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 s.e.xes separated.
Conversion of diclinous into hermaphrodite flowers.
Trees often have their s.e.xes separated.
In the introductory chapter I briefly specified the various means by which cross-fertilisation is favoured or ensured, namely, the separation of the s.e.xes,--the maturity of the male and female s.e.xual 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 s.e.xes 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.
Pa.s.siflora alata, racemosa, coerulea, edulis, laurifolia, and some individuals of P. quadrangularis (Pa.s.sifloraceae), 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 on.o.brychis) 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 quant.i.ty 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.