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 proc.u.mbens.--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 tabac.u.m (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 (l.a.b.i.atae).--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.
Verbasc.u.m thapsus (Scrophulariaceae).--Highly self-fertile.
Verbasc.u.m lychnitis.--Highly self-fertile.
Vandellia nummularifolia (Scrophulariaceae).--Perfect flowers produce a good many capsules.
Bartsia odont.i.tes (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.
MEANS OF CROSS-FERTILISATION.
The most important of all the means by which pollen is carried from the anthers to the stigma of the same flower, or from flower to flower, are insects, belonging to the orders of Hymenoptera, Lepidoptera, and Diptera; and in some parts of the world, birds. (10/1. I will here give all the cases known to me of birds fertilising flowers. In South Brazil, humming-birds certainly fertilise the various species of Abutilon, which are sterile without their aid (Fritz Muller "Jenaische Zeitschrift f.
Naturwiss." B. 7 1872 page 24.) Long-beaked humming-birds visit the flowers of Brugmansia, whilst some of the short-beaked species often penetrate its large corolla in order to obtain the nectar in an illegitimate manner, in the same manner as do bees in all parts of the world. It appears, indeed, that the beaks of humming-birds are specially adapted to the various kinds of flowers which they visit: on the Cordillera they suck the Salviae, and lacerate the flowers of the Tacsoniae; in Nicaragua, Mr. Belt saw them sucking the flowers of Marcgravia and Erythina, and thus they carried pollen from flower to flower. In North America they are said to frequent the flowers of Impatiens: (Gould "Introduction to the Trochilidae" 1861 pages 15, 120; "Gardeners" Chronicle" 1869 page 389; "The Naturalist in Nicaragua" page 129; "Journal of the Linnean Society Botany" volume 13 1872 page 151.) I may add that I often saw in Chile a Mimus with its head yellow with pollen from, as I believe, a Ca.s.sia. I have been a.s.sured that at the Cape of Good Hope, Strelitzia is fertilised by the Nectarinidae. There can hardly be a doubt that many Australian flowers are fertilised by the many honey-sucking birds of that country. Mr. Wallace remarks (address to the Biological Section, British a.s.sociation 1876) that he has "often observed the beaks and faces of the brush-tongued lories of the Moluccas covered with pollen." In New Zealand, many specimens of the Anthornis melanura had their heads coloured with pollen from the flowers of an endemic species of Fuchsia (Potts "Transactions of the New Zealand Inst.i.tute" volume 3 1870 page 72.) Next in importance, but in a quite subordinate degree, is the wind; and with some aquatic plants, according to Delpino, currents of water. The simple fact of the necessity in many cases of extraneous aid for the transport of the pollen, and the many contrivances for this purpose, render it highly probable that some great benefit is thus gained; and this conclusion has now been firmly established by the proved superiority in growth, vigour, and fertility of plants of crossed parentage over those of self-fertilised parentage.
But we should always keep in mind that two somewhat opposed ends have to be gained; the first and more important one being the production of seeds by any means, and the second, cross-fertilisation.