in competition. The most striking instances are those in which plants derived from a cross with a fresh stock are compared with plants of one of the later self-fertilised generations; yet there are some striking cases, as that of Viola, between the intercrossed plants of the same stock and the self-fertilised, even in the first generation. The results most to be trusted are those in which the productiveness of the plants was ascertained by the number of capsules produced by an equal number of plants, together with the actual or average number of seeds in each capsule. Of such cases there are twelve in the table, and the mean of their mean fertility is as 100 for the crossed plants, to 59 for the self-fertilised plants. The Primulaceae seem eminently liable to suffer in fertility from self-fertilisation. The following short table, Table 9/E, includes four cases which have already been partly given in the last table. TABLE 9/E.--INNATE FERTILITY OF PLANTS FROM A CROSS WITH A FRESH STOCK, COMPARED WITH THAT OF INTERCROSSED PLANTS OF THE SAME STOCK, AND WITH THAT OF SELF-FERTILISED PLANTS, ALL OF THE CORRESPONDING GENERATION. FERTILITY JUDGED OF BY THE NUMBER OR WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS.
Column 1: Name of plant and feature observed. Column 2: Plants from a cross with a fresh stock. Column 3: Intercrossed plants of the same stock. Column 4: Self-fertilised plants. Mimulus luteus--the intercrossed plants are derived from a cross between two plants of the 8th self-fertilised generation. The self-fertilised plants belong to the 9th generation: 100 : 4 : 3. Eschscholtzia californica--the intercrossed and self-fertilised plants belong to the 2nd generation: 100 : 45 : 40. Dianthus caryophyllus--the intercrossed plants are derived from self-fertilised of the 3rd generation, crossed by intercrossed plants of the 3rd generation. The self-fertilised plants belong to the 4th generation: 100 : 45 : 33. Petunia violacea--the intercrossed and self-fertilised plants belong to the 5th generation: 100 : 54 : 46. NB.--In the above cases, excepting in that of Eschscholtzia, the plants derived from a cross with a fresh stock belong on the mother-side to the same stock with the intercrossed and self-fertilised plants, and to the corresponding generation. These cases show us how greatly superior in innate fertility the seedlings from plants self-fertilised or intercrossed for several generations and then crossed by a fresh stock are, in comparison with the seedlings from plants of the old stock, either intercrossed or self-fertilised for the same number of generations. The three lots of plants in each case were left freely exposed to the visits of insects, and their flowers without doubt were cross-fertilised by them.
Table 9/E further shows us that in all four cases the intercrossed plants of the same stock still have a decided though small advantage in fertility over the self-fertilised plants. With respect to the state of the reproductive organs in the self-fertilised plants of Tables 9/D and 9/E, only a few observations were made. In the seventh and eighth generation of Ipomoea, the anthers in the flowers of the self-fertilised plants were plainly smaller than those in the flowers of the intercrossed plants. The tendency to sterility in these same plants was also shown by the first-formed flowers, after they had been carefully fertilised, often dropping off, in the same manner as frequently occurs with hybrids. The flowers likewise tended to be monstrous. In the fourth generation of Petunia, the pollen produced by the self-fertilised and intercrossed plants was compared, and they were far more empty and shrivelled grains in the former. RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE FIRST OR A SUCCEEDING GENERATION.
I will first treat of the parent-plants, which were raised from seeds purchased from nursery-gardens, or taken from plants growing in my garden, or growing wild, and surrounded in every case by many individuals of the same species. Plants thus circumstanced will commonly have been intercrossed by insects; so that the seedlings which were first experimented on will generally have been the product of a cross. Consequently any difference in the fertility of their flowers, when crossed and self-fertilised, will have been caused by the nature of the pollen employed; that is, whether it was taken from a distinct plant or from the same flower. The degrees of fertility shown in Table 9/F, were determined in each case by the average number of seeds per capsule, ascertained either by counting or weighing.
Another element ought properly to have been taken into account, namely, the proportion of flowers which yielded capsules when they were crossed and self-fertilised; and as crossed flowers generally produce a larger proportion of capsules, their superiority in fertility, if this element had been taken into account, would have been much more strongly marked than appears in Table 9/F. But had I thus acted, there would have been greater liability to error, as pollen applied to the stigma at the wrong time fails to produce any effect, independently of its greater or less potency. A good illustration of the great difference in the results which sometimes follows, if the number of capsules produced relatively to the number of flowers fertilised be included in the calculation, was afforded by Nolana prostrata. Thirty flowers on some plants of this species were crossed and produced twenty-seven capsules, each containing five seeds; thirty-two flowers on the same plants were self-fertilised and produced only six capsules, each containing five seeds. As the number of seeds per capsule is here the same, the fertility of the crossed and self-fertilised flowers is given in Table 9/F as equal, or as 100 to 100. But if the flowers which failed to produce capsules be included, the crossed flowers yielded on an average 4.50 seeds, whilst the self-fertilised flowers yielded only 0.94 seeds, so that their relative fertility would have been as 100 to 21. I should here state that it has been found convenient to reserve for separate discussion the cases of flowers which are usually quite sterile with their own pollen.
TABLE 9/f.--relative fertility of the flowers on the parent-plants used in my experiments, when fertilised with pollen from a distinct plant and with their own pollen. Fertility judged of by the average number of seeds per capsule. Fertility of crossed flowers taken as 100. Column 1: Name of plant and feature observed. Column 2: x, in the expression 100 to x. Ipomoea purpurea--crossed and self-fertilised flowers yielded seeds as (about): 100. Mimulus luteus--crossed and self-fertilised flowers yielded seeds as (by weight): 79. Linaria vulgaris--crossed and self-fertilised flowers yielded seeds as: 14. Vandellia nummularifolia--crossed and self-fertilised flowers yielded seeds as: 67? Gesneria pendulina--crossed and self-fertilised flowers yielded seeds as (by weight): 100. Salvia coccinea--crossed and self-fertilised flowers yielded seeds as (about): 100. Brassica oleracea--crossed and self-fertilised flowers yielded seeds as: 25. Eschscholtzia californica--(English stock) crossed and self-fertilised flowers yielded seeds as (by weight): 71. Eschscholtzia californica--(Brazilian stock grown in England) crossed and self-fertilised flowers yielded seeds (by weight) as (about): 15. Delphinium consolida--crossed and self-fertilised flowers (self-fertilised capsules spontaneously produced, but result supported by other evidence) yielded seeds as: 59. Viscaria oculata--crossed and self-fertilised flowers yielded seeds as (by weight): 38. Viscaria oculata--crossed and self-fertilised flowers (crossed capsules compared on following year with spontaneously self-fertilised capsules) yielded seeds as : 58. Dianthus caryophyllus--crossed and self-fertilised flowers yielded seeds as: 92. Tropaeolum minus--crossed and self-fertilised flowers yielded seeds as: 92.
Tropaeolum tricolorum--crossed and self-fertilised flowers yielded seeds as: 115. (9/1. Tropaeolum tricolorum and Cuphea purpurea have been introduced into this table, although seedlings were not raised from them; but of the Cuphea only six crossed and six self-fertilised capsules, and of the Tropaeolum only six crossed and eleven self-fertilised capsules, were compared. A larger proportion of the self-fertilised than of the crossed flowers of the Tropaeolum produced fruit.) Limnanthes douglasii--crossed and self-fertilised flowers yielded seeds as (about): 100. Sarothamnus scoparius--crossed and self-fertilised flowers yielded seeds as: 41. Ononis minutissima--crossed and self-fertilised flowers yielded seeds as: 65. Cuphea purpurea--crossed and self-fertilised flowers yielded seeds as: 113. Passiflora gracilis--crossed and self-fertilised flowers yielded seeds as: 85. Specularia speculum--crossed and self-fertilised flowers yielded seeds as: 72. Lobelia fulgens--crossed and self-fertilised flowers yielded seeds as (about): 100. Nemophila insignis--crossed and self-fertilised flowers yielded seeds as (by weight): 69. Borago officinalis--crossed and self-fertilised flowers yielded seeds as: 60. Nolana prostrata--crossed and self-fertilised flowers yielded seeds as: 100. Petunia violacea--crossed and self-fertilised flowers yielded seeds as (by weight): 67. Nicotiana tabacum--crossed and self-fertilised flowers yielded seeds as (by weight): 150. Cyclamen persicum--crossed and self-fertilised flowers yielded seeds as: 38. Anagallis collina--crossed and self-fertilised flowers yielded seeds as: 96. Canna warscewiczi--crossed and self-fertilised flowers (on three generations of crossed and self-fertilised plants taken all together) yielded seeds as: 85.
Table 9/G gives the relative fertility of flowers on crossed plants again cross-fertilised, and of flowers on self-fertilised plants again self-fertilised, either in the first or in a later generation. Here two causes combine to diminish the fertility of the self-fertilised flowers; namely, the lesser efficacy of pollen from the same flower, and the innate lessened fertility of plants derived from self-fertilised seeds, which as we have seen in the previous Table 9/D is strongly marked. The fertility was determined in the same manner as in Table 9/F, that is, by the average number of seeds per capsule; and the same remarks as before, with respect to the different proportion of flowers which set capsules when they are cross-fertilised and self-fertilised, are here likewise applicable. TABLE 9/G.--RELATIVE FERTILITY OF FLOWERS ON CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST OR SOME SUCCEEDING GENERATION; THE FORMER BEING AGAIN FERTILISED WITH POLLEN FROM A DISTINCT PLANT, AND THE LATTER AGAIN WITH THEIR OWN POLLEN. FERTILITY JUDGED OF BY THE AVERAGE NUMBER OF SEEDS PER CAPSULE. FERTILITY OF CROSSED FLOWERS TAKEN AS 100. Column 1: Name of plant and feature observed. Column 2: x, in the expression, 100 to x. Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the first generation yielded seeds as: 93. Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 3rd generation yielded seeds as: 94.
Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 4th generation yielded seeds as: 94. Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 5th generation yielded seeds as: 107. Mimulus luteus--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 3rd generation yielded seeds as (by weight): 65. Mimulus luteus--same plants of the 3rd generation treated in the same manner on the following year yielded seeds as (by weight): 34. Mimulus luteus--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 4th generation yielded seeds as (by weight): 40. Viola tricolor--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as: 69. Dianthus caryophyllus--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as: 65. Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd generation crossed by intercrossed plants, and other flowers again self-fertilised yielded seeds as: 97. Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as: 127. Lathytus odoratus--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as: 65. Lobelia ramosa--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 60.
Petunia violacea--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 68. Petunia violacea--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 4th generation yielded seeds as (by weight): 72. Petunia violacea--flowers on self-fertilised plants of the 4th generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as (by weight): 48. Nicotiana tabacum--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 97. Nicotiana tabacum--flowers on self-fertilised plants of the 2nd generation crossed by intercrossed plants, and other flowers again self-fertilised yielded seeds as (by estimation): 110. Nicotiana tabacum--flowers on self-fertilised plants of the 3rd generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as (by estimation): 110. Anagallis collina--flowers on red variety crossed by a blue variety, and other flowers on the red variety self-fertilised yielded seeds as: 48. Canna warscewiczi--crossed and self-fertilised flowers on the crossed and self-fertilised plants of three generations taken together yielded seeds as: 85.
Text from Project Gutenberg, public domain in the United States.