CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.
Six plants were raised in the usual manner from the crossed plants of the last generation (Table 2/10) again intercrossed, and from the self-fertilised again self-fertilised. As one of the crossed plants in Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and producing hardly any capsules, it and its opponent have been struck out of the table.
TABLE 2/11. Ipomoea purpurea (Tenth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 92 3/8 : 47 2/8. Pot 1 : 94 4/8 : 34 6/8.
Pot 2 : 87 : 54 4/8. Pot 2 : 89 5/8 : 49 2/8. Pot 2 : 105 : 66 2/8.
Total : 468.5 : 252.0.
The five crossed plants average 93.7 inches, and the five self-fertilised only 50.4, or as 100 to 54. This difference, however, is so great that it must be looked at as in part accidental. The six crossed plants (the diseased one here included) yielded spontaneously 101 capsules, and the six self-fertilised plants 88, the latter being chiefly produced by one of the plants. But as the diseased plant, which yielded hardly any seed, is here included, the ratio of 101 to 88 does not fairly give the relative fertility of the two lots. The stems of the six crossed plants looked so much finer than those of the six self-fertilised plants, that after the capsules had been gathered and most of the leaves had fallen off, they were weighed. Those of the crossed plants weighed 2,693 grains, whilst those of the self-fertilised plants weighed only 1,173 grains, or as 100 to 44; but as the diseased and dwarfed crossed plant is here included, the superiority of the former in weight was really greater.]
THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
In all the foregoing experiments, seedlings from flowers crossed by pollen from a distinct plant (though in the later generations more or less closely related) were put into competition with, and almost invariably proved markedly superior in height to the offspring from self-fertilised flowers. I wished, therefore, to ascertain whether a cross between two flowers on the same plant would give to the offspring any superiority over the offspring from flowers fertilised with their own pollen. I procured some fresh seed and raised two plants, which were covered with a net; and several of their flowers were crossed with pollen from a distinct flower on the same plant. Twenty-nine capsules thus produced contained on an average 4.86 seeds per capsule; and 100 of these seeds weighed 36.77 grains. Several other flowers were fertilised with their own pollen, and twenty-six capsules thus produced contained on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains. So that a cross of this kind appears to have increased slightly the number of seeds per capsule, in the ratio of 100 to 91; but these crossed seeds were lighter than the self-fertilised in the ratio of 86 to 100. I doubt, however, from other observations, whether these results are fully trustworthy. The two lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of nine pots, and were treated in every respect like the plants in the previous experiments. The remaining seeds, some in a state of germination and some not so, were sown on the opposite sides of a large pot (Number 10); and the four tallest plants on each side of this pot were measured. The result is shown in Table 2/12.
TABLE 2/12. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 82 : 77 4/8. Pot 1 : 75 : 87. Pot 1 : 65 : 64. Pot 1 : 76 : 87 2/8.
Pot 2 : 78 4/8 : 84. Pot 2 : 43 : 86 4/8. Pot 2 : 65 4/8 : 90 4/8.
Pot 3 : 61 2/8 : 86. Pot 3 : 85 : 69 4/8. Pot 3 : 89 : 87 4/8.
Pot 4 : 83 : 80 4/8. Pot 4 : 73 4/8 : 88 4/8. Pot 4 : 67 : 84 4/8.
Pot 5 : 78 : 66 4/8. Pot 5 : 76 6/8 : 77 4/8. Pot 5 : 57 : 81 4/8.
Pot 6 : 70 4/8 : 80. Pot 6 : 79 : 82 4/8. Pot 6 : 79 6/8 : 55 4/8.
Pot 7 : 76 : 77. Pot 7 : 84 4/8 : 83 4/8. Pot 7 : 79 : 73 4/8.
Pot 8 : 73 : 76 4/8. Pot 8 : 67 : 82. Pot 8 : 83 : 80 4/8.
Pot 9 : 73 2/8 : 78 4/8. Pot 9 : 78 : 67 4/8.
Pot 10 : 34 : 82 4/8. Pot 10 : 82 : 36 6/8. Pot 10 : 84 6/8 : 69 4/8. Pot 10 : 71 : 75 2/8. Crowded plants.
Total : 2270.25 : 2399.75.
The average height of the thirty-one crossed plants is 73.23 inches, and that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to 106. Looking to each pair, it may be seen that only thirteen of the crossed plants, whilst eighteen of the self-fertilised plants exceed their opponents. A record was kept with respect to the plant which flowered first in each pot; and only two of the crossed flowered before one of the self-fertilised in the same pot; whilst eight of the self-fertilised flowered first. It thus appears that the crossed plants are slightly inferior in height and in earliness of flowering to the self-fertilised. But the inferiority in height is so small, namely as 100 to 106, that I should have felt very doubtful on this head, had I not cut down all the plants (except those in the crowded pot Number 10) close to the ground and weighed them. The twenty-seven crossed plants weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20 1/2 ounces; and this gives a ratio of 100 to 124.
A self-fertilised plant of the same parentage as those in Table 2/12 had been raised in a separate pot for a distinct purpose; and it proved partially sterile, the anthers containing very little pollen. Several flowers on this plant were crossed with the little pollen which could be obtained from the other flowers on the same plant; and other flowers were self-fertilised. From the seeds thus produced four crossed and four self-fertilised plants were raised, which were planted in the usual manner on the opposite sides of two pots. All these four crossed plants were inferior in height to their opponents; they averaged 78.18 inches, whilst the four self-fertilised plants averaged 84.8 inches; or as 100 to 108. (2/2. From one of these self-fertilised plants, spontaneously self-fertilised, I gathered twenty-four capsules, and they contained on an average only 3.2 seeds per capsule; so that this plant had apparently inherited some of the sterility of its parent.) This case, therefore, confirms the last. Taking all the evidence together, we must conclude that these strictly self-fertilised plants grew a little taller, were heavier, and generally flowered before those derived from a cross between two flowers on the same plant. These latter plants thus present a wonderful contrast with those derived from a cross between two distinct individuals.
THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK BELONGING TO THE SAME VARIETY.
From the two foregoing series of experiments we see, firstly, the good effects during several successive generations of a cross between distinct plants, although these were in some degree inter-related and had been grown under nearly the same conditions; and, secondly, the absence of all such good effects from a cross between flowers on the same plant; the comparison in both cases being made with the offspring of flowers fertilised with their own pollen. The experiments now to be given show how powerfully and beneficially plants, which have been intercrossed during many successive generations, having been kept all the time under nearly uniform conditions, are affected by a cross with another plant belonging to the same variety, but to a distinct family or stock, which had grown under different conditions.
[Several flowers on the crossed plants of the ninth generation in Table 2/10, were crossed with pollen from another crossed plant of the same lot. The seedlings thus raised formed the tenth intercrossed generation, and I will call them the “INTERCROSSED PLANTS.” Several other flowers on the same crossed plants of the ninth generation were fertilised (not having been castrated) with pollen taken from plants of the same variety, but belonging to a distinct family, which had been grown in a distant garden at Colchester, and therefore under somewhat different conditions. The capsules produced by this cross contained, to my surprise, fewer and lighter seeds than did the capsules of the intercrossed plants; but this, I think, must have been accidental. The seedlings raised from them I will call the “COLCHESTER-CROSSED.” The two lots of seeds, after germinating on sand, were planted in the usual manner on the opposite sides of five pots, and the remaining seeds, whether or not in a state of germination, were thickly sown on the opposite sides of a very large pot, Number 6 in Table 2/13. In three of the six pots, after the young plants had twined a short way up their sticks, one of the Colchester-crossed plants was much taller than any one of the intercrossed plants on the opposite side of the same pot; and in the three other pots somewhat taller. I should state that two of the Colchester-crossed plants in Pot 4, when about two-thirds grown, became much diseased, and were, together with their intercrossed opponents, rejected. The remaining nineteen plants, when almost fully grown, were measured, with the following result:
TABLE 2/13. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Colchester-Crossed Plants.
Column 3: Intercrossed Plants of the Tenth Generation.
Pot 1 : 87 : 78. Pot 1 : 87 4/8 : 68 4/8. Pot 1 : 85 1/8 : 94 4/8.
Pot 2 : 93 6/8 : 60. Pot 2 : 85 4/8 : 87 2/8. Pot 2 : 90 5/8 : 45 4/8.
Pot 3 : 84 2/8 : 70 1/8. Pot 3 : 92 4/8 : 81 6/8. Pot 3 : 85 : 86 2/8.
Pot 4 : 95 6/8 : 65 1/8.
Pot 5 : 90 4/8 : 85 6/8. Pot 5 : 86 6/8 : 63. Pot 5 : 84 : 62 6/8.
Pot 6 : 90 4/8 : 43 4/8. Pot 6 : 75 : 39 6/8. Pot 6 : 71 : 30 2/8. Pot 6 : 83 6/8 : 86. Pot 6 : 63 : 53. Pot 6 : 65 : 48 6/8. Crowded plants in a very large pot.
Total : 1596.50 : 1249.75.
In sixteen out of these nineteen pairs, the Colchester-crossed plant exceeded in height its intercrossed opponent. The average height of the Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78 inches; or as 100 to 78. With respect to the fertility of the two lots, it was too troublesome to collect and count the capsules on all the plants; so I selected two of the best pots, 5 and 6, and in these the Colchester-crossed produced 269 mature and half-mature capsules, whilst an equal number of the intercrossed plants produced only 154 capsules; or as 100 to 57. By weight the capsules from the Colchester-crossed plants were to those from the intercrossed plants as 100 to 51; so that the former probably contained a somewhat larger average number of seeds.]
We learn from this important experiment that plants in some degree related, which had been intercrossed during the nine previous generations, when they were fertilised with pollen from a fresh stock, yielded seedlings as superior to the seedlings of the tenth intercrossed generation, as these latter were to the self-fertilised plants of the corresponding generation. For if we look to the plants of the ninth generation in Table 2/10 (and these offer in most respects the fairest standard of comparison) we find that the intercrossed plants were in height to the self-fertilised as 100 to 79, and in fertility as 100 to 26; whilst the Colchester-crossed plants are in height to the intercrossed as 100 to 78, and in fertility as 100 to 51.
[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH APPEARED IN THE SIXTH SELF-FERTILISED GENERATION.
In the five generations before the sixth, the crossed plant of each pair was taller than its self-fertilised opponent; but in the sixth generation (Table 2/7, Pot 2) the Hero appeared, which after a long and dubious struggle conquered its crossed opponent, though by only half an inch. I was so much surprised at this fact, that I resolved to ascertain whether this plant would transmit its powers of growth to its seedlings. Several flowers on Hero were therefore fertilised with their own pollen, and the seedlings thus raised were put into competition with self-fertilised and intercrossed plants of the corresponding generation. The three lots of seedlings thus all belong to the seventh generation. Their relative heights are shown in Tables 2/14 and 2/15.
TABLE 2/14. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of Hero.
Column 3: Self-fertilised Plants of the Seventh Generation.
Pot 1 : 74 : 89 4/8. Pot 1 : 60 : 61. Pot 1 : 55 2/8 : 49.
Pot 2 : 92 : 82. Pot 2 : 91 6/8 : 56. Pot 2 : 74 2/8 : 38.
Total : 447.25 : 375.50.
The average height of the six self-fertilised children of Hero is 74.54 inches, whilst that of the ordinary self-fertilised plants of the corresponding generation is only 62.58 inches, or as 100 to 84.
TABLE 2/15. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of Hero.
Column 3: Intercrossed Plants of the Seventh Generation.
Pot 3 : 92 : 76 6/8.
Pot 4 : 87 : 89. Pot 4 : 87 6/8 : 86 6/8.
Total : 266.75 : 252.50.
Here the average height of the three self-fertilised children of Hero is 88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100 to 95. We thus see that the self-fertilised children of Hero certainly inherit the powers of growth of their parents; for they greatly exceed in height the self-fertilised offspring of the other self-fertilised plants, and even exceed by a trifle the intercrossed plants,—all of the corresponding generation.
Several flowers on the self-fertilised children of Hero in Table 2/14 were fertilised with pollen from the same flower; and from the seeds thus produced, self-fertilised plants of the eighth generation (grandchildren of Hero) were raised. Several other flowers on the same plants were crossed with pollen from the other children of Hero. The seedlings raised from this cross may be considered as the offspring of the union of brothers and sisters. The result of the competition between these two sets of seedlings (namely self-fertilised and the offspring of brothers and sisters) is given in Table 2/16.
TABLE 2/16. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Grandchildren of Hero, from the Self-fertilised Children. Eighth Generation.
Column 3: Grandchildren from a cross between the self-fertilised children of Hero. Eighth Generation.
Pot 1 : 86 6/8 : 95 6/8. Pot 1 : 90 3/8 : 95 3/8.
Pot 2 : 96 : 85. Pot 2 : 77 2/8 : 93.
Pot 3 : 73 : 86 2/8. Pot 3 : 66 : 82 2/8. Pot 3 : 84 4/8 : 70 6/8.
Pot 4 : 88 1/8 : 66 3/8. Pot 4 : 84 : 15 4/8. Pot 4 : 36 2/8 : 38. Pot 4 : 74 : 78 3/8.
Pot 5 : 90 1/8 : 82 6/8. Pot 5 : 90 5/8 : 83 6/8.
Total : 1037.00 : 973.16.
The average height of the thirteen self-fertilised grandchildren of Hero is 79.76 inches, and that of the grandchildren from a cross between the self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of the crossed plants grew only to a height of 15 1/2 inches; and if this plant and its opponent are struck out, as would be the fairest plan, the average height of the crossed plants exceeds only by a fraction of an inch that of the self-fertilised plants. It is therefore clear that a cross between the self-fertilised children of Hero did not produce any beneficial effect worth notice; and it is very doubtful whether this negative result can be attributed merely to the fact of brothers and sisters having been united, for the ordinary intercrossed plants of the several successive generations must often have been derived from the union of brothers and sisters (as shown in Chapter 1), and yet all of them were greatly superior to the self-fertilised plants. We are therefore driven to the suspicion, which we shall soon see strengthened, that Hero transmitted to its offspring a peculiar constitution adapted for self-fertilisation.
It would appear that the self-fertilised descendants of Hero have not only inherited from Hero a power of growth equal to that of the ordinary intercrossed plants, but have become more fertile when self-fertilised than is usual with the plants of the present species. The flowers on the self-fertilised grandchildren of Hero in Table 2.16 (the eighth generation of self-fertilised plants) were fertilised with their own pollen and produced plenty of capsules, ten of which (though this is too few a number for a safe average) contained 5.2 seeds per capsule,—a higher average than was observed in any other case with the self-fertilised plants. The anthers produced by these self-fertilised grandchildren were also as well developed and contained as much pollen as those on the intercrossed plants of the corresponding generation; whereas this was not the case with the ordinary self-fertilised plants of the later generations. Nevertheless some few of the flowers produced by the grandchildren of Hero were slightly monstrous, like those of the ordinary self-fertilised plants of the later generations. In order not to recur to the subject of fertility, I may add that twenty-one self-fertilised capsules, spontaneously produced by the great-grandchildren of Hero (forming the ninth generation of self-fertilised plants), contained on an average 4.47 seeds; and this is as high an average as the self-fertilised flowers of any generation usually yielded.
Several flowers on the self-fertilised grandchildren of Hero in Table 2/16 were fertilised with pollen from the same flower; and the seedlings raised from them (great-grandchildren of Hero) formed the ninth self-fertilised generation. Several other flowers were crossed with pollen from another grandchild, so that they may be considered as the offspring of brothers and sisters, and the seedlings thus raised may be called the INTERCROSSED great-grandchildren. And lastly, other flowers were fertilised with pollen from a distinct stock, and the seedlings thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In my anxiety to see what the result would be, I unfortunately planted the three lots of seeds (after they had germinated on sand) in the hothouse in the middle of winter, and in consequence of this the seedlings (twenty in number of each kind) became very unhealthy, some growing only a few inches in height, and very few to their proper height. The result, therefore, cannot be fully trusted; and it would be useless to give the measurements in detail. In order to strike as fair an average as possible, I first excluded all the plants under 50 inches in height, thus rejecting all the most unhealthy plants. The six self-fertilised thus left were on an average 66.86 inches high; the eight intercrossed plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that there was not much difference between the three sets, the self-fertilised plants having a slight advantage. Nor was there any great difference when only the plants under 36 inches in height were excluded. Nor again when all the plants, however much dwarfed and unhealthy, were included. In this latter case the Colchester-crossed gave the lowest average of all; and if these plants had been in any marked manner superior to the other two lots, as from my former experience I fully expected they would have been, I cannot but think that some vestige of such superiority would have been evident, notwithstanding the very unhealthy condition of most of the plants. No advantage, as far as we can judge, was derived from intercrossing two of the grandchildren of Hero, any more than when two of the children were crossed. It appears therefore that Hero and its descendants have varied from the common type, not only in acquiring great power of growth, and increased fertility when subjected to self-fertilisation, but in not profiting from a cross with a distinct stock; and this latter fact, if trustworthy, is a unique case, as far as I have observed in all my experiments.]
A SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS.
In Table 2/17, we see the average or mean heights of the ten successive generations of the intercrossed and self-fertilised plants, grown in competition with each other; and in the right hand column we have the ratios of the one to the other, the height of the intercrossed plants being taken at 100. In the bottom line the mean height of the seventy-three intercrossed plants is shown to be 85.84 inches, and that of the seventy-three self-fertilised plants 66.02 inches, or as 100 to 77.
TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten generations.
Heights of Plants in inches:
Column 1: Name of Generation.
Column 2: Number of Crossed Plants.
Column 3: Average Height of Crossed Plants.
Column 4: Number of Self-fertilised Plants.
Column 5: Average Height of Self-fertilised Plants.
Column 6: n in Ratio between Average Heights of Crossed and Self-fertilised Plants, expressed as 100 to n.
First generation Table 2/1 : 6 : 86.00 : 6 : 65.66 : 76.
Second generation Table 2/2 : 6 : 84.16 : 6 : 66.33 : 79.
Third generation Table 2/3 : 6 : 77.41 : 6 : 52.83 : 68.
Fourth generation Table 2/5 : 7 : 69.78 : 7 : 60.14 : 86.
Fifth generation Table 2/6 : 6 : 82.54 : 6 : 62.33 : 75.
Sixth generation Table 2/7 : 6 : 87.50 : 6 : 63.16 : 72.
Seventh generation Table 2/8 : 9 : 83.94 : 9 : 68.25 : 81.
Eighth generation Table 2/9 : 8 : 113.25 : 8 : 96.65 : 85.
Ninth generation Table 2/10 : 14 : 81.39 : 14 : 64.07 : 79.
Tenth generation Table 2/11 : 5 : 93.70 : 5 : 50.40 : 54.
All ten generations together : 73 : 85.84 : 73 : 66.02 : 77.
(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and self-fertilised plants of Ipomoea purpurea in the ten generations; the mean height of the crossed plants being taken as 100. On the right hand, the mean heights of the crossed and self-fertilised plants of all the generations taken together are shown (as eleven pairs of unequal vertical lines.))
The mean height of the self-fertilised plants in each of the ten generations is also shown in the diagram 2/1, that of the intercrossed plants being taken at 100, and on the right side we see the relative heights of the seventy-three intercrossed plants, and of the seventy-three self-fertilised plants. The difference in height between the crossed and self-fertilised plants will perhaps be best appreciated by an illustration: If all the men in a country were on an average 6 feet high, and there were some families which had been long and closely interbred, these would be almost dwarfs, their average height during ten generations being only 4 feet 8 1/4 inches.
It should be especially observed that the average difference between the crossed and self-fertilised plants is not due to a few of the former having grown to an extraordinary height, or to a few of the self-fertilised being extremely short, but to all the crossed plants having surpassed their self-fertilised opponents, with the few following exceptions. The first occurred in the sixth generation, in which the plant named “Hero” appeared; two in the eighth generation, but the self-fertilised plants in this generation were in an anomalous condition, as they grew at first at an unusual rate and conquered for a time the opposed crossed plants; and two exceptions in the ninth generation, though one of these plants only equalled its crossed opponent. Therefore, of the seventy-three crossed plants, sixty-eight grew to a greater height than the self-fertilised plants, to which they were opposed.
In the right-hand column of figures, the difference in height between the crossed and self-fertilised plants in the successive generations is seen to fluctuate much, as might indeed have been expected from the small number of plants measured in each generation being insufficient to give a fair average. It should be remembered that the absolute height of the plants goes for nothing, as each pair was measured as soon as one of them had twined up to the summit of its rod. The great difference in the tenth generation, namely, 100 to 54, no doubt was partly accidental, though, when these plants were weighed, the difference was even greater, namely, 100 to 44. The smallest amount of difference occurred in the fourth and the eighth generations, and this was apparently due to both the crossed and self-fertilised plants having become unhealthy, which prevented the former attaining their usual degree of superiority. This was an unfortunate circumstance, but my experiments were not thus vitiated, as both lots of plants were exposed to the same conditions, whether favourable or unfavourable.
There is reason to believe that the flowers of this Ipomoea, when growing out of doors, are habitually crossed by insects, so that the first seedlings which I raised from purchased seeds were probably the offspring of a cross. I infer that this is the case, firstly from humble-bees often visiting the flowers, and from the quantity of pollen left by them on the stigmas of such flowers; and, secondly, from the plants raised from the same lot of seed varying greatly in the colour of their flowers, for as we shall hereafter see, this indicates much intercrossing. (2/3. Verlot says ‘Sur la Production des Variétés’ 1865 page 66, that certain varieties of a closely allied plant, the Convolvulus tricolor, cannot be kept pure unless grown at a distance from all other varieties.) It is, therefore, remarkable that the plants raised by me from flowers which were, in all probability, self-fertilised for the first time after many generations of crossing, should have been so markedly inferior in height to the intercrossed plants as they were, namely, as 76 to 100. As the plants which were self-fertilised in each succeeding generation necessarily became much more closely interbred in the later than in the earlier generations, it might have been expected that the difference in height between them and the crossed plants would have gone on increasing; but, so far is this from being the case, that the difference between the two sets of plants in the seventh, eighth, and ninth generations taken together is less than in the first and second generations together. When, however, we remember that the self-fertilised and crossed plants are all descended from the same mother-plant, that many of the crossed plants in each generation were related, often closely related, and that all were exposed to the same conditions, which, as we shall hereafter find, is a very important circumstance, it is not at all surprising that the difference between them should have somewhat decreased in the later generations. It is, on the contrary, an astonishing fact, that the crossed plants should have been victorious, even to a slight degree, over the self-fertilised plants of the later generations.
The much greater constitutional vigour of the crossed than of the self-fertilised plants, was proved on five occasions in various ways; namely, by exposing them, while young, to a low temperature or to a sudden change of temperature, or by growing them, under very unfavourable conditions, in competition with full-grown plants of other kinds.
With respect to the productiveness of the crossed and self-fertilised plants of the successive generations, my observations unfortunately were not made on any uniform plan, partly from the want of time, and partly from not having at first intended to observe more than a single generation. A summary of the results is here given in a tabulated form, the fertility of the crossed plants being taken as 100.
TABLE 2/18. Ratio of productiveness of crossed and self-fertilised plants. Ipomoea purpurea.
FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN COMPETITION WITH ONE ANOTHER.
Sixty-five capsules produced from flowers on five crossed plants fertilised by pollen from a distinct plant, and fifty-five capsules produced from flowers on five self-fertilised plants fertilised by their own pollen, contained seeds in the proportion of : 100 to 93.
Fifty-six spontaneously self-fertilised capsules on the above five crossed plants, and twenty-five spontaneously self-fertilised capsules on the above five self-fertilised plants, yielded seeds in the proportion of : 100 to 99.
Combining the total number of capsules produced by these plants, and the average number of seeds in each, the above crossed and self-fertilised plants yielded seeds in the proportion of : 100 to 64.
Other plants of this first generation grown under unfavourable conditions and spontaneously self-fertilised, yielded seeds in the proportion of : 100 to 45.