CHAPTER X. MEANS OF FERTILISATION.
Sterility and fertility of plants when insects are excluded.
The means by which flowers are cross-fertilised.
Structures favourable to self-fertilisation.
Relation between the structure and conspicuousness of flowers, the
visits of insects, and the advantages of cross-fertilisation.
The means by which flowers are fertilised with pollen from a distinct
plant.
Greater fertilising power of such pollen.
Anemophilous species.
Conversion of anemophilous species into entomophilous.
Origin of nectar.
Anemophilous plants generally have their sexes separated.
Conversion of diclinous into hermaphrodite flowers.
Trees often have their sexes separated.
In the introductory chapter I briefly specified the various means by which cross-fertilisation is favoured or ensured, namely, the separation of the sexes,—the maturity of the male and female sexual elements at different periods,—the heterostyled or dimorphic and trimorphic condition of certain plants,—many mechanical contrivances,—the more or less complete inefficiency of a flower’s own pollen on the stigma,—and the prepotency of pollen from any other individual over that from the same plant. Some of these points require further consideration; but for full details I must refer the reader to the several excellent works mentioned in the introduction. I will in the first place give two lists: the first, of plants which are either quite sterile or produce less than about half the full complement of seeds, when insects are excluded; and a second list of plants which, when thus treated, are fully fertile or produce at least half the full complement of seeds. These lists have been compiled from the several previous tables, with some additional cases from my own observations and those of others. The species are arranged nearly in the order followed by Lindley in his ‘Vegetable Kingdom.’ The reader should observe that the sterility or fertility of the plants in these two lists depends on two wholly distinct causes; namely, the absence or presence of the proper means by which pollen is applied to the stigma, and its less or greater efficiency when thus applied. As it is obvious that with plants in which the sexes are separate, pollen must be carried by some means from flower to flower, such species are excluded from the lists; as are likewise dimorphic and trimorphic plants, in which the same necessity occurs to a limited extent. Experience has proved to me that, independently of the exclusion of insects, the seed-bearing power of a plant is not lessened by covering it while in flower under a thin net supported on a frame; and this might indeed have been inferred from the consideration of the two following lists, as they include a considerable number of species belonging to the same genera, some of which are quite sterile and others quite fertile when protected by a net from the access of insects.
[LIST OF PLANTS WHICH, WHEN INSECTS ARE EXCLUDED, ARE EITHER QUITE STERILE, OR PRODUCE, AS FAR AS I COULD JUDGE, LESS THAN HALF THE NUMBER OF SEEDS PRODUCED BY UNPROTECTED PLANTS.
Passiflora alata, racemosa, coerulea, edulis, laurifolia, and some individuals of P. quadrangularis (Passifloraceae), are quite sterile under these conditions: see ‘Variation of Animals and Plants under Domestication’ chapter 17 2nd edition volume 2 page 118.
Viola canina (Violaceae).—Perfect flowers quite sterile unless fertilised by bees, or artificially fertilised.
Viola tricolor.—Sets very few and poor capsules.
Reseda odorata (Resedaceae).—Some individuals quite sterile.
Reseda lutea.—Some individuals produce very few and poor capsules.
Abutilon darwinii (Malvaceae).—Quite sterile in Brazil: see previous discussion on self-sterile plants.
Nymphaea (Nymphaeaceae).—Professor Caspary informs me that some of the species are quite sterile if insects are excluded.
Euryale amazonica (Nymphaeaceae).—Mr. J. Smith, of Kew, informs me that capsules from flowers left to themselves, and probably not visited by insects, contained from eight to fifteen seeds; those from flowers artificially fertilised with pollen from other flowers on the same plant contained from fifteen to thirty seeds; and that two flowers fertilised with pollen brought from another plant at Chatsworth contained respectively sixty and seventy-five seeds. I have given these statements because Professor Caspary advances this plant as a case opposed to the doctrine of the necessity or advantage of cross-fertilisation: see Sitzungsberichte der Phys.-okon. Gesell.zu Konigsberg, B.6 page 20.)
Delphinium consolida (Ranunculaceae).—Produces many capsules, but these contain only about half the number of seeds compared with capsules from flowers naturally fertilised by bees.
Eschscholtzia californica (Papaveraceae).—Brazilian plants quite sterile: English plants produce a few capsules.
Papaver vagum (Papaveraceae).—In the early part of the summer produced very few capsules, and these contained very few seeds.
Papaver alpinum.—H. Hoffmann (‘Speciesfrage’ 1875 page 47) states that this species produced seeds capable of germination only on one occasion.
Corydalis cava (Fumariaceae).—Sterile: see the previous discussion on self-sterile plants.
Corydalis solida.—I had a single plant in my garden (1863), and saw many hive-bees sucking the flowers, but not a single seed was produced. I was much surprised at this fact, as Professor Hildebrand’s discovery that C. cava is sterile with its own pollen had not then been made. He likewise concludes from the few experiments which he made on the present species that it is self-sterile. The two foregoing cases are interesting, because botanists formerly thought (see, for instance, Lecoq, ‘De la Fecondation et de l’Hybridation’ 1845 page 61 and Lindley ‘Vegetable Kingdom’ 1853 page 436) that all the species of the Fumariaceae were specially adapted for self-fertilisation.
Corydalis lutea.—A covered-up plant produced (1861) exactly half as many capsules as an exposed plant of the same size growing close alongside. When humble-bees visit the flowers (and I repeatedly saw them thus acting) the lower petals suddenly spring downwards and the pistil upwards; this is due to the elasticity of the parts, which takes effect, as soon as the coherent edges of the hood are separated by the entrance of an insect. Unless insects visit the flowers the parts do not move. Nevertheless, many of the flowers on the plants which I had protected produced capsules, notwithstanding that their petals and pistils still retained their original position; and I found to my surprise that these capsules contained more seeds than those from flowers, the petals of which had been artificially separated and allowed to spring apart. Thus, nine capsules produced by undisturbed flowers contained fifty-three seeds; whilst nine capsules from flowers, the petals of which had been artificially separated, contained only thirty-two seeds. But we should remember that if bees had been permitted to visit these flowers, they would have visited them at the best time for fertilisation. The flowers, the petals of which had been artificially separated, set their capsules before those which were left undisturbed under the net. To show with what certainty the flowers are visited by bees, I may add that on one occasion all the flowers on some unprotected plants were examined, and every single one had its petals separated; and, on a second occasion, forty-one out of forty-three flowers were in this state. Hildebrand states (Pring. Jahr. f. wiss. Botanik, B. 7 page 450) that the mechanism of the parts in this species is nearly the same as in C. ochroleuca, which he has fully described.
Hypecoum grandiflorum (Fumariaceae).—Highly self-sterile (Hildebrand, ibid.).
Kalmia latifolia (Ericaceae).—Mr. W.J. Beal says (‘American Naturalist’ 1867) that flowers protected from insects wither and drop off, with “most of the anthers still remaining in the pockets.”
Pelargonium zonale (Geraniaceae).—Almost sterile; one plant produced two fruits. It is probable that different varieties would differ in this respect, as some are only feebly dichogamous.
Dianthus caryophyllus (Caryophyllaceae).—Produces very few capsules which contain any good seeds.
Phaseolus multiflorus (Leguminosae).—Plants protected from insects produced on two occasions about one-third and one-eighth of the full number of seeds: see my article in ‘Gardeners’ Chronicle’ 1857 page 225 and 1858 page 828; also ‘Annals and Magazine of Natural History’ 3rd series volume 2 1858 page 462. Dr. Ogle (‘Popular Science Review’ 1870 page 168) found that a plant was quite sterile when covered up. The flowers are not visited by insects in Nicaragua, and, according to Mr. Belt, the species is there quite sterile: ‘The Naturalist in Nicaragua’ page 70.
Vicia faba (Leguminosae).—Seventeen covered-up plants yielded 40 beans, whilst seventeen plants left unprotected and growing close alongside produced 135 beans; these latter plants were, therefore, between three and four times more fertile than the protected plants: see ‘Gardeners’ Chronicle’ for fuller details, 1858 page 828.
Erythrina (sp.?) (Leguminosae).—Sir W. MacArthur informed me that in New South Wales the flowers do not set, unless the petals are moved in the same manner as is done by insects.
Lathyrus grandiflorus (Leguminosae).—Is in this country more or less sterile. It never sets pods unless the flowers are visited by humble-bees (and this happens only rarely), or unless they are artificially fertilised: see my article in ‘Gardeners’ Chronicle’ 1858 page 828.
Sarothamnus scoparius (Leguminosae).—Extremely sterile when the flowers are neither visited by bees, nor disturbed by being beaten by the wind against the surrounding net.
Melilotus officinalis (Leguminosae).—An unprotected plant visited by bees produced at least thirty times more seeds than a protected one. On this latter plant many scores of racemes did not produce a single pod; several racemes produced each one or two pods; five produced three; six produced four; and one produced six pods. On the unprotected plant each of several racemes produced fifteen pods; nine produced between sixteen and twenty-two pods, and one produced thirty pods.
Lotus corniculatus (Leguminosae).—Several covered-up plants produced only two empty pods, and not a single good seed.
Trifolium repens (Leguminosae).—Several plants were protected from insects, and the seeds from ten flowers-heads on these plants, and from ten heads on other plants growing outside the net (which I saw visited by bees), were counted; and the seeds from the latter plants were very nearly ten times as numerous as those from the protected plants. The experiment was repeated on the following year; and twenty protected heads now yielded only a single aborted seed, whilst twenty heads on the plants outside the net (which I saw visited by bees) yielded 2290 seeds, as calculated by weighing all the seed, and counting the number in a weight of two grains.
Trifolium pratense.—One hundred flower-heads on plants protected by a net did not produce a single seed, whilst 100 heads on plants growing outside, which were visited by bees, yielded 68 grains weight of seeds; and as eighty seeds weighed two grains, the 100 heads must have yielded 2720 seeds. I have often watched this plant, and have never seen hive-bees sucking the flowers, except from the outside through holes bitten by humble-bees, or deep down between the flowers, as if in search of some secretion from the calyx, almost in the same manner as described by Mr. Farrer, in the case of Coronilla (‘Nature’ 1874 July 2 page 169). I must, however, except one occasion, when an adjoining field of sainfoin (Hedysarum onobrychis) had just been cut down, and when the bees seemed driven to desperation. On this occasion most of the flowers of the clover were somewhat withered, and contained an extraordinary quantity of nectar, which the bees were able to suck. An experienced apiarian, Mr. Miner, says that in the United States hive-bees never suck the red clover; and Mr. R. Colgate informs me that he has observed the same fact in New Zealand after the introduction of the hive-bee into that island. On the other hand, H. Muller (‘Befruchtung’ page 224) has often seen hive-bees visiting this plant in Germany, for the sake both of pollen and nectar, which latter they obtained by breaking apart the petals. It is at least certain that humble-bees are the chief fertilisers of the common red clover.
Trifolium incarnatum.—The flower-heads containing ripe seeds, on some covered and uncovered plants, appeared equally fine, but this was a false appearance; 60 heads on the latter yielded 349 grains weight of seeds, whereas 60 on the covered-up plants yielded only 63 grains, and many of the seeds in the latter lot were poor and aborted. Therefore the flowers which were visited by bees produced between five and six times as many seeds as those which were protected. The covered-up plants not having been much exhausted by seed-bearing, bore a second considerable crop of flower-stems, whilst the exposed plants did not do so.
Cytisus laburnum (Leguminosae).—Seven flower-racemes ready to expand were enclosed in a large bag made of net, and they did not seem in the least injured by this treatment. Only three of them produced any pods, each a single one; and these three pods contained one, four, and five seeds. So that only a single pod from the seven racemes included a fair complement of seeds.
Cuphea purpurea (Lythraceae).—Produced no seeds. Other flowers on the same plant artificially fertilised under the net yielded seeds.
Vinca major (Apocynaceae).—Is generally quite sterile, but sometimes sets seeds when artificially cross-fertilised: see my notice ‘Gardeners’ Chronicle’ 1861 page 552.
Vinca rosea.—Behaves in the same manner as the last species: ‘Gardeners’ Chronicle’ 1861 page 699, 736, 831.
Tabernaemontana echinata (Apocynaceae).—Quite sterile.
Petunia violacea (Solanaceae).—Quite sterile, as far as I have observed.
Solanum tuberosum (Solanaceae).—Tinzmann says (‘Gardeners’ Chronicle’ 1846 page 183) that some varieties are quite sterile unless fertilised by pollen from another variety.
Primula scotica (Primulaceae).—A non-dimorphic species, which is fertile with its own pollen, but is extremely sterile if insects are excluded. J. Scott in ‘Journal of the Linnean Society Botany’ volume 8 1864 page 119.
Cortusa matthioli (Primulaceae).—Protected plants completely sterile; artificially self-fertilised flowers perfectly fertile. J. Scott ibid. page 84.
Cyclamen persicum (Primulaceae).—During one season several covered-up plants did not produce a single seed.
Borago officinalis (Boraginaceae).—Protected plants produced about half as many seeds as the unprotected.
Salvia tenori (Labiatae).—Quite sterile; but two or three flowers on the summits of three of the spikes, which touched the net when the wind blew, produced a few seeds. This sterility was not due to the injurious effects of the net, for I fertilised five flowers with pollen from an adjoining plant, and these all yielded fine seeds. I removed the net, whilst one little branch still bore a few not completely faded flowers, and these were visited by bees and yielded seeds.
Salvia coccinea.—Some covered-up plants produced a good many fruits, but not, I think, half as many as did the uncovered plants; twenty-eight of the fruits spontaneously produced by the protected plant contained on an average only 1.45 seeds, whilst some artificially self-fertilised fruits on the same plant contained more than twice as many, namely 3.3 seeds.
Bignonia (unnamed species) (Bignoniaceae).—Quite sterile: see my account of self-sterile plants.
Digitalis purpurea (Scrophulariaceae).—Extremely sterile, only a few poor capsules being produced.
Linaria vulgaris (Scrophulariaceae).—Extremely sterile.
Antirrhinum majus, red var. (Scrophulariaceae).—Fifty pods gathered from a large plant under a net contained 9.8 grains weight of seeds; but many (unfortunately not counted) of the fifty pods contained no seeds. Fifty pods on a plant fully exposed to the visits of humble-bees contained 23.1 grains weight of seed, that is, more than twice the weight; but in this case again, several of the fifty pods contained no seeds.
Antirrhinum majus (white var., with a pink mouth to the corolla).—Fifty pods, of which only a very few were empty, on a covered-up plant contained 20 grains weight of seed; so that this variety seems to be much more self-fertile than the previous one. With Dr. W. Ogle (‘Popular Science Review’ January 1870 page 52) a plant of this species was much more sterile when protected from insects than with me, for it produced only two small capsules. As showing the efficiency of bees, I may add that Mr. Crocker castrated some young flowers and left them uncovered; and these produced as many seeds as the unmutilated flowers.
Antirrhinum majus (peloric var.).—This variety is quite fertile when artificially fertilised with its own pollen, but is utterly sterile when left to itself and uncovered, as humble-bees cannot crawl into the narrow tubular flowers.
Verbascum phoeniceum (Scrophulariaceae).—Quite sterile. See my account of self-sterile plants.
Verbascum nigrum.—Quite sterile. See my account of self-sterile plants.
Campanula carpathica (Lobeliaceae).—Quite sterile.
Lobelia ramosa (Lobeliaceae).—Quite sterile.
Lobelia fulgens.—This plant is never visited in my garden by bees, and is quite sterile; but in a nursery-garden at a few miles’ distance I saw humble-bees visiting the flowers, and they produced some capsules.
Isotoma (a white-flowered var.) (Lobeliaceae).—Five plants left unprotected in my greenhouse produced twenty-four fine capsules, containing altogether 12.2 grains weight of seed, and thirteen other very poor capsules, which were rejected. Five plants protected from insects, but otherwise exposed to the same conditions as the above plants, produced sixteen fine capsules, and twenty other very poor and rejected ones. The sixteen fine capsules contained seeds by weight in such proportion that twenty-four would have yielded 4.66 grains. So that the unprotected plants produced nearly thrice as many seeds by weight as the protected plants.
Leschenaultia formosa (Goodeniaceae).—Quite sterile. My experiments on this plant, showing the necessity of insect aid, are given in the ‘Gardeners’ Chronicle’ 1871 page 1166.
Senecio cruentus (Compositae).—Quite sterile: see my account of self-sterile plants.
Heterocentron mexicanum (Malastomaceae).—Quite sterile; but this species and the following members of the group produce plenty of seed when artificially self-fertilised.
Rhexia glandulosa (Melastomaceae).—Set spontaneously only two or three capsules.
Centradenia floribunda (Melastomaceae).—During some years produced spontaneously two or three capsules, sometimes none.
Pleroma (unnamed species from Kew) (Melastomaceae).—During some years produced spontaneously two or three capsules, sometimes none.
Monochaetum ensiferum (Melastomaceae).—During some years produced spontaneously two or three capsules, sometimes none.
Hedychium (unnamed species) (Marantaceae).—Almost self-sterile without aid.
Orchideae.—An immense proportion of the species sterile, if insects are excluded.
PLANTS, WHICH WHEN PROTECTED FROM INSECTS ARE EITHER QUITE FERTILE, OR YIELD MORE THAN HALF THE NUMBER OF SEEDS PRODUCED BY UNPROTECTED PLANTS.
Passiflora gracilis (Passifloraceae).—Produces many fruits, but these contain fewer seeds than fruits from intercrossed flowers.
Brassica oleracea (Cruciferae).—Produces many capsules, but these generally not so rich in seed as those on uncovered plants.
Raphanus sativus (Cruciferae).—Half of a large branching plant was covered by a net, and was as thickly covered with capsules as the other and unprotected half; but twenty of the capsules on the latter contained on an average 3.5 seeds, whilst twenty of the protected capsules contained only 1.85 seeds, that is, only a little more than half the number. This plant might perhaps have been more properly included in the former list.
Iberis umbellata (Cruciferae).—Highly fertile.
Iberis amara.—Highly fertile.
Reseda odorata and lutea (Resedaceae).—Certain individuals completely self-fertile.
Euryale ferox (Nymphaeaceae).—Professor Caspary informs me that this plant is highly self-fertile when insects are excluded. He remarks in the paper before referred to, that his plants (as well as those of the Victoria regia) produce only one flower at a time; and that as this species is an annual, and was introduced in 1809, it must have been self-fertilised for the last fifty-six generations; but Dr. Hooker assures me that to his knowledge it has been repeatedly introduced, and that at Kew the same plant both of the Euryale and of the Victoria produce several flowers at the same time.
Nymphaea (Nymphaeaceae).—Some species, as I am informed by Professor Caspary, are quite self-fertile when insects are excluded.
Adonis aestivalis (Ranunculaceae).—Produces, according to Professor H. Hoffmann (‘Speciesfrage’ page 11), plenty of seeds when protected from insects.
Ranunculus acris (Ranunculaceae).—Produces plenty of seeds under a net.
Papaver somniferum (Papaveraceae).—Thirty capsules from uncovered plants yielded 15.6 grains weight of seed, and thirty capsules from covered-up plants, growing in the same bed, yielded 16.5 grains weight; so that the latter plants were more productive than the uncovered. Professor H. Hoffmann (‘Speciesfrage’ 1875 page 53) also found this species self-fertile when protected from insects.
Papaver vagum.—Produced late in the summer plenty of seeds, which germinated well.
Papaver argemonoides.—According to Hildebrand (‘Jahrbuch fur w. Bot.’ B.7 page 466), spontaneously self-fertilised flowers are by no means sterile.
Glaucium luteum (Papaveraceae).—According to Hildebrand (‘Jahrbuch fur w. Bot.’ B.7 page 466), spontaneously self-fertilised flowers are by no means sterile.
Argemone ochroleuca (Papaveraceae).—According to Hildebrand (‘Jahrbuch fur w. Bot.’ B.7 page 466), spontaneously self-fertilised flowers are by no means sterile.
Adlumia cirrhosa (Fumariaceae).—Sets an abundance of capsules.
Hypecoum procumbens (Fumariaceae).—Hildebrand says (idem), with respect to protected flowers, that “eine gute Fruchtbildung eintrete.”
Fumaria officinalis (Fumariaceae).—Covered-up and unprotected plants apparently produced an equal number of capsules, and the seeds of the former seemed to the eye equally good. I have often watched this plant, and so has Hildebrand, and we have never seen an insect visit the flowers. Hermann Muller has likewise been struck with the rarity of the visits of insects to it, though he has sometimes seen hive-bees at work. The flowers may perhaps be visited by small moths, as is probably the case with the following species.
Fumaria capreolata.—Several large beds of this plant growing wild were watched by me during many days, but the flowers were never visited by any insects, though a humble-bee was once seen closely to inspect them. Nevertheless, as the nectary contains much nectar, especially in the evening, I felt convinced that they were visited, probably by moths. The petals do not naturally separate or open in the least; but they had been opened by some means in a certain proportion of the flowers, in the same manner as follows when a thick bristle is pushed into the nectary; so that in this respect they resemble the flowers of Corydalis lutea. Thirty-four heads, each including many flowers, were examined, and twenty of them had from one to four flowers, whilst fourteen had not a single flower thus opened. It is therefore clear that some of the flowers had been visited by insects, while the majority had not; yet almost all produced capsules.
Linum usitatissimum (Linaceae).—Appears to be quite fertile. H. Hoffmann ‘Botanische Zeitung’ 1876 page 566.
Impatiens barbigerum (Balsaminaceae).—The flowers, though excellently adapted for cross-fertilisation by the bees which freely visit them, set abundantly under a net.
Impatiens noli-me-tangere (Balsaminaceae).—This species produces cleistogene and perfect flowers. A plant was covered with a net, and some perfect flowers, marked with threads, produced eleven spontaneously self-fertilised capsules, which contained on an average 3.45 seeds. I neglected to ascertain the number of seeds produced by perfect flowers exposed to the visits of insects, but I believe it is not greatly in excess of the above average. Mr. A.W. Bennett has carefully described the structure of the flowers of I. fulva in ‘Journal of the Linnean Society’ volume 13 Bot. 1872 page 147. This latter species is said to be sterile with its own pollen (‘Gardeners’ Chronicle’ 1868 page 1286), and if so, it presents a remarkable contrast with I. barbigerum and noli-me-tangere.
Limnanthes douglasii (Geraniaceae).—Highly fertile.
Viscaria oculata (Caryophyllaceae).—Produces plenty of capsules with good seeds.
Stellaria media (Caryophyllaceae).—Covered-up and uncovered plants produced an equal number of capsules, and the seeds in both appeared equally numerous and good.
Beta vulgaris (Chenopodiaceae).—Highly self-fertile.
Vicia sativa (Leguminosae).—Protected and unprotected plants produced an equal number of pods and equally fine seeds. If there was any difference between the two lots, the covered-up plants were the most productive.
Vicia hirsuta.—This species bears the smallest flowers of any British leguminous plant. The result of covering up plants was exactly the same as in the last species.
Pisum sativum (Leguminosae).—Fully fertile.
Lathyrus odoratus (Leguminosae).—Fully fertile.
Lathyrus nissolia.—Fully fertile.
Lupinus luteus (Leguminosae).—Fairly productive.
Lupinus pilosus.—Produced plenty of pods.
Ononis minutissima (Leguminosae).—Twelve perfect flowers on a plant under a net were marked by threads, and produced eight pods, containing on an average 2.38 seeds. Pods produced by flowers visited by insects would probably have contained on an average 3.66 seeds, judging from the effects of artificial cross-fertilisation.
Phaseolus vulgaris (Leguminosae).—Quite fertile.
Trifolium arvense (Leguminosae).—The excessively small flowers are incessantly visited by hive and humble-bees. When insects were excluded the flower-heads seemed to produce as many and as fine seeds as the exposed heads.
Trifolium procumbens.—On one occasion covered-up plants seemed to yield as many seeds as the uncovered. On a second occasion sixty uncovered flower-heads yielded 9.1 grains weight of seeds, whilst sixty heads on protected plants yielded no less than 17.7 grains; so that these latter plants were much more productive; but this result I suppose was accidental. I have often watched this plant, and have never seen the flowers visited by insects; but I suspect that the flowers of this species, and more especially of Trifolium minus, are frequented by small nocturnal moths which, as I hear from Mr. Bond, haunt the smaller clovers.
Medicago lupulina (Leguminosae).—On account of the danger of losing the seeds, I was forced to gather the pods before they were quite ripe; 150 flower-heads on plants visited by bees yielded pods weighing 101 grains; whilst 150 heads on protected plants yielded pods weighing 77 grains. The inequality would probably have been greater if the mature seeds could have been all safely collected and compared. Ig. Urban (Keimung, Bluthen, etc., bei Medicago 1873) has described the means of fertilisation in this genus, as has the Reverend G. Henslow in the ‘Journal of the Linnean Society Botany’ volume 9 1866 pages 327 and 355.
Nicotiana tabacum (Solanaceae).—Fully self-fertile.
Ipomoea purpurea (Convolvulaceae).—Highly self-fertile.
Leptosiphon androsaceus (Polemoniacae).—Plants under a net produced a good many capsules.
Primula mollis (Primulaceae).—A non-dimorphic species, self-fertile: J. Scott, in ‘Journal of the Linnean Society Botany’ volume 8 1864 page 120.
Nolana prostrata (Nolanaceae).—Plants covered up in the greenhouse, yielded seeds by weight compared with uncovered plants, the flowers of which were visited by many bees, in the ratio of 100 to 61.
Ajuga reptans (Labiatae).—Set a good many seeds; but none of the stems under a net produced so many as several uncovered stems growing closely by.
Euphrasia officinalis (Scrophulariaceae).—Covered-up plants produced plenty of seed; whether less than the exposed plants I cannot say. I saw two small Dipterous insects (Dolichopos nigripennis and Empis chioptera) repeatedly sucking the flowers; as they crawled into them, they rubbed against the bristles which project from the anthers, and became dusted with pollen.
Veronica agrestis (Scrophulariaceae).—Covered-up plants produced an abundance of seeds. I do not know whether any insects visit the flowers; but I have observed Syrphidae repeatedly covered with pollen visiting the flowers of V. hederaefolia and chamoedrys.
Mimulus luteus (Scrophulariaceae).—Highly self-fertile.
Calceolaria (greenhouse variety) (Scrophulariaceae).—Highly self-fertile.
Verbascum thapsus (Scrophulariaceae).—Highly self-fertile.
Verbascum lychnitis.—Highly self-fertile.
Vandellia nummularifolia (Scrophulariaceae).—Perfect flowers produce a good many capsules.
Bartsia odontites (Scrophulariaceae).—Covered-up plants produced a good many seeds; but several of these were shrivelled, nor were they so numerous as those produced by unprotected plants, which were incessantly visited by hive and humble-bees.
Specularia speculum (Lobeliaceae).—Covered plants produced almost as many capsules as the uncovered.
Lactuca sativa (Compositae).—Covered plants produced some seeds, but the summer was wet and unfavourable.
Galium aparine (Rubiaceae).—Covered plants produced quite as many seeds as the uncovered.
Apium petroselinum (Umbelliferae).—Covered plants apparently were as productive as the uncovered.
Zea mays (Gramineae).—A single plant in the greenhouse produced a good many grains.
Canna warscewiczi (Marantaceae).—Highly self-fertile.
Orchidaceae.—In Europe Ophrys apifera is as regularly self-fertilised as is any cleistogene flower. In the United States, South Africa, and Australia there are a few species which are perfectly self-fertile. These several cases are given in the second edition of my work on the Fertilisation of Orchids.
Allium cepa (blood red var.) (Liliaceae).—Four flower-heads were covered with a net, and they produced somewhat fewer and smaller capsules than those on the uncovered heads. The capsules were counted on one uncovered head, and were 289 in number; whilst those on a fine head from under the net were only 199.]
Each of these lists contains by a mere accident the same number of genera, namely, forty-nine. The genera in the first list include sixty-five species, and those in the second sixty species; the Orchideae in both being excluded. If the genera in this latter order, as well as in the Asclepiadae and Apocynaceae, had been included, the number of species which are sterile if insects are excluded would have been greatly increased; but the lists are confined to species which were actually experimented on. The results can be considered as only approximately accurate, for fertility is so variable a character, that each species ought to have been tried many times. The above number of species, namely, 125, is as nothing to the host of living plants; but the mere fact of more than half of them being sterile within the specified degree, when insects are excluded, is a striking one; for whenever pollen has to be carried from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertilisation. I do not, however, believe that if all known plants were tried in the same manner, half would be found to be sterile within the specified limits; for many flowers were selected for experiment which presented some remarkable structure; and such flowers often require insect-aid. Thus out of the forty-nine genera in the first list, about thirty-two have flowers which are asymmetrical or present some remarkable peculiarity; whilst in the second list, including species which are fully or moderately fertile when insects were excluded, only about twenty-one out of the forty-nine are asymmetrical or present any remarkable peculiarity.