ON THE POWER OF MUTUAL FERTILISATION BETWEEN THE THREE FORMS.
Nothing shows more clearly the extraordinary complexity of the reproductive system of this plant, than the necessity of making eighteen distinct unions in order to ascertain the relative fertilising power of the three forms. Thus the long-styled form has to be fertilised with pollen from its own two kinds of anthers, from the two in the mid-styled, and from the two in the short-styled form. The same process has to be repeated with the mid-styled and short-styled forms. It might have been thought sufficient to have tried on each stigma the green pollen, for instance, from either the mid- or short-styled longest stamens, and not from both; but the result proves that this would have been insufficient, and that it was necessary to try all six kinds of pollen on each stigma. As in fertilising flowers there will always be some failures, it would have been advisable to have repeated each of the eighteen unions a score of times; but the labour would have been too great; as it was, I made 223 unions, i.e. on an average I fertilised above a dozen flowers in the eighteen different methods. Each flower was castrated; the adjoining buds had to be removed, so that the flowers might be safely marked with thread, wool, etc.; and after each fertilisation the stigma was examined with a lens to see that there was sufficient pollen on it. Plants of all three forms were protected during two years by large nets on a framework; two plants were used during one or both years, in order to avoid any individual peculiarity in a particular plant. As soon as the flowers had withered, the nets were removed; and in the autumn the capsules were daily inspected and gathered, the ripe seeds being counted under the microscope. I have given these details that confidence may be placed in the following tables, and as some excuse for two blunders which, I believe, were made. These blunders are referred to, with their probable cause, in two footnotes to the tables. The erroneous numbers, however, are entered in the tables, that it may not be supposed that I have in any one instance tampered with the results.
A few words explanatory of the three tables must be given. Each is devoted to one of the three forms, and is divided into six compartments. The two upper ones in each table show the number of good seeds resulting from the application to the stigma of pollen from the two sets of stamens which correspond in length with the pistil of that form, and which are borne by the other two forms. Such unions are of a legitimate nature. The two next lower compartments show the result of the application of pollen from the two sets of stamens, not corresponding in length with the pistil, and which are borne by the other two forms. These unions are illegitimate. The two lowest compartments show the result of the application of each form’s own two kinds of pollen from the two sets of stamens belonging to the same form, and which do not equal the pistil in length. These unions are likewise illegitimate. The term own-form pollen here used does not mean pollen from the flower to be fertilised—for this was never used—but from another flower on the same plant, or more commonly from a distinct plant of the same form. The figure “0” means that no capsule was produced, or if a capsule was produced that it contained no good seed. In some part of each row of figures in each compartment, a short horizontal line may be seen; the unions above this line were made in 1862, and below it in 1863. It is of importance to observe this, as it shows that the same general result was obtained during two successive years; but more especially because 1863 was a very hot and dry season, and the plants had occasionally to be watered. This did not prevent the full complement of seed being produced from the more fertile unions; but it rendered the less fertile ones even more sterile than they otherwise would have been. I have seen striking instances of this fact in making illegitimate and legitimate unions with Primula; and it is well known that the conditions of life must be highly favourable to give any chance of success in producing hybrids between species which are crossed with difficulty.
TABLE 4.23. Lythrum salicaria, long-styled form.
TABLE 4.23.1. Legitimate union.
13 flowers fertilised by the longest stamens of the mid-styled. These stamens equal in length the pistil of the long-styled.
Product of good seed in each capsule.
36 53
81 0
0 0
0 0
0 0
- 0
45
41
38 percent of these flowers yielded capsules. Each capsule contained, on an average, 51.2 seeds.
TABLE 4.23.2. Legitimate union.
13 flowers fertilised by the longest stamens of the short-styled. These stamens equal in length the pistil of the long-styled.
Product of good seed in each capsule.
159 104
43 119
96 poor seed. 96
103 99
0 131
0 116
-
114
84 percent of these flowers yielded capsules. Each capsule contained, on an average, 107.3 seeds.
TABLE 4.23.3. Illegitimate union.
14 flowers fertilised by the shortest stamens of the mid-styled.
3 0
0 0
0 0
0 0
0 0
- 0
0 0
0
Too sterile for any average.
TABLE 4.23.4. Illegitimate union.
12 flowers fertilised by the mid-length stamens of the short-styled.
20 0
0 0
0 0
0 0
- 0
0 0
0
Too sterile for any average.
TABLE 4.23.5. Illegitimate union.
15 flowers fertilised by own-form mid-length stamens.
2 -
10 0
23 0
0 0
0 0
0 0
0 0
0 0
Too sterile for any average.
TABLE 4.23.6. Illegitimate union.
15 flowers fertilised by own-form shortest stamens.
4 -
8 0
4 0
0 0
0 0
0 0
0 0
0 0
Too sterile for any average.
Besides the above experiments, I fertilised a considerable number of long-styled flowers with pollen, taken by a camel’s-hair brush, from both the mid-length and shortest stamens of their own form: only 5 capsules were produced, and these yielded on an average 14.5 seeds. In 1863 I tried a much better experiment: a long-styled plant was grown by itself, miles away from any other plant, so that the flowers could have received only their own two kinds of pollen. The flowers were incessantly visited by bees, and their stigmas must have received successive applications of pollen on the most favourable days and at the most favourable hours: all who have crossed plants know that this highly favours fertilisation. This plant produced an abundant crop of capsules; I took by chance 20 capsules, and these contained seeds in number as follows:—
20 20 35 21 19
26 24 12 23 10
7 30 27 29 13
20 12 29 19 35
This gives an average of 21.5 seeds per capsule. As we know that the long-styled form, when standing near plants of the other two forms and fertilised by insects, produces on an average 93 seeds per capsule, we see that this form, fertilised by its own two pollens, yields only between one-fourth and one-fifth of the full number of seed. I have spoken as if the plant had received both its own kinds of pollen, and this is, of course, possible; but, from the enclosed position of the shortest stamens, it is much more probable that the stigma received exclusively pollen from the mid-length stamens; and this, as may be seen in Table 4.23.5, is the more fertile of the two self-unions.
TABLE 4.24. Lythrum salicaria, mid-styled form.
TABLE 4.24.1. Legitimate union.
12 flowers fertilised by the mid-length stamens of the long-styled. These stamens equal in length the pistil of the mid-styled.
Product of good seed in each capsule.
138 122
149 50
147 151
109 119
133 138
144 0
-
92 percent of these flowers (probably 100 per cent) yielded capsules. Each capsule contained, on an average, 127.3 seeds.
TABLE 4.24.2. Legitimate union.
12 flowers fertilised by the mid-length stamens of the short-styled. These stamens equal in length the pistil of the mid-styled.
Product of good seed in each capsule.
112 109
130 143
143 124
100 145
33 12
- 141
104
100 percent of these flowers yielded capsules. Each capsule contained, on an average, 108.0 seeds; or, excluding capsules with less than 20 seeds, the average is 116.7 seeds.
TABLE 4.24.3. Illegitimate union.
13 flowers fertilised by the shortest stamens of the long-styled.
83 12
0 19
0 85 seeds small and poor.
- 0
44 0
44 0
45 0
54 percent of these flowers yielded capsules. Each capsule contained, on an average, 47.4 seeds; or, excluding capsules with less than 20 seeds, the average is 60.2 seeds.
TABLE 4.24.4. Illegitimate union.
15 flowers fertilised by the longest stamens of the short-styled.
130 86
115 113
14 29
6 17
2 113
9 79
- 128
132 0
93 percent of these flowers yielded capsules. Each capsule contained, on an average, 69.5 seeds; or, excluding capsules with less than 20 seeds, the average is 102.8 seeds.
TABLE 4.24.5. Illegitimate union.
12 flowers fertilised by own-form longest stamens.
92 0
9 0
63 0
- 0
136?* 0
0 0
0
(4/6. * I have hardly a doubt that this result of 136 seeds in Table 4.24.5 was due to a gross error. The flowers to be fertilised by their own longest stamens were first marked by “white thread,” and those by the mid-length stamens of the long-styled form by “white silk;” a flower fertilised in the later manner would have yielded about 136 seeds, and it may be observed that one such pod is missing, namely at the bottom of Table 4.24.1. Therefore I have hardly any doubt that I fertilised a flower marked with “white thread” as if it had been marked with “white silk.” With respect to the capsule which yielded 92 seeds, in the same column with that which yielded 136, I do not know what to think. I endeavoured to prevent pollen dropping from an upper to a lower flower, and I tried to remember to wipe the pincers carefully after each fertilisation; but in making eighteen different unions, sometimes on windy days, and pestered by bees and flies buzzing about, some few errors could hardly be avoided. One day I had to keep a third man by me all the time to prevent the bees visiting the uncovered plants, for in a few seconds’ time they might have done irreparable mischief. It was also extremely difficult to exclude minute Diptera from the net. In 1862 I made the great mistake of placing a mid-styled and long-styled under the same huge net: in 1863 I avoided this error.)
Excluding the capsule with 136 seeds, 25 percent of the flowers yielded capsules, and each capsule contained, on an average, 54.6 seeds; or, excluding capsules with less than 20 seeds, the average is 77.5.
TABLE 4.24.6. Illegitimate union.
12 flowers fertilised by own-form shortest stamens.
0 0
0 0
0 0
- 0
0 0
0 0
0
Not one flower yielded a capsule.
Besides the experiments in Table 4.24, I fertilised a considerable number of mid-styled flowers with pollen, taken by a camel’s-hair brush, from both the longest and shortest stamens of their own form: only 5 capsules were produced, and these yielded on an average 11.0 seeds.
TABLE 4.25. Lythrum salicaria, short-styled form.
TABLE 4.25.1. Legitimate union.
12 flowers fertilised by the shortest stamens of the long-styled. These stamens equal in length the pistil of the short-styled.
69 56
61 88
88 112
66 111
0 62
0 100
-
83 percent of the flowers yielded capsules. Each capsule contained, on an average, 81.3 seeds.
TABLE 4.25.2. Legitimate union.
13 flowers fertilised by the shortest stamens of the mid-styled. These stamens equal in length the pistil of the short-styled.
93 69
77 69
48 53
43 9
0 0
0 0
- 0
61 percent of the flowers yielded capsules. Each capsule contained, on an average, 64.6 seeds.
TABLE 4.25.3. Illegitimate union.
10 flowers fertilised by the mid-length stamens of the long-styled.
0 14
0 0
0 0
0 0
- 0
23
Too sterile for any average.
TABLE 4.25.4. Illegitimate union. 10 flowers fertilised by the longest stamens of the mid-styled.
0 0
0 0
0 0
0 0
- 0
0
Too sterile for any average.
TABLE 4.25.5. Illegitimate union.
10 flowers fertilised by own-form longest stamens.
0 0
0 0
0 0
- 0
0 0
0
Too sterile for any average.
TABLE 4.25.6. Illegitimate union.
10 flowers fertilised by own-form mid-length stamens.
64?* 0
0 0
0 0
- 0
21 0
9
(4/7. *I suspect that by mistake I fertilised this flower in Table 4.25.6 with pollen from the shortest stamens of the long-styled form, and it would then have yielded about 64 seeds. Flowers to be thus fertilised were marked with black silk; those with pollen from the mid-length stamens of the short-styled with black thread; and thus probably the mistake arose.)
Too sterile for any average.
Besides the experiments in the table, I fertilised a number of flowers without particular care with their own two kinds of pollen, but they did not produce a single capsule.