Fourteen flowers on B were crossed with pollen from A, C or D, and all produced capsules; some of these were not very fine, yet they contained plenty of seeds. Eighteen flowers were fertilised with pollen from other flowers on the same plant, and produced not one capsule.
Ten flowers on C were crossed with pollen from A, B, D or E, and produced nine fine capsules. Nineteen flowers were fertilised with pollen from other flowers on the same plant, and produced no capsules.
Ten flowers on D were crossed with pollen from A, B, C or E, and produced nine fine capsules. Eighteen flowers were fertilised with pollen from other flowers on the same plant, and produced no capsules.
Seven flowers on E were crossed with pollen from A, C, or D, and all produced fine capsules. Eight flowers were fertilised with pollen from other flowers on the same plant, and produced no capsules.
On the plants F and G no flowers were crossed, but very many (number not recorded) were fertilised with pollen from other flowers on the same plants, and these did not produce a single capsule.
We thus see that fifty-five flowers on five of the above plants were reciprocally crossed in various ways; several flowers on each of these plants being fertilised with pollen from several of the other plants.
These fifty-five flowers produced fifty-two capsules, almost all of which were of full size and contained an abundance of seeds. On the other hand, seventy-nine flowers (besides many others not recorded) were fertilised with pollen from other flowers on the same plants, and these did not produce a single capsule. In one case in which I examined the stigmas of the flowers fertilised with their own pollen, these were penetrated by the pollen-tubes, although such penetration produced no effect. Pollen falls generally, and I believe always, from the anthers on the stigmas of the same flower; yet only three out of the above seven protected plants produced spontaneously any capsules, and these it might have been thought must have been self-fertilised. There were altogether seven such capsules; but as they were all seated close to the artificially crossed flowers, I can hardly doubt that a few grains of foreign pollen had accidentally fallen on their stigmas. Besides the above seven plants, four others were kept covered under the SAME large net; and some of these produced here and there in the most capricious manner little groups of capsules; and this makes me believe that a bee, many of which settled on the outside of the net, being attracted by the odour, had on some one occasion found an entrance, and had intercrossed a few of the flowers.
In the spring of 1869 four plants raised from fresh seeds were carefully protected under separate nets; and now the result was widely different to what it was before. Three of these protected plants became actually loaded with capsules, especially during the early part of the summer; and this fact indicates that temperature produces some effect, but the experiment given in the following paragraph shows that the innate const.i.tution of the plant is a far more important element. The fourth plant produced only a few capsules, many of them of small size; yet it was far more self-fertile than any of the seven plants tried during the previous year. The flowers on four small branches of this semi-self-sterile plant were smeared with pollen from one of the other plants, and they all produced fine capsules.
As I was much surprised at the difference in the results of the trials made during the two previous years, six fresh plants were protected by separate nets in the year 1870. Two of these proved almost completely self-sterile, for on carefully searching them I found only three small capsules, each containing either one or two seeds of small size, which, however, germinated. A few flowers on both these plants were reciprocally fertilised with each other"s pollen, and a few with pollen from one of the following self-fertile plants, and all these flowers produced fine capsules. The four other plants whilst still remaining protected beneath the nets presented a wonderful contrast (though one of them in a somewhat less degree than the others), for they became actually covered with spontaneously self-fertilised capsules, as numerous as, or very nearly so, and as fine as those on the unprotected plants growing near.
The above three spontaneously self-fertilised capsules produced by the two almost completely self-sterile plants, contained altogether five seeds; and from these I raised in the following year (1871) five plants, which were kept under separate nets. They grew to an extraordinarily large size, and on August 29th were examined. At first sight they appeared entirely dest.i.tute of capsules; but on carefully searching their many branches, two or three capsules were found on three of the plants, half-a-dozen on the fourth, and about eighteen on the fifth plant. But all these capsules were small, some being empty; the greater number contained only a single seed, and very rarely more than one.
After this examination the nets were taken off, and the bees immediately carried pollen from one of these almost self-sterile plants to the other, for no other plants grew near. After a few weeks the ends of the branches on all five plants became covered with capsules, presenting a curious contrast with the lower and naked parts of the same long branches. These five plants therefore inherited almost exactly the same s.e.xual const.i.tution as their parents; and without doubt a self-sterile race of Mignonette could have been easily established.
Reseda lutea.
Plants of this species were raised from seeds gathered from a group of wild plants growing at no great distance from my garden. After casually observing that some of these plants were self-sterile, two plants taken by hazard were protected under separate nets. One of these soon became covered with spontaneously self-fertilised capsules, as numerous as those on the surrounding unprotected plants; so that it was evidently quite self-fertile. The other plant was partially self-sterile, producing very few capsules, many of which were of small size. When, however, this plant had grown tall, the uppermost branches became pressed against the net and grew crooked, and in this position the bees were able to suck the flowers through the meshes, and brought pollen to them from the neighbouring plants. These branches then became loaded with capsules; the other and lower branches remaining almost bare. The s.e.xual const.i.tution of this species is therefore similar to that of Reseda odorata.
CONCLUDING REMARKS ON SELF-STERILE PLANTS.
In order to favour as far as possible the self-fertilisation of some of the foregoing plants, all the flowers on Reseda odorata and some of those on the Abutilon were fertilised with pollen from other flowers on the same plant, instead of with their own pollen, and in the case of the Senecio with pollen from other flowers on the same corymb; but this made no difference in the result. Fritz Muller tried both kinds of self-fertilisation in the case of Bignonia, Tabernaemontana and Abutilon, likewise with no difference in the result. With Eschscholtzia, however, he found that pollen from other flowers on the same plant was a little more effective than pollen from the same flower. So did Hildebrand in Germany; as thirteen out of fourteen flowers of Eschscholtzia thus fertilised set capsules, these containing on an average 9.5 seeds; whereas only fourteen flowers out of twenty-one fertilised with their own pollen set capsules, these containing on an average 9.0 seeds. (9/11. "Pringsheim"s Jahrbuch fur wiss. Botanik" 7 page 467.) Hildebrand found a trace of a similar difference with Corydalis cava, as did Fritz Muller with an Oncidium. (9/12. "Variation under Domestication" chapter 17 2nd edition volume 2 pages 113-115.)
In considering the several cases above given of complete or almost complete self-sterility, we are first struck with their wide distribution throughout the vegetable kingdom. Their number is not at present large, for they can be discovered only by protecting plants from insects and then fertilising them with pollen from another plant of the same species and with their own pollen; and the latter must be proved to be in an efficient state by other trials. Unless all this be done, it is impossible to know whether their self-sterility may not be due to the male or female reproductive organs, or to both, having been affected by changed conditions of life. As in the course of my experiments I have found three new cases, and as Fritz Muller has observed indications of several others, it is probable that they will hereafter be proved to be far from rare. (9/13. Mr. Wilder, the editor of a horticultural journal in the United States quoted in "Gardeners" Chronicle" 1868 page 1286, states that Lilium auratum, Impatiens pallida and fulva, and Forsythia viridissima, cannot be fertilised with their own pollen.)
As with plants of the same species and parentage, some individuals are self-sterile and others self-fertile, of which fact Reseda odorata offers the most striking instances, it is not at all surprising that species of the same genus differ in this same manner. Thus Verbasc.u.m phoeniceum and nigrum are self-sterile, whilst V. thapsus and lychnitis are quite self-fertile, as I know by trial. There is the same difference between some of the species of Papaver, Corydalis, and of other genera.
Nevertheless, the tendency to self-sterility certainly runs to a certain extent in groups, as we see in the genus Pa.s.siflora, and with the Vandeae amongst Orchids.
Self-sterility differs much in degree in different plants. In those extraordinary cases in which pollen from the same flower acts on the stigma like a poison, it is almost certain that the plants would never yield a single self-fertilised seed. Other plants, like Corydalis cava, occasionally, though very rarely, produce a few self-fertilised seeds. A large number of species, as may be seen in Table 9/F, are less fertile with their own pollen than with that from another plant; and lastly, some species are perfectly self-fertile. Even with the individuals of the same species, as just remarked, some are utterly self-sterile, others moderately so, and some perfectly self-fertile. The cause, whatever it may be, which renders many plants more or less sterile with their own pollen, that is, when they are self-fertilised, must be different, at least to a certain extent, from that which determines the difference in height, vigour, and fertility of the seedlings raised from self-fertilised and crossed seeds; for we have already seen that the two cla.s.ses of cases do not by any means run parallel. This want of parallelism would be intelligible, if it could be shown that self-sterility depended solely on the incapacity of the pollen-tubes to penetrate the stigma of the same flower deeply enough to reach the ovules; whilst the greater or less vigorous growth of the seedlings no doubt depends on the nature of the contents of the pollen-grains and ovules. Now it is certain that with some plants the stigmatic secretion does not properly excite the pollen-grains, so that the tubes are not properly developed, if the pollen is taken from the same flower. This is the case according to Fritz Muller with Eschscholtzia, for he found that the pollen-tubes did not penetrate the stigma deeply; and with the Orchidaceous genus Notylia they failed altogether to penetrate it.
(9/14. "Botanische Zeitung" 1868 pages 114, 115.)
With dimorphic and trimorphic species, an illegitimate union between plants of the same form presents the closest a.n.a.logy with self-fertilisation, whilst a legitimate union closely resembles cross-fertilisation; and here again the lessened fertility or complete sterility of an illegitimate union depends, at least in part, on the incapacity for interaction between the pollen-grains and stigma. Thus with Linum grandiflorum, as I have elsewhere shown, not more than two or three out of hundreds of pollen-grains, either of the long-styled or short-styled form, when placed on the stigma of their own form, emit their tubes, and these do not penetrate deeply; nor does the stigma itself change colour, as occurs when it is legitimately fertilised.
(9/15. "Journal of the Linnean Society Botany" volume 7 1863 pages 73-75.)
On the other hand the difference in innate fertility, as well as in growth between plants raised from crossed and self-fertilised seeds, and the difference in fertility and growth between the legitimate and illegitimate offspring of dimorphic and trimorphic plants, must depend on some incompatibility between the s.e.xual elements contained within the pollen-grains and ovules, as it is through their union that new organisms are developed.
If we now turn to the more immediate cause of self-sterility, we clearly see that in most cases it is determined by the conditions to which the plants have been subjected. Thus Eschscholtzia is completely self-sterile in the hot climate of Brazil, but is perfectly fertile there with the pollen of any other individual. The offspring of Brazilian plants became in England in a single generation partially self-fertile, and still more so in the second generation. Conversely, the offspring of English plants, after growing for two seasons in Brazil, became in the first generation quite self-sterile. Again, Abutilon darwinii, which is self-sterile in its native home of Brazil, became moderately self-fertile in a single generation in an English hothouse. Some other plants are self-sterile during the early part of the year, and later in the season become self-fertile. Pa.s.siflora alata lost its self-sterility when grafted on another species. With Reseda, however, in which some individuals of the same parentage are self-sterile and others are self-fertile, we are forced in our ignorance to speak of the cause as due to spontaneous variability; but we should remember that the progenitors of these plants, either on the male or female side, may have been exposed to somewhat different conditions. The power of the environment thus to affect so readily and in so peculiar a manner the reproductive organs, is a fact which has many important bearings; and I have therefore thought the foregoing details worth giving. For instance, the sterility of many animals and plants under changed conditions of life, such as confinement, evidently comes within the same general principle of the s.e.xual system being easily affected by the environment. It has already been proved, that a cross between plants which have been self-fertilised or intercrossed during several generations, having been kept all the time under closely similar conditions, does not benefit the offspring; and on the other hand, that a cross between plants that have been subjected to different conditions benefits the offspring to an extraordinary degree. We may therefore conclude that some degree of differentiation in the s.e.xual system is necessary for the full fertility of the parent-plants and for the full vigour of their offspring. It seems also probable that with those plants which are capable of complete self-fertilisation, the male and female elements and organs already differ to an extent sufficient to excite their mutual interaction; but that when such plants are taken to another country, and become in consequence self-sterile, their s.e.xual elements and organs are so acted on as to be rendered too uniform for such interaction, like those of a self-fertilised plant long cultivated under the same conditions. Conversely, we may further infer that plants which are self-sterile in their native country, but become self-fertile under changed conditions, have their s.e.xual elements so acted on, that they become sufficiently differentiated for mutual interaction.
We know that self-fertilised seedlings are inferior in many respects to those from a cross; and as with plants in a state of nature pollen from the same flower can hardly fail to be often left by insects or by the wind on the stigma, it seems at first sight highly probable that self-sterility has been gradually acquired through natural selection in order to prevent self-fertilisation. It is no valid objection to this belief that the structure of some flowers, and the dichogamous condition of many others, suffice to prevent the pollen reaching the stigma of the same flower; for we should remember that with most species many flowers expand at the same time, and that pollen from the same plant is equally injurious or nearly so as that from the same flower. Nevertheless, the belief that self-sterility is a quality which has been gradually acquired for the special purpose of preventing self-fertilisation must, I believe, be rejected. In the first place, there is no close correspondence in degree between the sterility of the parent-plants when self-fertilised, and the extent to which their offspring suffer in vigour by this process; and some such correspondence might have been expected if self-sterility had been acquired on account of the injury caused by self-fertilisation. The fact of individuals of the same parentage differing greatly in their degree of self-sterility is likewise opposed to such a belief; unless, indeed, we suppose that certain individuals have been rendered self-sterile to favour intercrossing, whilst other individuals have been rendered self-fertile to ensure the propagation of the species. The fact of self-sterile individuals appearing only occasionally, as in the case of Lobelia, does not countenance this latter view. But the strongest argument against the belief that self-sterility has been acquired to prevent self-fertilisation, is the immediate and powerful effect of changed conditions in either causing or in removing self-sterility. We are not therefore justified in admitting that this peculiar state of the reproductive system has been gradually acquired through natural selection; but we must look at it as an incidental result, dependent on the conditions to which the plants have been subjected, like the ordinary sterility caused in the case of animals by confinement, and in the case of plants by too much manure, heat, etc. I do not, however, wish to maintain that self-sterility may not sometimes be of service to a plant in preventing self-fertilisation; but there are so many other means by which this result might be prevented or rendered difficult, including as we shall see in the next chapter the prepotency of pollen from a distinct individual over a plant"s own pollen, that self-sterility seems an almost superfluous acquirement for this purpose.
Finally, the most interesting point in regard to self-sterile plants is the evidence which they afford of the advantage, or rather of the necessity, of some degree or kind of differentiation in the s.e.xual elements, in order that they should unite and give birth to a new being.
It was ascertained that the five plants of Reseda odorata which were selected by chance, could be perfectly fertilised by pollen taken from any one of them, but not by their own pollen; and a few additional trials were made with some other individuals, which I have not thought worth recording. So again, Hildebrand and Fritz Muller frequently speak of self-sterile plants being fertile with the pollen of any other individual; and if there had been any exceptions to the rule, these could hardly have escaped their observation and my own. We may therefore confidently a.s.sert that a self-sterile plant can be fertilised by the pollen of any one out of a thousand or ten thousand individuals of the same species, but not by its own. Now it is obviously impossible that the s.e.xual organs and elements of every individual can have been specialised with respect to every other individual. But there is no difficulty in believing that the s.e.xual elements of each differ slightly in the same diversified manner as do their external characters; and it has often been remarked that no two individuals are absolutely alike.
Therefore we can hardly avoid the conclusion, that differences of an a.n.a.logous and indefinite nature in the reproductive system are sufficient to excite the mutual action of the s.e.xual elements, and that unless there be such differentiation fertility fails.
THE APPEARANCE OF HIGHLY SELF-FERTILE VARIETIES.
We have just seen that the degree to which flowers are capable of being fertilised with their own pollen differs much, both with the species of the same genus, and sometimes with the individuals of the same species.
Some allied cases of the appearance of varieties which, when self-fertilised, yield more seed and produce offspring growing taller than their self-fertilised parents, or than the intercrossed plants of the corresponding generation, will now be considered.
Firstly, in the third and fourth generations of Mimulus luteus, a tall variety, often alluded to, having large white flowers blotched with crimson, appeared amongst both the intercrossed and self-fertilised plants. It prevailed in all the later self-fertilised generations to the exclusion of every other variety, and transmitted its characters faithfully, but disappeared from the intercrossed plants, owing no doubt to their characters being repeatedly blended by crossing. The self-fertilised plants belonging to this variety were not only taller, but more fertile than the intercrossed plants; though these latter in the earlier generations were much taller and more fertile than the self-fertilised plants. Thus in the fifth generation the self-fertilised plants were to the intercrossed in height as 126 to 100. In the sixth generation they were likewise much taller and finer plants, but were not actually measured; they produced capsules compared with those on the intercrossed plants, in number, as 147 to 100; and the self-fertilised capsules contained a greater number of seeds. In the seventh generation the self-fertilised plants were to the crossed in height as 137 to 100; and twenty flowers on these self-fertilised plants fertilised with their own pollen yielded nineteen very fine capsules,--a degree of self-sterility which I have not seen equalled in any other case. This variety seems to have become specially adapted to profit in every way by self-fertilisation, although this process was so injurious to the parent-plants during the first four generations. It should however be remembered that seedlings raised from this variety, when crossed by a fresh stock, were wonderfully superior in height and fertility to the self-fertilised plants of the corresponding generation.
Secondly, in the sixth self-fertilised generation of Ipomoea a single plant named the Hero appeared, which exceeded by a little in height its intercrossed opponent,--a case which had not occurred in any previous generation. Hero transmitted the peculiar colour of its flowers, as well as its increased tallness and a high degree of self-fertility, to its children, grandchildren, and great-grandchildren. The self-fertilised children of Hero were in height to other self-fertilised plants of the same stock as 100 to 85. Ten self-fertilised capsules produced by the grandchildren contained on an average 5.2 seeds; and this is a higher average than was yielded in any other generation by the capsules of self-fertilised flowers. The great-grandchildren of Hero derived from a cross with a fresh stock were so unhealthy, from having been grown at an unfavourable season, that their average height in comparison with that of the self-fertilised plants cannot be judged of with any safety; but it did not appear that they had profited even by a cross of this kind.
Thirdly, the plants of Nicotiana on which I experimented appear to come under the present cla.s.s of cases; for they varied in their s.e.xual const.i.tution and were more or less highly self-fertile. They were probably the offspring of plants which had been spontaneously self-fertilised under gla.s.s for several generations in this country. The flowers on the parent-plants which were first fertilised by me with their own pollen yielded half again as many seeds as did those which were crossed; and the seedlings raised from these self-fertilised seeds exceeded in height those raised from the crossed seeds to an extraordinary degree. In the second and third generations, although the self-fertilised plants did not exceed the crossed in height, yet their self-fertilised flowers yielded on two occasions considerably more seeds than the crossed flowers, even than those which were crossed with pollen from a distinct stock or variety.
Lastly, as certain individual plants of Reseda odorata and lutea are incomparably more self-fertile than other individuals, the former might be included under the present heading of the appearance of new and highly self-fertile varieties. But in this case we should have to look at these two species as normally self-sterile; and this, judging by my experience, appears to be the correct view.
We may therefore conclude from the facts now given, that varieties sometimes arise which when self-fertilised possess an increased power of producing seeds and of growing to a greater height, than the intercrossed or self-fertilised plants of the corresponding generation--all the plants being of course subjected to the same conditions. The appearance of such varieties is interesting, as it bears on the existence under nature of plants which regularly fertilise themselves, such as Ophrys apifera and a few other orchids, or as Leersia oryzoides, which produces an abundance of cleistogene flowers, but most rarely flowers capable of cross-fertilisation.
Some observations made on other plants lead me to suspect that self-fertilisation is in some respects beneficial; although the benefit thus derived is as a rule very small compared with that from a cross with a distinct plant. Thus we have seen in the last chapter that seedlings of Ipomoea and Mimulus raised from flowers fertilised with their own pollen, which is the strictest possible form of self-fertilisation, were superior in height, weight, and in early flowering to the seedlings raised from flowers crossed with pollen from other flowers on the same plant; and this superiority apparently was too strongly marked to be accidental. Again, the cultivated varieties of the common pea are highly self-fertile, although they have been self-fertilised for many generations; and they exceeded in height seedlings from a cross between two plants belonging to the same variety in the ratio of 115 to 100; but then only four pairs of plants were measured and compared. The self-fertility of Primula veris increased after several generations of illegitimate fertilisation, which is a process closely a.n.a.logous to self-fertilisation, but only as long as the plants were cultivated under the same favourable conditions. I have also elsewhere shown that with Primula veris and sinensis, equal-styled varieties occasionally appear which possess the s.e.xual organs of the two forms combined in the same flower. (9/16. "Journal of the Linnean Society Botany" volume 10 1867 pages 417, 419.) Consequently they fertilise themselves in a legitimate manner and are highly self-fertile; but the remarkable fact is that they are rather more fertile than ordinary plants of the same species legitimately fertilised by pollen from a distinct individual. Formerly it appeared to me probable, that the increased fertility of these dimorphic plants might be accounted for by the stigma lying so close to the anthers that it was impregnated at the most favourable age and time of the day; but this explanation is not applicable to the above given cases, in which the flowers were artificially fertilised with their own pollen.
Considering the facts now adduced, including the appearance of those varieties which are more fertile and taller than their parents and than the intercrossed plants of the corresponding generation, it is difficult to avoid the suspicion that self-fertilisation is in some respects advantageous; though if this be really the case, any such advantage is as a rule quite insignificant compared with that from a cross with a distinct plant, and especially with one of a fresh stock. Should this suspicion be hereafter verified, it would throw light, as we shall see in the next chapter, on the existence of plants bearing small and inconspicuous flowers which are rarely visited by insects, and therefore are rarely intercrossed.
RELATIVE WEIGHT AND PERIOD OF GERMINATION OF SEEDS FROM CROSSED AND SELF-FERTILISED FLOWERS.
An equal number of seeds from flowers fertilised with pollen from another plant, and from flowers fertilised with their own pollen, were weighed, but only in sixteen cases. Their relative weights are given in the following list; that of the seeds from the crossed flowers being taken as 100.
Column 1: Name of Plant.
Column 2: x, in the expression, 100 to x.
Ipomoea purpurea (parent plants): 127.
Ipomoea purpurea (third generation): 87.
Salvia coccinea: 100.
Bra.s.sica oleracea: 103.
Iberis umbellata (second generation): 136.
Delphinium consolida: 45.
Hibiscus africa.n.u.s: 105.
Tropaeolum minus: 115.
Lathyrus odoratus (about): 100.
Sarothamnus scoparius: 88.
Specularia speculum: 86.
Nemophila insignis: 105.
Borago officinalis: 111.
Cyclamen persic.u.m (about): 50.
f.a.gopyrum esculentum: 82.
Canna warscewiczi (3 generations): 102.
It is remarkable that in ten out of these sixteen cases the self-fertilised seeds were either superior or equal to the crossed in weight; nevertheless, in six out of the ten cases (namely, with Ipomoea, Salvia, Bra.s.sica, Tropaeolum, Lathyrus, and Nemophila) the plants raised from these self-fertilised seeds were very inferior in height and in other respects to those raised from the crossed seeds. The superiority in weight of the self-fertilised seeds in at least six out of the ten cases, namely, with Bra.s.sica, Hibiscus, Tropaeolum, Nemophila, Borago, and Canna, may be accounted for in part by the self-fertilised capsules containing fewer seeds; for when a capsule contains only a few seeds, these will be apt to be better nourished, so as to be heavier, than when many are contained in the same capsule. It should, however, be observed that in some of the above cases, in which the crossed seeds were the heaviest, as with Sarothamnus and Cyclamen, the crossed capsules contained a larger number of seeds. Whatever may be the explanation of the self-fertilised seeds being often the heaviest, it is remarkable in the case of Bra.s.sica, Tropaeolum, Nemophila, and of the first generation of Ipomoea, that the seedlings raised from them were inferior in height and in other respects to the seedlings raised from the crossed seeds.
This fact shows how superior in const.i.tutional vigour the crossed seedlings must have been, for it cannot be doubted that heavy and fine seeds tend to yield the finest plants. Mr. Galton has shown that this holds good with Lathyrus odoratus; as has Mr. A.J. Wilson with the Swedish turnip, Bra.s.sica campestris ruta baga. Mr. Wilson separated the largest and smallest seeds of this latter plant, the ratio between the weights of the two lots being as 100 to 59, and he found that the seedlings "from the larger seeds took the lead and maintained their superiority to the last, both in height and thickness of stem." (9/17.
"Gardeners" Chronicle" 1867 page 107. Loiseleur-Deslongchamp "Les Cereales" 1842 pages 208-219, was led by his observations to the extraordinary conclusion that the smaller grains of cereals produce as fine plants as the large. This conclusion is, however, contradicted by Major Hallet"s great success in improving wheat by the selection of the finest grains. It is possible, however, that man, by long-continued selection, may have given to the grains of the cereals a greater amount of starch or other matter, than the seedlings can utilise for their growth. There can be little doubt, as Humboldt long ago remarked, that the grains of cereals have been rendered attractive to birds in a degree which is highly injurious to the species.) Nor can this difference in the growth of the seedling turnips be attributed to the heavier seeds having been of crossed, and the lighter of self-fertilised origin, for it is known that plants belonging to this genus are habitually intercrossed by insects.
With respect to the relative period of germination of crossed and self-fertilised seeds, a record was kept in only twenty-one cases; and the results are very perplexing. Neglecting one case in which the two lots germinated simultaneously, in ten cases or exactly one-half many of the self-fertilised seeds germinated before the crossed, and in the other half many of the crossed before the self-fertilised. In four out of these twenty cases, seeds derived from a cross with a fresh stock were compared with self-fertilised seeds from one of the later self-fertilised generations; and here again in half the cases the crossed seeds, and in the other half the self-fertilised seeds, germinated first. Yet the seedlings of Mimulus raised from such self-fertilised seeds were inferior in all respects to the crossed seedlings, and in the case of Eschscholtzia they were inferior in fertility. Unfortunately the relative weight of the two lots of seeds was ascertained in only a few instances in which their germination was observed; but with Ipomoea and I believe with some of the other species, the relative lightness of the self-fertilised seeds apparently determined their early germination, probably owing to the smaller ma.s.s being favourable to the more rapid completion of the chemical and morphological changes necessary for germination. On the other hand, Mr.
Galton gave me seeds (no doubt all self-fertilised) of Lathyrus odoratus, which were divided into two lots of heavier and lighter seeds; and several of the former germinated first. It is evident that many more observations are necessary before anything can be decided with respect to the relative period of germination of crossed and self-fertilised seeds.