The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 7

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 const.i.tution 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.]

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 compet.i.tion 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 ill.u.s.tration: 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 surpa.s.sed 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 circ.u.mstance, 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 quant.i.ty 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 Varietes' 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 circ.u.mstance, 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 astonis.h.i.+ng 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 const.i.tutional 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 compet.i.tion 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 COMPEt.i.tION 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.

THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Crossed capsules compared with self-fertilised capsules contained seeds in the ratio of : 100 to 94.

An equal number of crossed and self-fertilised plants, both spontaneously self-fertilised, produced capsules in the ratio of : 100 to 38.

And these capsules contained seeds in the ratio of : 100 to 94.