The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗
- Author: Charles Robert Darwin
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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.
SUMMARY OF THE RESULTS.
LONG-STYLED FORM.
Twenty-six flowers fertilised legitimately by the stamens of corresponding length, borne by the mid-and short-styled forms, yielded 61.5 per cent of capsules, which contained on an average 89.7 seeds.
Twenty-six long-styled flowers fertilised illegitimately by the other stamens of the mid-and short-styled forms yielded only two very poor capsules.
Thirty long-styled flowers fertilised illegitimately by their own-form two sets of stamens yielded only eight very poor capsules; but long-styled flowers fertilised by bees with pollen from their own stamens produced numerous capsules containing on an average 21.5 seeds.
MID-STYLED FORM.
Twenty-four flowers legitimately fertilised by the stamens of corresponding length, borne by the long and short-styled forms, yielded 96 (probably 100) per cent of capsules, which contained (excluding one capsule with 12 seeds) on an average 117.2 seeds.
Fifteen mid-styled flowers fertilised illegitimately by the longest stamens of the short-styled form yielded 93 per cent of capsules, which (excluding four capsules with less than 20 seeds) contained on an average 102.8 seeds.
Thirteen mid-styled flowers fertilised illegitimately by the mid-length stamens of the long-styled form yielded 54 per cent of capsules, which (excluding one with 19 seeds) contained on an average 60.2 seeds.
Twelve mid-styled flowers fertilised illegitimately by their own-form longest stamens yielded 25 per cent of capsules, which (excluding one with 9 seeds) contained on an average 77.5 seeds.
Twelve mid-styled flowers fertilised illegitimately by their own-form shortest stamens yielded not a single capsule.
SHORT-STYLED FORM.
Twenty-five flowers fertilised legitimately by the stamens of corresponding length, borne by the long and mid-styled forms, yielded 72 per cent of capsules, which (excluding one capsule with only 9 seeds) contained on an average 70.8 seeds.
Twenty short-styled flowers fertilised illegitimately by the other stamens of the long and mid-styled forms yielded only two very poor capsules.
Twenty short-styled flowers fertilised illegitimately by their own stamens yielded only two poor (or perhaps three) capsules.
If we take all six legitimate unions together, and all twelve illegitimate unions together, we get the following results:
TABLE 4.26.
Column 1: Nature of union. Column 2: Number of Flowers fertilised. Column 3: Number of Capsules produced. Column 4: Average Number of Seeds per Capsule. Column 5: Average Number of Seeds per Flower fertilised.
The six legitimate unions : 75 : 56 : 96.29 : 71.89. The twelve illegitimate unions : 146 : 36 : 44.72 : 11.03.
Therefore the fertility of the legitimate unions to that of the illegitimate, as judged by the proportion of the fertilised flowers which yielded capsules, is as 100 to 33; and judged by the average number of seeds per capsule, as 100 to 46.
From this summary and the several foregoing tables we see that it is only pollen from the longest stamens which can fully fertilise the longest pistil; only that from the mid-length stamens, the mid-length pistil; and only that from the shortest stamens, the shortest pistil. And now we can comprehend the meaning of the almost exact correspondence in length between the pistil in each form and a set of six stamens in two of the other forms; for the stigma of each form is thus rubbed against that part of the insect's body which becomes charged with the proper pollen. It is also evident that the stigma of each form, fertilised in three different ways with pollen from the longest, mid-length, and shortest stamens, is acted on very differently, and conversely that the pollen from the twelve longest, twelve mid-length, and twelve shortest stamens acts very differently on each of the three stigmas; so that there are three sets of female and of male organs. Moreover, in most cases the six stamens of each set differ somewhat in their fertilising power from the six corresponding ones in one of the other forms. We may further draw the remarkable conclusion that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilisation, by so much is the sterility of the union increased. There are no exceptions to this rule. To understand what follows the reader should look to Tables 4.23, 4.24 and 4.25, and to the diagram Figure 4.10. In the long-styled form the short stamens obviously differ in length from the pistil to a greater degree than do the mid-length stamens; and the capsules produced by the use of pollen from the shortest stamens contain fewer seeds than those produced by the pollen from the mid-length stamens. The same result follows with the long-styled form, from the use of the pollen of shortest stamens of the mid-styled form and of the mid-length stamens of the short-styled form. The same rule also holds good with the mid-styled and short- styled forms, when illegitimately fertilised with pollen from the stamens more or less unequal in length to their pistils. Certainly the difference in sterility in these several cases is slight; but, as far as we are enabled to judge, it always increases with the increasing inequality of length between the pistil and the stamens which are used in each case.
The correspondence in length between the pistil in each form and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of high service to the species by leading to full and legitimate fertilisation. But the rule of the increased sterility of the illegitimate unions according to the greater inequality in length between the pistils and stamens employed for the union can be of no service. With some heterostyled dimorphic plants the difference of fertility between the two illegitimate unions appears at first sight to be related to the facility of self-fertilisation; so that when from the position of the parts the liability in one form to self- fertilisation is greater than in the other, a union of this kind has been checked by having been rendered the more sterile of the two. But this explanation does not apply to Lythrum; thus the stigma of the long-styled form is more liable to be illegitimately fertilised with pollen from its own mid- length stamens, or with pollen from the mid-length stamens of the short-styled form, than by its own shortest stamens or those of the mid-styled form; yet the two former unions, which it might have been expected would have been guarded against by increased sterility, are much less likely to be effected. The same relation holds good even in a more striking manner with the mid-styled form, and with the short-styled form as far as the extreme sterility of all its illegitimate unions allows of any comparison. We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens, is a purposeless result, incidental on those changes through which the species has passed in acquiring certain characters fitted to ensure the legitimate fertilisation of the three forms.
Another conclusion which may be drawn from Tables 4.23, 4.24, and 4.25, even from a glance at them, is that the mid-styled form differs from both the others in its much higher capacity for fertilisation in various ways. Not only did the twenty-four flowers legitimately fertilised by the stamens of corresponding lengths, all, or all but one, yield capsules rich in seed; but of the other four illegitimate unions, that by the longest stamens of the short-styled form was highly fertile, though less so than the two legitimate unions, and that by the mid-length stamens of the long-styled form was fertile to a considerable degree; the remaining two illegitimate unions, namely, with this form's own pollen, were sterile, but in different degrees. So that the mid-styled form, when fertilised in the six different possible methods, evinces five grades of fertility. By comparing Tables 4.24.3 and 4.24.6 we may see that the action of the pollen from the shortest stamens of the long-styled and mid-styled forms is widely different; in the one case above half the fertilised flowers yielded capsules containing a fair number of seeds; in the other case not one capsule was produced. So, again, the green, large-grained pollen from the longest stamens of the short-styled and mid-styled forms (in Tables 4.24.4 and 4.24.5) is widely different. In both these cases the difference in action is so plain that it cannot be mistaken, but it can be corroborated. If we look to Table 4.25 to the legitimate action of the shortest stamens of the long- and mid-styled forms on the short-styled form, we again see a similar but slighter difference, the pollen of the shortest stamens of the mid-styled form yielding a smaller average of seed during the two years of 1862 and 1863 than that from the shortest stamens of the long-styled form. Again, if we look to Table 4.23, to the legitimate action on the long-styled form of the green pollen of the two sets of longest stamens, we shall find exactly the same result, namely, that the pollen from the longest stamens of the mid-styled form yielded during both years fewer seeds than that from the longest stamens of the short-styled form. Hence it is certain that the two kinds of pollen produced by the mid-styled form are less potent than the two similar kinds of pollen produced by the corresponding stamens of the other two forms.
In close connection with the lesser potency of the two kinds of pollen of the mid-styled form is the fact that, according to H. Muller, the grains of both are a little less in diameter than the corresponding grains produced by
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.
SUMMARY OF THE RESULTS.
LONG-STYLED FORM.
Twenty-six flowers fertilised legitimately by the stamens of corresponding length, borne by the mid-and short-styled forms, yielded 61.5 per cent of capsules, which contained on an average 89.7 seeds.
Twenty-six long-styled flowers fertilised illegitimately by the other stamens of the mid-and short-styled forms yielded only two very poor capsules.
Thirty long-styled flowers fertilised illegitimately by their own-form two sets of stamens yielded only eight very poor capsules; but long-styled flowers fertilised by bees with pollen from their own stamens produced numerous capsules containing on an average 21.5 seeds.
MID-STYLED FORM.
Twenty-four flowers legitimately fertilised by the stamens of corresponding length, borne by the long and short-styled forms, yielded 96 (probably 100) per cent of capsules, which contained (excluding one capsule with 12 seeds) on an average 117.2 seeds.
Fifteen mid-styled flowers fertilised illegitimately by the longest stamens of the short-styled form yielded 93 per cent of capsules, which (excluding four capsules with less than 20 seeds) contained on an average 102.8 seeds.
Thirteen mid-styled flowers fertilised illegitimately by the mid-length stamens of the long-styled form yielded 54 per cent of capsules, which (excluding one with 19 seeds) contained on an average 60.2 seeds.
Twelve mid-styled flowers fertilised illegitimately by their own-form longest stamens yielded 25 per cent of capsules, which (excluding one with 9 seeds) contained on an average 77.5 seeds.
Twelve mid-styled flowers fertilised illegitimately by their own-form shortest stamens yielded not a single capsule.
SHORT-STYLED FORM.
Twenty-five flowers fertilised legitimately by the stamens of corresponding length, borne by the long and mid-styled forms, yielded 72 per cent of capsules, which (excluding one capsule with only 9 seeds) contained on an average 70.8 seeds.
Twenty short-styled flowers fertilised illegitimately by the other stamens of the long and mid-styled forms yielded only two very poor capsules.
Twenty short-styled flowers fertilised illegitimately by their own stamens yielded only two poor (or perhaps three) capsules.
If we take all six legitimate unions together, and all twelve illegitimate unions together, we get the following results:
TABLE 4.26.
Column 1: Nature of union. Column 2: Number of Flowers fertilised. Column 3: Number of Capsules produced. Column 4: Average Number of Seeds per Capsule. Column 5: Average Number of Seeds per Flower fertilised.
The six legitimate unions : 75 : 56 : 96.29 : 71.89. The twelve illegitimate unions : 146 : 36 : 44.72 : 11.03.
Therefore the fertility of the legitimate unions to that of the illegitimate, as judged by the proportion of the fertilised flowers which yielded capsules, is as 100 to 33; and judged by the average number of seeds per capsule, as 100 to 46.
From this summary and the several foregoing tables we see that it is only pollen from the longest stamens which can fully fertilise the longest pistil; only that from the mid-length stamens, the mid-length pistil; and only that from the shortest stamens, the shortest pistil. And now we can comprehend the meaning of the almost exact correspondence in length between the pistil in each form and a set of six stamens in two of the other forms; for the stigma of each form is thus rubbed against that part of the insect's body which becomes charged with the proper pollen. It is also evident that the stigma of each form, fertilised in three different ways with pollen from the longest, mid-length, and shortest stamens, is acted on very differently, and conversely that the pollen from the twelve longest, twelve mid-length, and twelve shortest stamens acts very differently on each of the three stigmas; so that there are three sets of female and of male organs. Moreover, in most cases the six stamens of each set differ somewhat in their fertilising power from the six corresponding ones in one of the other forms. We may further draw the remarkable conclusion that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilisation, by so much is the sterility of the union increased. There are no exceptions to this rule. To understand what follows the reader should look to Tables 4.23, 4.24 and 4.25, and to the diagram Figure 4.10. In the long-styled form the short stamens obviously differ in length from the pistil to a greater degree than do the mid-length stamens; and the capsules produced by the use of pollen from the shortest stamens contain fewer seeds than those produced by the pollen from the mid-length stamens. The same result follows with the long-styled form, from the use of the pollen of shortest stamens of the mid-styled form and of the mid-length stamens of the short-styled form. The same rule also holds good with the mid-styled and short- styled forms, when illegitimately fertilised with pollen from the stamens more or less unequal in length to their pistils. Certainly the difference in sterility in these several cases is slight; but, as far as we are enabled to judge, it always increases with the increasing inequality of length between the pistil and the stamens which are used in each case.
The correspondence in length between the pistil in each form and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of high service to the species by leading to full and legitimate fertilisation. But the rule of the increased sterility of the illegitimate unions according to the greater inequality in length between the pistils and stamens employed for the union can be of no service. With some heterostyled dimorphic plants the difference of fertility between the two illegitimate unions appears at first sight to be related to the facility of self-fertilisation; so that when from the position of the parts the liability in one form to self- fertilisation is greater than in the other, a union of this kind has been checked by having been rendered the more sterile of the two. But this explanation does not apply to Lythrum; thus the stigma of the long-styled form is more liable to be illegitimately fertilised with pollen from its own mid- length stamens, or with pollen from the mid-length stamens of the short-styled form, than by its own shortest stamens or those of the mid-styled form; yet the two former unions, which it might have been expected would have been guarded against by increased sterility, are much less likely to be effected. The same relation holds good even in a more striking manner with the mid-styled form, and with the short-styled form as far as the extreme sterility of all its illegitimate unions allows of any comparison. We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens, is a purposeless result, incidental on those changes through which the species has passed in acquiring certain characters fitted to ensure the legitimate fertilisation of the three forms.
Another conclusion which may be drawn from Tables 4.23, 4.24, and 4.25, even from a glance at them, is that the mid-styled form differs from both the others in its much higher capacity for fertilisation in various ways. Not only did the twenty-four flowers legitimately fertilised by the stamens of corresponding lengths, all, or all but one, yield capsules rich in seed; but of the other four illegitimate unions, that by the longest stamens of the short-styled form was highly fertile, though less so than the two legitimate unions, and that by the mid-length stamens of the long-styled form was fertile to a considerable degree; the remaining two illegitimate unions, namely, with this form's own pollen, were sterile, but in different degrees. So that the mid-styled form, when fertilised in the six different possible methods, evinces five grades of fertility. By comparing Tables 4.24.3 and 4.24.6 we may see that the action of the pollen from the shortest stamens of the long-styled and mid-styled forms is widely different; in the one case above half the fertilised flowers yielded capsules containing a fair number of seeds; in the other case not one capsule was produced. So, again, the green, large-grained pollen from the longest stamens of the short-styled and mid-styled forms (in Tables 4.24.4 and 4.24.5) is widely different. In both these cases the difference in action is so plain that it cannot be mistaken, but it can be corroborated. If we look to Table 4.25 to the legitimate action of the shortest stamens of the long- and mid-styled forms on the short-styled form, we again see a similar but slighter difference, the pollen of the shortest stamens of the mid-styled form yielding a smaller average of seed during the two years of 1862 and 1863 than that from the shortest stamens of the long-styled form. Again, if we look to Table 4.23, to the legitimate action on the long-styled form of the green pollen of the two sets of longest stamens, we shall find exactly the same result, namely, that the pollen from the longest stamens of the mid-styled form yielded during both years fewer seeds than that from the longest stamens of the short-styled form. Hence it is certain that the two kinds of pollen produced by the mid-styled form are less potent than the two similar kinds of pollen produced by the corresponding stamens of the other two forms.
In close connection with the lesser potency of the two kinds of pollen of the mid-styled form is the fact that, according to H. Muller, the grains of both are a little less in diameter than the corresponding grains produced by
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