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.
LIST OF 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.
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 Cassia. I have been assured 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 Association 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
Institute’ 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.
The advantages derived from cross-fertilisation throw a flood of light
on most of the chief characters of flowers. We can thus understand their
large size and bright colours, and in some cases the bright tints of the
adjoining parts, such as the peduncles, bracteae, etc. By this means
they are rendered conspicuous to insects, on the same principle that
almost every fruit which is devoured by birds presents a strong contrast
in colour with the green foliage, in order that it may be seen, and its
seeds freely disseminated. With some flowers conspicuousness is gained
at the expense even of the reproductive organs, as with the ray-florets
of many Compositae, the exterior flowers of Hydrangea, and the terminal
flowers of the Feather-hyacinth or Muscari. There is also reason to
believe, and this was the opinion of Sprengel, that flowers differ in
colour in accordance with the kinds of insects which frequent them.
Not only do the bright colours of flowers serve to attract insects, but
dark-coloured streaks and marks are often present, which Sprengel long
ago maintained served as guides to the nectary. These marks follow the
veins in the petals, or lie between them. They may occur on only one, or
on all excepting one or more of the upper or lower petals; or they may
form a dark ring round the tubular part of the corolla, or be confined
to the lips of an irregular flower. In the white varieties of many
flowers, such as of Digitalis purpurea, Antirrhinum majus, several
species of Dianthus, Phlox, Myosotis, Rhododendron, Pelargonium, Primula
and Petunia, the marks generally persist, whilst the rest of the corolla
has become of a pure white; but this may be due merely to their colour
being more intense and thus less readily obliterated. Sprengel’s notion
of the use of these marks as guides appeared to me for a long time
fanciful; for insects, without such aid, readily discover and bite holes
through the nectary from the outside. They also discover the minute
nectar-secreting glands on the stipules and leaves of certain plants.
Moreover, some few plants, such as certain poppies, which are not
nectariferous, have guiding marks; but we might perhaps expect that some
few plants would retain traces of a former nectariferous condition. On
the other hand, these marks are much more common on asymmetrical
flowers, the entrance into which would be apt to puzzle insects, than on
regular flowers. Sir J. Lubbock has also proved that bees readily
distinguish colours, and that they lose much time if the position of
honey which they have once visited be in the least changed. (10/2.
‘British Wild Flowers in relation to Insects’ 1875 page 44.) The
following case affords, I think, the best evidence that these marks have
really been developed in correlation with the nectary. The two upper
petals of the common Pelargonium are thus marked near their bases; and I
have repeatedly observed that when the flowers vary so as to become
peloric or regular, they lose their nectaries and at the same time the
dark marks. When the nectary is only partially aborted, only one of the
upper petals loses its mark. Therefore the nectary and these marks
clearly stand in some sort of close relation to one another; and the
simplest view is that they were developed together for a special
purpose; the only conceivable one being that the marks serve as a guide
to the nectary. It is, however, evident from what has been already said,
that insects could discover the nectar without the aid of guiding marks.
They are of service to the plant, only by aiding insects to visit and
suck a greater number of flowers within a given time than would
otherwise be possible; and thus there will be a better chance of
fertilisation by pollen brought from a distinct plant, and this we know
is of paramount importance.
The odours emitted by flowers attract insects, as I have observed in the
case of plants covered by a muslin net. Nageli affixed artificial
flowers to branches, scenting some with essential oils and leaving
others unscented; and insects were attracted to the former in an
unmistakable manner. (10/3. ‘Enstehung etc. der Naturhist. Art.’ 1865
page 23.) Not a few flowers are both conspicuous and odoriferous. Of all
colours, white is the prevailing one; and of white flowers a
considerably larger proportion smell sweetly than of any other colour,
namely, 14.6 per cent; of red, only 8.2 per cent are odoriferous. (10/4.
The colours and odours of the flowers of 4200 species have been
tabulated by Landgrabe and by Schubler and Kohler. I have not seen their
original works, but a very full abstract is given in Loudon’s
‘Gardeners’ Magazine’ volume 13 1837 page 367.) The fact of a larger
proportion of white flowers smelling sweetly may depend in part on those
which are fertilised by moths requiring the double aid of
conspicuousness in the dusk and of odour. So great is the economy of
nature, that most flowers which are fertilised by crepuscular or
nocturnal insects emit their odour chiefly or exclusively in the
evening. Some flowers, however, which are highly odoriferous depend
solely on this quality for their fertilisation, such as the
night-flowering stock (Hesperis) and some species of Daphne; and these
present the rare case of flowers which are fertilised by insects being
obscurely coloured.
The storage of a supply of nectar in a protected place is manifestly
connected with the visits of insects. So is the position which the
stamens and pistils occupy, either permanently or at the proper period
through their own movements; for when mature they invariably stand in
the pathway leading to the nectary. The shape of the nectary and of the
adjoining parts are likewise related to the particular kinds of insects
which habitually visit the flowers; this has been well shown by Hermann
Muller by his comparison of lowland species which are chiefly visited by
bees, with alpine species belonging to the same genera which are visited
by butterflies. (10/5. ‘Nature’ 1874 page 110, 1875 page 190, 1876 pages
210, 289.) Flowers may also be adapted to certain kinds of insects, by
secreting nectar particularly attractive to them, and unattractive to
other kinds; of which fact Epipactis latifolia offers the most striking
instance known to me, as it is visited exclusively by wasps. Structures
also exist, such as the hairs within the corolla of the fox glove
(Digitalis), which apparently serve to exclude insects that are not well
fitted to bring pollen from one flower to another. (10/6. Belt ‘The
Naturalist in Nicaragua’ 1874 page 132.) I need say nothing here of the
endless contrivances, such as the viscid glands attached to the
pollen-masses of the Orchideae and Asclepiadae, or the viscid or
roughened state of the pollen-grains of many plants, or the irritability
of their stamens which move when touched by insects etc.--as all these
contrivances evidently favour or ensure cross-fertilisation.
All ordinary flowers are so far open that insects can force an entrance
into them, notwithstanding that some, like the Snapdragon (Antirrhinum),
various Papilionaceous and Fumariaceous flowers, are in appearance
closed. It cannot be maintained that their openness is necessary for
fertility, as cleistogene flowers which are permanently closed yield a
full complement of seeds. Pollen contains much nitrogen and
phosphorus--the two most precious of all the elements for the growth of
plants--but in the case of most open flowers, a large quantity of pollen
is consumed by pollen-devouring insects, and a large quantity is
destroyed during long-continued rain. With many plants this latter evil
is guarded against, as far as is possible, by the anthers opening only
during dry weather (10/7. Mr. Blackley observed that the ripe anthers of
rye did not dehisce whilst kept under a bell-glass in a damp atmosphere,
whilst other anthers exposed to the same temperature in the open air
dehisced freely. He also found much more pollen adhering to the sticky
slides, which were attached to kites and sent high up in the atmosphere,
during the first fine and dry days after wet weather, than at other
times: ‘Experimental Researches on Hay Fever’ 1873 page 127.)--by the
position and form of some or all of the petals,--by the presence of
hairs, etc., and as Kerner has shown in his interesting essay, by the
movements of the petals or of the whole flower during cold and wet
weather. (10/8. ‘Die Schutzmittel des Pollens’ 1873.) In order to
compensate the loss of pollen in so many ways, the anthers produce a far
larger amount than is necessary for the fertilisation of the same
flower. I know this from my own experiments on Ipomoea, given in the
Introduction; and it is still more plainly shown by the astonishingly
small quantity produced by cleistogene flowers, which lose none of their
pollen, in comparison with that produced by the open flowers borne by
the same plants; and yet this small quantity suffices for the
fertilisation of all their numerous seeds. Mr. Hassall took pains in
estimating the number of pollen-grains produced by a flower of the
Dandelion (Leontodon), and found the number to be 243,600, and in a
Paeony 3,654,000 grains. (10/9. ‘Annals and Magazine of Natural History’
volume 8 1842 page 108.) The editor of the ‘Botanical Register’ counted
the ovules in the flowers of Wistaria sinensis, and carefully estimated
the number of pollen-grains, and he found that for each ovule there were
7000 grains. (10/10. Quoted in ‘Gardeners’ Chronicle’ 1846 page 771.)
With Mirabilis, three or four of the very large pollen-grains are
sufficient to fertilise an ovule; but I do not know how many grains a
flower produces. With Hibiscus, Kolreuter found that sixty grains were
necessary to fertilise all the ovules of a flower, and he calculated
that 4863 grains were produced by a single flower, or eighty-one times
too many. With Geum urbanum, however, according to Gartner, the pollen
is only ten times too much. (10/11. Kolreuter ‘Vorlaufige Nachricht’
1761 page 9. Gartner ‘Beitrage zur Kenntniss’ etc. page 346.) As we thus
see that the open state of all ordinary flowers, and the consequent loss
of much pollen, necessitate the development of so prodigious an excess
of this precious substance, why, it may be asked, are flowers always
left open? As many plants exist throughout the vegetable kingdom which
bear cleistogene flowers, there can hardly be a doubt that all open
flowers might easily have been converted into closed ones. The graduated
steps by which this process could have been effected may be seen at the
present time in Lathyrus nissolia, Biophytum sensitivum, and several
other plants. The answer to the above question obviously is, that with
permanently closed flowers there could be no cross-fertilisation.
The frequency, almost regularity, with which pollen is transported by
insects from flower to flower, often from a considerable distance, well
deserves attention. (10/12. An experiment made by Kolreuter ‘Forsetsung’
etc. 1763 page 69, affords good evidence on this head. Hibiscus
vesicarius is strongly dichogamous, its pollen being shed before the
stigmas are mature. Kolreuter marked 310 flowers, and put pollen from
other flowers on their stigmas every day, so that they were thoroughly
fertilised; and he left the same number of other flowers to the agency
of insects. Afterwards he counted the seeds of both lots: the flowers
which he had fertilised with such astonishing care produced 11,237
seeds, whilst those left to the insects produced 10,886; that is, a less
number by only 351; and this small inferiority is fully accounted for by
the insects not having worked during some days, when the weather was
cold with continued rain.) This is best shown by the impossibility in
many cases of raising two varieties of the same species pure, if they
grow at all near together; but to this subject I shall presently return;
also by the many cases of hybrids which have appeared spontaneously both
in gardens and a state of nature. With respect to the distance from
which pollen is often brought, no one who has had any experience would
expect to obtain pure cabbage-seed, for instance, if a plant of another
variety grew within two or three hundred yards. An accurate observer,
the late Mr. Masters of Canterbury, assured me that he once had his
whole stock of seeds “seriously affected with purple bastards,” by some
plants of purple kale which flowered in a cottager’s garden at the
distance of half a mile; no other plant of this variety growing any
nearer. (10/13. Mr. W.C. Marshall caught no less than seven specimens of
a moth (Cucullia umbratica) with the pollinia of the butterfly-orchis
(Habenaria chlorantha) sticking to their eyes, and, therefore, in the
proper position for fertilising the flowers of this species, on an
island in Derwentwater, at the distance of half a mile from any place
where this plant grew: ‘Nature’ 1872 page 393.) But the most striking
case which has been recorded is that by M. Godron, who shows by the
nature of the hybrids produced that Primula grandiflora must have been
crossed with pollen brought by bees from P. officinalis, growing at the
distance of above two kilometres, or of about one English mile and a
quarter. (10/14. ‘Revue des Sc. Nat.’ 1875 page 331.)
All those who have long attended to hybridisation, insist in the
strongest terms on the liability of castrated flowers to be fertilised
by pollen brought from distant plants of the same species. (10/15. See,
for instance, the remarks by Herbert ‘Amaryllidaceae’ 1837 page 349.
Also Gartner’s strong expressions on this subject in his
‘Bastarderzeugung’ 1849 page 670 and ‘Kenntniss der Befruchtung’ 1844
pages 510, 573. Also Lecoq ‘De la Fecondation’ etc. 1845 page 27. Some
statements have been published during late years of the extraordinary
tendency of hybrid plants to revert to their parent forms; but as it is
not said how the flowers were protected from insects, it may be
suspected that they were often fertilised with pollen brought from a
distance from the parent-species.) The following case shows this in the
clearest manner: Gartner, before he had gained much experience,
castrated and fertilised 520 flowers on various species with pollen of
other genera or other species, but left them unprotected; for, as he
says, he thought it a laughable idea that pollen should be brought from
flowers of the same species, none of which grew nearer than between 500
and 600 yards. (10/16. ‘Kenntniss der Befruchtung’ pages 539, 550, 575,
576.) The result was that 289 of these 520 flowers yielded no seed, or
none that germinated; the seed of 29 flowers produced hybrids, such as
might have been expected from the nature of the pollen employed; and
lastly, the seed of the remaining 202 flowers produced perfectly pure
plants, so that these flowers must have been fertilised by pollen
brought by insects from a distance of between 500 and 600 yards. (10/17.
Henschel’s experiments quoted by Gartner ‘Kenntniss’ etc. page 574,
which are worthless in all other respects, likewise show how largely
flowers are intercrossed by insects. He castrated many flowers on
thirty-seven species, belonging to twenty-two genera, and put on their
stigmas either no pollen, or pollen from distinct genera, yet they all
seeded, and all the seedlings raised from them were of course pure.) It
is of course possible that some of these 202 flowers might have been
fertilised by pollen left accidentally in them when they were castrated;
but to show how improbable this is, I may add that Gartner, during the
next eighteen years, castrated no less than 8042 flowers and hybridised
them in a closed room; and the seeds from only seventy of these, that is
considerably less than 1 per cent, produced pure or unhybridised
offspring. (10/18. ‘Kenntniss’ etc. pages 555, 576.)
From the various facts now given, it is evident that most flowers are
adapted in an admirable manner for cross-fertilisation. Nevertheless,
the greater number likewise present structures which are manifestly
adapted, though not in so striking a manner, for self-fertilisation. The
chief of these is their hermaphrodite condition; that is, their
including within the same corolla both the male and female reproductive
organs. These often stand close together and are mature at the same
time; so that pollen from the same flower cannot fail to be deposited at
the proper period on the stigma. There are also various details of
structure adapted for self-fertilisation. (10/19. Hermann Muller ‘Die
Befruchtung’ etc. page 448.) Such structures are best shown in those
curious cases discovered by Hermann Muller, in which a species exists
under two forms,--one bearing conspicuous flowers fitted for
cross-fertilisation, and the other smaller flowers fitted for
self-fertilisation, with many parts in the latter slightly modified for
this special purpose. (10/20. ‘Nature’ 1873 pages 44, 433.)
As two objects in most respects opposed, namely, cross-fertilisation and
self-fertilisation, have in many cases to be gained, we can understand
the co-existence in so many flowers of structures which appear at first
sight unnecessarily complex and of an opposed nature. We can thus
understand the great contrast in structure between cleistogene flowers,
which are adapted exclusively for self-fertilisation, and ordinary
flowers on the same plant, which are adapted so as to allow of at least
occasional cross-fertilisation. (10/21. Fritz Muller has discovered in
the animal kingdom ‘Jenaische Zeitschr.’ B. 4 page 451, a case curiously
analogous to that of the plants which bear cleistogene and perfect
flowers. He finds in the nests of termites in Brazil, males and females
with imperfect wings, which do not leave the nests and propagate the
species in a cleistogene manner, but only if a fully-developed queen
after swarming does not enter the old nest. The fully-developed males
and females are winged, and individuals from distinct nests can hardly
fail often to intercross. In the act of swarming they are destroyed in
almost infinite numbers by a host of enemies, so that a queen may often
fail to enter an old nest; and then the imperfectly developed males and
females propagate and keep up the stock.) The former are always minute,
completely closed, with their petals more or less rudimentary and never
brightly coloured; they never secrete nectar, never are odoriferous,
have very small anthers which produce only a few grains of pollen, and
their stigmas are but little developed. Bearing in mind that some
flowers are cross-fertilised by the wind (called anemophilous by
Delpino), and others by insects (called entomophilous), we can further
understand, as was pointed out by me several years ago, the great
contrast in appearance between these two classes of flowers. (10/22.
‘Journal of the Linnean Society’ volume 7 Botany 1863 page 77.)
Anemophilous flowers resemble in many respects cleistogene flowers, but
differ widely in not being closed, in producing an extraordinary amount
of pollen which is always incoherent, and in the stigma often being
largely developed or plumose. We certainly owe the beauty and odour of
our flowers and the storage of a large supply of honey to the existence
of insects.
ON THE RELATION BETWEEN THE STRUCTURE AND CONSPICUOUSNESS OF FLOWERS,
THE VISITS OF INSECTS, AND THE ADVANTAGES OF CROSS-FERTILISATION.
It has already been shown that there is no close relation between the
number of seeds produced by flowers when crossed and self-fertilised,
and the degree to which their offspring are aaffected by the two
processes. I have also given reasons for believing that the inefficiency
of a plant’s own pollen is in most cases an incidental result, or has
not been specially acquired for the sake of preventing
self-fertilisation. On the other hand, there can hardly be a doubt that
dichogamy, which prevails according to Hildebrand in the greater number
of species (10/23. ‘Die Geschlecter Vertheiling’ etc. page 32.),--that
the heterostyled condition of certain plants,--and that many mechanical
structures--have all been acquired so as both to check
self-fertilisation and to favour cross-fertilisation. The means for
favouring cross-fertilisation must have been acquired before those which
prevent self-fertilisation; as it would manifestly be injurious to a
plant that its stigma should fail to receive its own pollen, unless it
had already become well adapted for receiving pollen from another
individual. It should be observed that many plants still possess a high
power of self-fertilisation, although their flowers are excellently
constructed for cross-fertilisation--for instance, those of many
papilionaceous species.
It may be admitted as almost certain that some structures, such as a
narrow elongated nectary, or a long tubular corolla, have been developed
in order that certain kinds of insects alone should obtain the nectar.
These insects would thus find a store of nectar preserved from the
attacks of other insects; and they would thus be led to visit frequently
such flowers and to carry pollen from one to the other. (10/24. See the
interesting discussion on this subject by Hermann Muller, ‘Die
Befruchtung’ etc. page 431.) It might perhaps have been expected that
plants having their flowers thus peculiarly constructed would profit in
a greater degree by being crossed, than ordinary or simple flowers; but
this does not seem to hold good. Thus Tropaeolum minus has a long
nectary and an irregular corolla, whilst Limnanthes douglasii has a
regular flower and no proper nectary, yet the crossed seedlings of both
species are to the self-fertilised in height as 100 to 79. Salvia
coccinea has an irregular corolla, with a curious apparatus by which
insects depress the stamens, while the flowers of Ipomoea are regular;
and the crossed seedlings of the former are in height to the
self-fertilised as 100 to 76, whilst those of the Ipomoea are as 100 to
77. Fagopyrum is dimorphic, and Anagallis collina is non-dimorphic, and
the crossed seedlings of both are in height to the self-fertilised as
100 to 69.
With all European plants, excepting the comparatively rare anemophilous
kinds, the possibility of distinct individuals intercrossing depends on
the visits of insects; and Hermann Muller has proved by his valuable
observations, that large conspicuous flowers are visited much more
frequently and by many more kinds of insects, than are small
inconspicuous flowers. He further remarks that the flowers which are
rarely visited must be capable of self-fertilisation, otherwise they
would quickly become extinct. (10/25. ‘Die Befruchtung’ etc. page 426.
‘Nature’ 1873 page 433.) There is, however, some liability to error in
forming a judgment on this head, from the extreme difficulty of
ascertaining whether flowers which are rarely or never visited during
the day (as in the above given case of Fumaria capreolata) are not
visited by small nocturnal Lepidoptera, which are known to be strongly
attracted by sugar. (10/26. In answer to a question by me, the editor of
an entomological journal writes--“The Depressariae, as is notorious to
every collector of Noctuae, come very freely to sugar, and no doubt
naturally visit flowers:” the ‘Entomologists’ Weekly Intelligencer’ 1860
page 103.) The two lists given in the early part of this chapter support
Muller’s conclusion that small and inconspicuous flowers are completely
self-fertile: for only eight or nine out of the 125 species in the two
lists come under this head, and all of these were proved to be highly
fertile when insects were excluded. The singularly inconspicuous flowers
of the Fly Ophrys (O. muscifera), as I have elsewhere shown, are rarely
visited by insects; and it is a strange instance of imperfection, in
contradiction to the above rule, that these flowers are not
self-fertile, so that a large proportion of them do not produce seeds.
The converse of the rule that plants bearing small and inconspicuous
flowers are self-fertile, namely, that plants with large and conspicuous
flowers are self-sterile, is far from true, as may be seen in our second
list of spontaneously self-fertile species; for this list includes such
species as Ipomoea purpurea, Adonis aestivalis, Verbascum thapsus, Pisum
sativum, Lathyrus odoratus, some species of Papaver and of Nymphaea, and
others.
The rarity of the visits of insects to small flowers, does not depend
altogether on their inconspicuousness, but likewise on the absence of
some sufficient attraction; for the flowers of Trifolium arvense are
extremely small, yet are incessantly visited by hive and humble-bees, as
are the small and dingy flowers of the asparagus. The flowers of Linaria
cymbalaria are small and not very conspicuous, yet at the proper time
they are freely visited by hive-bees. I may add that, according to Mr.
Bennett, there is another and quite distinct class of plants which
cannot be much frequented by insects, as they flower either exclusively
or often during the winter, and these seem adapted for
self-fertilisation, as they shed their pollen before the flowers expand.
(10/27. ‘Nature’ 1869 page 11.)
That many flowers have been rendered conspicuous for the sake of guiding
insects to them is highly probable or almost certain; but it may be
asked, have other flowers been rendered inconspicuous so that they may
not be frequently visited, or have they merely retained a former and
primitive condition? If a plant were much reduced in size, so probably
would be the flowers through correlated growth, and this may possibly
account for some cases; but the size and colour of the corolla are both
extremely variable characters, and it can hardly be doubted that if
large and brightly-coloured flowers were advantageous to any species,
these could be acquired through natural selection within a moderate
lapse of time, as indeed we see with most alpine plants. Papilionaceous
flowers are manifestly constructed in relation to the visits of insects,
and it seems improbable, from the usual character of the group, that the
progenitors of the genera Vicia and Trifolium produced such minute and
unattractive flowers as those of V. hirsuta and T. procumbens. We are
thus led to infer that some plants either have not had their flowers
increased in size, or have actually had them reduced and purposely
rendered inconspicuous, so that they are now but little visited by
insects. In either case they must also have acquired or retained a high
degree of self-fertility.
If it became from any cause advantageous to a species to have its
capacity for self-fertilisation increased, there is little difficulty in
believing that this could readily be effected; for three cases of plants
varying in such a manner as to be more fertile with their own pollen
than they originally were, occurred in the course of my few experiments,
namely, with Mimulus, Ipomoea, and Nicotiana. Nor is there any reason to
doubt that many kinds of plants are capable under favourable
circumstances of propagating themselves for very many generations by
self-fertilisation. This is the case with the varieties of Pisum sativum
and of Lathyrus odoratus which are cultivated in England, and with
Ophrys apifera and some other plants in a state of nature. Nevertheless,
most or all of these plants retain structures in an efficient state
which cannot be of the least use excepting for cross-fertilisation. We
have also seen reason to suspect that self-fertilisation is in some
peculiar manner beneficial to certain plants; but if this be really the
case, the benefit thus derived is far more than counter-balanced by a
cross with a fresh stock or with a slightly different variety.
Notwithstanding the several considerations just advanced, it seems to me
highly improbable that plants bearing small and inconspicuous flowers
have been or should continue to be subjected to self-fertilisation for a
long series of generations. I think so, not from the evil which
manifestly follows from self-fertilisation, in many cases even in the
first generation, as with Viola tricolor, Sarothamnus, Nemophila,
Cyclamen, etc.; nor from the probability of the evil increasing after
several generations, for on this latter head I have not sufficient
evidence, owing to the manner in which my experiments were conducted.
But if plants bearing small and inconspicuous flowers were not
occasionally intercrossed, and did not profit by the process, all their
flowers would probably have been rendered cleistogene, as they would
thus have largely benefited by having to produce only a small quantity
of safely-protected pollen. In coming to this conclusion, I have been
guided by the frequency with which plants belonging to distinct orders
have been rendered cleistogene. But I can hear of no instance of a
species with all its flowers rendered permanently cleistogene. Leersia
makes the nearest approach to this state; but as already stated, it has
been known to produce perfect flowers in one part of Germany. Some other
plants of the cleistogene class, for instance Aspicarpa, have failed to
produce perfect flowers during several years in a hothouse; but it does
not follow that they would fail to do so in their native country, any
more than with Vandellia, which with me produced only cleistogene
flowers during certain years. Plants belonging to this class commonly
bear both kinds of flowers every season, and the perfect flowers of
Viola canina yield fine capsules, but only when visited by bees. We have
also seen that the seedlings of Ononis minutissima, raised from the
perfect flowers fertilised with pollen from another plant, were finer
than those from self-fertilised flowers; and this was likewise the case
to a certain extent with Vandellia. As therefore no species which at one
time bore small and inconspicuous flowers has had all its flowers
rendered cleistogene, I must believe that plants now bearing small and
inconspicuous flowers profit by their still remaining open, so as to be
occasionally intercrossed by insects. It has been one of the greatest
oversights in my work that I did not experimentise on such flowers,
owing to the difficulty of fertilising them, and to my not having seen
the importance of the subject. (10/28. Some of the species of Solanum
would be good ones for such experiments, for they are said by Hermann
Muller ‘Befruchtung’ page 434, to be unattractive to insects from not
secreting nectar, not producing much pollen, and not being very
conspicuous. Hence probably it is that, according to Verlot ‘Production
des Varieties’ 1865 page 72, the varieties of “les aubergines et les
tomates” (species of Solanum) do not intercross when they are cultivated
near together; but it should be remembered that these are not endemic
species. On the other hand, the flowers of the common potato (S.
tuberosum), though they do not secrete nectar Kurr ‘Bedeutung der
Nektarien’ 1833 page 40, yet cannot be considered as inconspicuous, and
they are sometimes visited by diptera (Muller), and, as I have seen, by
humble-bees. Tinzmann (as quoted in ‘Gardeners’ Chronicle’ 1846 page
183, found that some of the varieties did not bear seed when fertilised
with pollen from the same variety, but were fertile with that from
another variety.)
It should be remembered that in two of the cases in which highly
self-fertile varieties appeared amongst my experimental plants, namely,
with Mimulus and Nicotiana, such varieties were greatly benefited by a
cross with a fresh stock or with a slightly different variety; and this
likewise was the case with the cultivated varieties of Pisum sativum and
Lathyrus odoratus, which have been long propagated by
self-fertilisation. Therefore until the contrary is distinctly proved, I
must believe that as a general rule small and inconspicuous flowers are
occasionally intercrossed by insects; and that after long-continued
self-fertilisation, if they are crossed with pollen brought from a plant
growing under somewhat different conditions, or descended from one thus
growing, their offspring would profit greatly. It cannot be admitted,
under our present state of knowledge, that self-fertilisation continued
during many successive generations is ever the most beneficial method of
reproduction.
THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN
FROM A DISTINCT PLANT.
We have seen in four cases that seedlings raised from a cross between
flowers on the same plant, even on plants appearing distinct from having
been propagated by stolons or cuttings, were not superior to seedlings
from self-fertilised flowers; and in a fifth case (Digitalis) superior
only in a slight degree. Therefore we might expect that with plants
growing in a state of nature a cross between the flowers on distinct
individuals, and not merely between the flowers on the same plant, would
generally or often be effected by some means. The fact of bees and of
some Diptera visiting the flowers of the same species as long as they
can, instead of promiscuously visiting various species, favours the
intercrossing of distinct plants. On the other hand, insects usually
search a large number of flowers on the same plant before they fly to
another, and this is opposed to cross-fertilisation. The extraordinary
number of flowers which bees are able to search within a very short
space of time, as will be shown in a future chapter, increases the
chance of cross-fertilisation; as does the fact that they are not able
to perceive without entering a flower whether other bees have exhausted
the nectar. For instance, Hermann Muller found that four-fifths of the
flowers of Lamium album which a humble-bee visited had been already
exhausted of their nectar. (10/29. ‘Die Befruchtung’ etc. page 311.) In
order that distinct plants should be intercrossed, it is of course
indispensable that two or more individuals should grow near one another;
and this is generally the case. Thus A. de Candolle remarks that in
ascending a mountain the individuals of the same species do not commonly
disappear near its upper limit quite gradually, but rather abruptly.
This fact can hardly be explained by the nature of the conditions, as
these graduate away in an insensible manner, and it probably depends in
large part on vigorous seedlings being produced only as high up the
mountain as many individuals can subsist together.
With respect to dioecious plants, distinct individuals must always
fertilise each other. With monoecious plants, as pollen has to be
carried from flower to flower, there will always be a good chance of its
being carried from plant to plant. Delpino has also observed the curious
fact that certain individuals of the monoecious walnut (Juglans regia)
are proterandrous, and others proterogynous, and these will reciprocally
fertilise each other. (10/30. ‘Ult. Osservazioni’ etc. part 2 fasc 2
page 337.) So it is with the common nut (Corylus avellana) (10/31.
‘Nature’ 1875 page 26.), and, what is more surprising, with some few
hermaphrodite plants, as observed by Hermann Muller. (10/32. ‘Die
Befruchtung’ etc. pages 285, 339.) These latter plants cannot fail to
act on each other like dimorphic or trimorphic species, in which the
union of two individuals is necessary for full and normal fertility.
With ordinary hermaphrodite species, the expansion of only a few flowers
at the same time is one of the simplest means for favouring the
intercrossing of distinct individuals; but this would render the plants
less conspicuous to insects, unless the flowers were of large size, as
in the case of several bulbous plants. Kerner thinks that it is for this
object that the Australian Villarsia parnassifolia produces daily only a
single flower. (10/33. ‘Die Schutzmittel’ etc page 23.) Mr. Cheeseman
also remarks, that as certain Orchids in New Zealand which require
insect-aid for their fertilisation bear only a single flower, distinct
plants cannot fail to intercross. (10/34. ‘Transactions of the New
Zealand Institute’ volume 5 1873 page 356.)
Dichogamy, which prevails so extensively throughout the vegetable
kingdom, much increases the chance of distinct individuals
intercrossing. With proterandrous species, which are far more ccommon
than proterogynous, the young flowers are exclusively male in function,
and the older ones exclusively female; and as bees habitually alight low
down on the spikes of flowers in order to crawl upwards, they get dusted
with pollen from the uppermost flowers, which they carry to the stigmas
of the lower and older flowers on the next spike which they visit. The
degree to which distinct plants will thus be intercrossed depends on the
number of spikes in full flower at the same time on the same plant. With
proterogynous flowers and with depending racemes, the manner in which
insects visit the flowers ought to be reversed in order that distinct
plants should be intercrossed. But this whole subject requires further
investigation, as the great importance of crosses between distinct
individuals, instead of merely between distinct flowers, has hitherto
been hardly recognised.
In some few cases the special movements of certain organs almost ensure
pollen being carried from plant to plant. Thus with many orchids, the
pollen-masses after becoming attached to the head or proboscis of an
insect do not move into the proper position for striking the stigma,
until ample time has elapsed for the insect to fly to another plant.
With Spiranthes autumnalis, the pollen-masses cannot be applied to the
stigma until the labellum and rostellum have moved apart, and this
movement is very slow. (10/35. ‘The Various Contrivances by which
British and Foreign Orchids are fertilised’ first edition page 128.)
With Posoqueria fragrans (one of the Rubiaceae) the same end is gained
by the movement of a specially constructed stamen, as described by Fritz
Muller.
We now come to a far more general and therefore more important means by
which the mutual fertilisation of distinct plants is effected, namely,
the fertilising power of pollen from another variety or individual being
greater than that of a plant’s own pollen. The simplest and best known
case of prepotent action in pollen, though it does not bear directly on
our present subject, is that of a plant’s own pollen over that from a
distinct species. If pollen from a distinct species be placed on the
stigma of a castrated flower, and then after the interval of several
hours, pollen from the same species be placed on the stigma, the effects
of the former are wholly obliterated, excepting in some rare cases. If
two varieties are treated in the same manner, the result is analogous,
though of directly opposite nature; for pollen from any other variety is
often or generally prepotent over that from the same flower. I will give
some instances: the pollen of Mimulus luteus regularly falls on the
stigma of its own flower, for the plant is highly fertile when insects
are excluded. Now several flowers on a remarkably constant whitish
variety were fertilised without being castrated with pollen from a
yellowish variety; and of the twenty-eight seedlings thus raised, every
one bore yellowish flowers, so that the pollen of the yellow variety
completely overwhelmed that of the mother-plant. Again, Iberis umbellata
is spontaneously self-fertile, and I saw an abundance of pollen from
their own flowers on the stigmas; nevertheless, of thirty seedlings
raised from non-castrated flowers of a crimson variety crossed with
pollen from a pink variety, twenty-four bore pink flowers, like those of
the male or pollen-bearing parent.
In these two cases flowers were fertilised with pollen from a distinct
variety, and this was shown to be prepotent by the character of the
offspring. Nearly similar results often follow when two or more
self-fertile varieties are allowed to grow near one another and are
visited by insects. The common cabbage produces a large number of
flowers on the same stalk, and when insects are excluded these set many
capsules, moderately rich in seeds. I planted a white Kohl-rabi, a
purple Kohl-rabi, a Portsmouth broccoli, a Brussels sprout, and a
Sugar-loaf cabbage near together and left them uncovered. Seeds
collected from each kind were sown in separate beds; and the majority of
the seedlings in all five beds were mongrelised in the most complicated
manner, some taking more after one variety, and some after another. The
effects of the Kohl-rabi were particularly plain in the enlarged stems
of many of the seedlings. Altogether 233 plants were raised, of which
155 were mongrelised in the plainest manner, and of the remaining 78 not
half were absolutely pure. I repeated the experiment by planting near
together two varieties of cabbage with purple-green and white-green
lacinated leaves; and of the 325 seedlings raised from the purple-green
variety, 165 had white-green and 160 purple-green leaves. Of the 466
seedlings raised from the white-green variety, 220 had purple-green and
246 white-green leaves. These cases show how largely pollen from a
neighbouring variety of the cabbage effaces the action of the plant’s
own pollen. We should bear in mind that pollen must be carried by the
bees from flower to flower on the same large branching stem much more
abundantly than from plant to plant; and in the case of plants the
flowers of which are in some degree dichogamous, those on the same stem
would be of different ages, and would thus be as ready for mutual
fertilisation as the flowers on distinct plants, were it not for the
prepotency of pollen from another variety. (10/36. A writer in the
‘Gardeners’ Chronicle’ 1855 page 730, says that he planted a bed of
turnips (Brassica rapa) and of rape (B. napus) close together, and sowed
the seeds of the former. The result was that scarcely one seedling was
true to its kind, and several closely resembled rape.)
Several varieties of the radish (Raphanus sativus), which is moderately
self-fertile when insects are excluded, were in flower at the same time
in my garden. Seed was collected from one of them, and out of twenty-two
seedlings thus raised only twelve were true to their kind. (10/37.
Duhamel as quoted by Godron ‘De l’Espece’ tome 2 page 50, makes an
analogous statement with respect to this plant.)
The onion produces a large number of flowers, all crowded together into
a large globular head, each flower having six stamens; so that the
stigmas receive plenty of pollen from their own and the adjoining
anthers. Consequently the plant is fairly self-fertile when protected
from insects. A blood-red, silver, globe and Spanish onion were planted
near together; and seedlings were raised from each kind in four separate
beds. In all the beds mongrels of various kinds were numerous, except
amongst the ten seedlings from the blood-red onion, which included only
two. Altogether forty-six seedlings were raised, of which thirty-one had
been plainly crossed.
A similar result is known to follow with the varieties of many other
plants, if allowed to flower near together: I refer here only to species
which are capable of fertilising themselves, for if this be not the
case, they would of course be liable to be crossed by any other variety
growing near. Horticulturists do not commonly distinguish between the
effects of variability and intercrossing; but I have collected evidence
on the natural crossing of varieties of the tulip, hyacinth, anemone,
ranunculus, strawberry, Leptosiphon androsaceus, orange, rhododendron
and rhubarb, all of which plants I believe to be self-fertile. (10/38.
With respect to tulips and some other flowers, see Godron ‘De l’Espece’
tome 1 page 252. For anemones ‘Gardeners’ Chronicle’ 1859 page 98. For
strawberries see Herbert in ‘Transactions of the Horticultural Society’
volume 4 page 17. The same observer elsewhere speaks of the spontaneous
crossing of rhododendrons. Gallesio makes the same statement with
respect to oranges. I have myself known extensive crossing to occur with
the common rhubarb. For Leptosiphon, Verlot ‘Des Varieties’ 1865 page
20. I have not included in my list the Carnation, Nemophila, or
Antirrhinum, the varieties of which are known to cross freely, because
these plants are not always self-fertile. I know nothing about the
self-fertility of Trollius Lecoq ‘De la Fecondation’ 1862 page 93,
Mahonia, and Crinum, in which genera the species intercross largely.
With respect to Mahonia it is now scarcely possible to procure in this
country pure specimens of M. aquifolium or repens; and the various
species of Crinum sent by Herbert ‘Amaryllidaceae’ page 32, to Calcutta,
crossed there so freely that pure seed could not be saved.) Much other
indirect evidence could be given with respect to the extent to which
varieties of the same species spontaneously intercross.
Gardeners who raise seed for sale are compelled by dearly bought
experience to take extraordinary precautions against intercrossing. Thus
Messrs. Sharp “have land engaged in the growth of seed in no less than
eight parishes.” The mere fact of a vast number of plants belonging to
the same variety growing together is a considerable protection, as the
chances are strong in favour of plants of the same variety
intercrossing; and it is in chief part owing to this circumstance, that
certain villages have become famous for pure seed of particular
varieties. (10/39. With respect to Messrs. Sharp see ‘Gardeners’
Chronicle’ 1856 page 823. Lindley’s ‘Theory of Horticulture’ page 319.)
Only two trials were made by me to ascertain after how long an interval
of time, pollen from a distinct variety would obliterate more or less
completely the action of a plant’s own pollen. The stigmas in two lately
expanded flowers on a variety of cabbage, called Ragged Jack, were well
covered with pollen from the same plant. After an interval of
twenty-three hours, pollen from the Early Barnes Cabbage growing at a
distance was placed on both stigmas; and as the plant was left
uncovered, pollen from other flowers on the Ragged Jack would certainly
have been left by the bees during the next two or three days on the same
two stigmas. Under these circumstances it seemed very unlikely that the
pollen of the Barnes cabbage would produce any effect; but three out of
the fifteen plants raised from the two capsules thus produced were
plainly mongrelised: and I have no doubt that the twelve other plants
were affected, for they grew much more vigorously than the
self-fertilised seedlings from the Ragged Jack planted at the same time
and under the same conditions. Secondly, I placed on several stigmas of
a long-styled cowslip (Primula veris) plenty of pollen from the same
plant, and after twenty-four hours added some from a short-styled
dark-red Polyanthus, which is a variety of the cowslip. From the flowers
thus treated thirty seedlings were raised, and all these without
exception bore reddish flowers; so that the effect of the plant’s own
pollen, though placed on the stigmas twenty-four hours previously, was
quite destroyed by that of the red variety. It should, however, be
observed that these plants are dimorphic, and that the second union was
a legitimate one, whilst the first was illegitimate; but flowers
illegitimately fertilised with their own pollen yield a moderately fair
supply of seeds.
We have hitherto considered only the prepotent fertilising power of
pollen from a distinct variety over a plants’ own pollen,--both kinds of
pollen being placed on the same stigma. It is a much more remarkable
fact that pollen from another individual of the same variety is
prepotent over a plant’s own pollen, as shown by the superiority of the
seedlings raised from a cross of this kind over seedlings from
self-fertilised flowers. Thus in Tables 7/A, B, and C, there are at
least fifteen species which are self-fertile when insects are excluded;
and this implies that their stigmas must receive their own pollen;
nevertheless, most of the seedlings which were raised by fertilising the
non-castrated flowers of these fifteen species with pollen from another
plant were greatly superior, in height, weight, and fertility, to the
self-fertilised offspring. (10/40. These fifteen species consist of
Brassica oleracea, Reseda odorata and lutea, Limnanthes douglasii,
Papaver vagum, Viscaria oculata, Beta vulgaris, Lupinus luteus, Ipomoea
purpurea, Mimulus luteus, Calceolaria, Verbascum thapsus, Vandellia
nummularifolia, Lactuca sativa, and Zea mays.) For instance, with
Ipomoea purpurea every single intercrossed plant exceeded in height its
self-fertilised opponent until the sixth generation; and so it was with
Mimulus luteus until the fourth generation. Out of six pairs of crossed
and self-fertilised cabbages, every one of the former was much heavier
than the latter. With Papaver vagum, out of fifteen pairs, all but two
of the crossed plants were taller than their self-fertilised opponents.
Of eight pairs of Lupinus luteus, all but two of the crossed were
taller; of eight pairs of Beta vulgaris all but one; and of fifteen
pairs of Zea mays all but two were taller. Of fifteen pairs of
Limnanthes douglasii, and of seven pairs of Lactuca sativa, every single
crossed plant was taller than its self-fertilised opponent. It should
also be observed that in these experiments no particular care was taken
to cross-fertilise the flowers immediately after their expansion; it is
therefore almost certain that in many of these cases some pollen from
the same flower will have already fallen on and acted on the stigma.
There can hardly be a doubt that several other species of which the
crossed seedlings are more vigorous than the self-fertilised, as shown
in Tables 7/A, 7/B and 7/C, besides the above fifteen, must have
received their own pollen and that from another plant at nearly the same
time; and if so, the same remarks as those just given are applicable to
them. Scarcely any result from my experiments has surprised me so much
as this of the prepotency of pollen from a distinct individual over each
plant’s own pollen, as proved by the greater constitutional vigour of
the crossed seedlings. The evidence of prepotency is here deduced from
the comparative growth of the two lots of seedlings; but we have similar
evidence in many cases from the much greater fertility of the
non-castrated flowers on the mother-plant, when these received at the
same time their own pollen and that from a distinct plant, in comparison
with the flowers which received only their own pollen.
From the various facts now given on the spontaneous intercrossing of
varieties growing near together, and on the effects of cross-fertilising
flowers which are self-fertile and have not been castrated, we may
conclude that pollen brought by insects or by the wind from a distinct
plant will generally prevent the action of pollen from the same flower,
even though it may have been applied some time before; and thus the
intercrossing of plants in a state of nature will be greatly favoured or
ensured.
The case of a great tree covered with innumerable hermaphrodite flowers
seems at first sight strongly opposed to the belief in the frequency of
intercrosses between distinct individuals. The flowers which grow on the
opposite sides of such a tree will have been exposed to somewhat
different conditions, and a cross between them may perhaps be in some
degree beneficial; but it is not probable that it would be nearly so
beneficial as a cross between flowers on distinct trees, as we may infer
from the inefficiency of pollen taken from plants which have been
propagated from the same stock, though growing on separate roots. The
number of bees which frequent certain kinds of trees when in full flower
is very great, and they may be seen flying from tree to tree more
frequently than might have been expected. Nevertheless, if we consider
how numerous are the flowers, for instance, on a horse-chestnut or
lime-tree, an incomparably larger number of flowers must be fertilised
by pollen brought from other flowers on the same tree, than from flowers
on a distinct tree. But we should bear in mind that with the
horse-chestnut, for instance, only one or two of the several flowers on
the same peduncle produce a seed; and that this seed is the product of
only one out of several ovules within the same ovarium. Now we know from
the experiments of Herbert and others that if one flower is fertilised
with pollen which is more efficient than that applied to the other
flowers on the same peduncle, the latter often drop off (10/41.
‘Variation under Domestication’ chapter 17 2nd edition volume 2 page
120.); and it is probable that this would occur with many of the
self-fertilised flowers on a large tree, if other and adjoining flowers
were cross-fertilised. Of the flowers annually produced by a great tree,
it is almost certain that a large number would be self-fertilised; and
if we assume that the tree produced only 500 flowers, and that this
number of seeds were requisite to keep up the stock, so that at least
one seedling should hereafter struggle to maturity, then a large
proportion of the seedlings would necessarily be derived from
self-fertilised seeds. But if the tree annually produced 50,000 flowers,
of which the self-fertilised dropped off without yielding seeds, then
the cross-fertilised flowers might yield seeds in sufficient number to
keep up the stock, and most of the seedlings would be vigorous from
being the product of a cross between distinct individuals. In this
manner the production of a vast number of flowers, besides serving to
entice numerous insects and to compensate for the accidental destruction
of many flowers by spring-frosts or otherwise, would be a very great
advantage to the species; and when we behold our orchard-trees covered
with a white sheet of bloom in the spring, we should not falsely accuse
nature of wasteful expenditure, though comparatively little fruit is
produced in the autumn.
ANEMOPHILOUS PLANTS.
The nature and relations of plants which are fertilised by the wind have
been admirably discussed by Delpino and Hermann Muller; and I have
already made some remarks on the structure of their flowers in contrast
with those of entomophilous species. (10/42. Delpino ‘Ult. Osservazioni
sulla Dicogamia’ part 2 fasc. 1 1870 and ‘Studi sopra un Lignaggio
anemofilo’ etc. 1871. Hermann Muller ‘Die Befruchtung’ etc. pages 412,
442. Both these authors remark that plants must have been anemophilous
before they were entomophilous. Hermann Muller further discusses in a
very interesting manner the steps by which entomophilous flowers became
nectariferous and gradually acquired their present structure through
successive beneficial changes.) There is good reason to believe that the
first plants which appeared on this earth were cryptogamic; and judging
from what now occurs, the male fertilising element must either have
possessed the power of spontaneous movement through the water or over
damp surfaces, or have been carried by currents of water to the female
organs. That some of the most ancient plants, such as ferns, possessed
true sexual organs there can hardly be a doubt; and this shows, as
Hildebrand remarks, at how early a period the sexes were separated.
(10/43. ‘Die Geschlechter-Vertheilung’ 1867 pages 84-90.) As soon as
plants became phanerogamic and grew on the dry ground, if they were ever
to intercross, it would be indispensable that the male fertilising
element should be transported by some means through the air; and the
wind is the simplest means of transport. There must also have been a
period when winged insects did not exist, and plants would not then have
been rendered entomophilous. Even at a somewhat later period the more
specialised orders of the Hymenoptera, Lepidoptera, and Diptera, which
are now chiefly concerned with the transport of pollen, did not exist.
Therefore the earliest terrestrial plants known to us, namely, the
Coniferae and Cycadiae, no doubt were anemophilous, like the existing
species of these same groups. A vestige of this early state of things is
likewise shown by some other groups of plants which are anemophilous, as
these on the whole stand lower in the scale than entomophilous species.
There is no great difficulty in understanding how an anemophilous plant
might have been rendered entomophilous. Pollen is a nutritious
substance, and would soon have been discovered and devoured by insects;
and if any adhered to their bodies it would have been carried from the
anthers to the stigma of the same flower, or from one flower to another.
One of the chief characteristics of the pollen of anemophilous plants is
its incoherence; but pollen in this state can adhere to the hairy bodies
of insects, as we see with some Leguminosae, Ericaceae, and
Melastomaceae. We have, however, better evidence of the possibility of a
transition of the above kind in certain plants being now fertilised
partly by the wind and partly by insects. The common rhubarb (Rheum
rhaponticum) is so far in an intermediate condition, that I have seen
many Diptera sucking the flowers, with much pollen adhering to their
bodies; and yet the pollen is so incoherent, that clouds of it are
emitted if the plant be gently shaken on a sunny day, some of which
could hardly fail to fall on the large stigmas of the neighbouring
flowers. According to Delpino and Hermann Muller, some species of
Plantago are in a similar intermediate condition. (10/44. ‘Die
Befruchtung’ etc. page 342.)
Although it is probable that pollen was aboriginally the sole attraction
to insects, and although many plants now exist whose flowers are
frequented exclusively by pollen-devouring insects, yet the great
majority secrete nectar as the chief attraction. Many years ago I
suggested that primarily the saccharine matter in nectar was excreted as
a waste product of chemical changes in the sap; and that when the
excretion happened to occur within the envelopes of a flower, it was
utilised for the important object of cross-fertilisation, being
subsequently much increased in quantity and stored in various ways.
(10/45. Nectar was regarded by De Candolle and Dunal as an excretion, as
stated by Martinet in ‘Annal des Sc. Nat.’ 1872 tome 14 page 211.) This
view is rendered probable by the leaves of some trees excreting, under
certain climatic conditions, without the aid of special glands, a
saccharine fluid, often called honey-dew. This is the case with the
leaves of the lime; for although some authors have disputed the fact, a
most capable judge, Dr. Maxwell Masters, informs me that, after having
heard the discussions on this subject before the Horticultural Society,
he feels no doubt on this head. The leaves, as well as the cut stems, of
the manna ash (Fraxinus ornus) secrete in a like manner saccharine
matter. (10/46. ‘Gardeners’ Chronicle’ 1876 page 242.) According to
Treviranus, so do the upper surfaces of the leaves of Carduus arctioides
during hot weather. Many analogous facts could be given. (10/47. Kurr
‘Untersuchungen uber die Bedeutung der Nektarien’ 1833 page 115.) There
are, however, a considerable number of plants which bear small glands on
their leaves, petioles, phyllodia, stipules, bracteae, or flower
peduncles, or on the outside of their calyx, and these glands secrete
minute drops of a sweet fluid, which is eagerly sought by sugar-loving
insects, such as ants, hive-bees, and wasps. (10/48. A large number of
cases are given by Delpino in the ‘Bulletino Entomologico’ Anno 6 1874.
To these may be added those given in my text, as well as the excretion
of saccharine matter from the calyx of two species of Iris, and from the
bracteae of certain Orchideae: see Kurr ‘Bedeutung der Nektarien’ 1833
pages 25, 28. Belt ‘Nicaragua’ page 224, also refers to a similar
excretion by many epiphytal orchids and passion-flowers. Mr. Rodgers has
seen much nectar secreted from the bases of the flower-peduncles of
Vanilla. Link says that the only example of a hypopetalous nectary known
to him is externally at the base of the flowers of Chironia decussata:
see ‘Reports on Botany, Ray Society’ 1846 page 355. An important memoir
bearing on this subject has lately appeared by Reinke ‘Gottingen
Nachrichten’ 1873 page 825, who shows that in many plants the tips of
the serrations on the leaves in the bud bear glands which secrete only
at a very early age, and which have the same morphological structure as
true nectar-secreting glands. He further shows that the nectar-secreting
glands on the petioles of Prunus avium are not developed at a very early
age, yet wither away on the old leaves. They are homologous with those
on the serrations of the blades of the same leaves, as shown by their
structure and by transition-forms; for the lowest serrations on the
blades of most of the leaves secrete nectar instead of resin (harz).) In
the case of the glands on the stipules of Vicia sativa, the excretion
manifestly depends on changes in the sap, consequent on the sun shining
brightly; for I repeatedly observed that as soon as the sun was hidden
behind clouds the secretion ceased, and the hive-bees left the field;
but as soon as the sun broke out again, they returned to their feast.
(10/49. I published a brief notice of this case in the ‘Gardeners’
Chronicle’ 1855 July 21 page 487, and afterwards made further
observations. Besides the hive-bee, another species of bee, a moth,
ants, and two kinds of flies sucked the drops of fluid on the stipules.
The larger drops tasted sweet. The hive-bees never even looked at the
flowers which were open at the same time; whilst two species of
humble-bees neglected the stipules and visited only the flowers.) I have
observed an analogous fact with the secretion of true nectar in the
flowers of Lobelia erinus.
Delpino, however, maintains that the power of secreting a sweet fluid by
any extra-floral organ has been in every case specially gained, for the
sake of attracting ants and wasps as defenders of the plant against
their enemies; but I have never seen any reason to believe that this is
so with the three species observed by me, namely, Prunus laurocerasus,
Vicia sativa, and V. faba. No plant is so little attacked by enemies of
any kind as the common bracken-fern (Pteris aquilina); and yet, as my
son Francis has discovered, the large glands at the bases of the fronds,
but only whilst young, excrete much sweetish fluid, which is eagerly
sought by innumerable ants, chiefly belonging to Myrmica; and these ants
certainly do not serve as a protection against any enemy. Delpino argues
that such glands ought not to be considered as excretory, because if
they were so, they would be present in every species; but I cannot see
much force in this argument, as the leaves of some plants excrete sugar
only during certain states of the weather. That in some cases the
secretion serves to attract insects as defenders of the plant, and may
have been developed to a high degree for this special purpose, I have
not the least doubt, from the observations of Delpino, and more
especially from those of Mr. Belt on Acacia sphaerocephala, and on
passion-flowers. This acacia likewise produces, as an additional
attraction to ants, small bodies containing much oil and protoplasm, and
analogous bodies are developed by a Cecropia for the same purpose, as
described by Fritz Muller. (10/50. Mr. Belt ‘The Naturalist in
Nicaragua’ 1874 page 218, has given a most interesting account of the
paramount importance of ants as defenders of the above Acacia. With
respect to the Cecropia see ‘Nature’ 1876 page 304. My son Francis has
described the microscopical structure and development of these wonderful
food-bodies in a paper read before the Linnean Society.)
The excretion of a sweet fluid by glands seated outside of a flower is
rarely utilised as a means for cross-fertilisation by the aid of
insects; but this occurs with the bracteae of the Marcgraviaceae, as the
late Dr. Cruger informed me from actual observation in the West Indies,
and as Delpino infers with much acuteness from the relative position of
the several parts of their flowers. (10/51. ‘Ult. Osservaz. Dicogamia’
1868-69 page 188.) Mr. Farrer has also shown that the flowers of
Coronilla are curiously modified, so that bees may fertilise them whilst
sucking the fluid secreted from the outside of the calyx. (10/52.
‘Nature’ 1874 page 169.) It further appears probable from the
observations of the Reverend W.A. Leighton, that the fluid so abundantly
secreted by glands on the phyllodia of the Australian Acacia magnifica,
which stand near the flowers, is connected with their fertilisation.
(10/53. ‘Annals and Magazine of Natural History’ volume 16 1865 page 14.
In my work on the ‘Fertilisation of Orchids’ and in a paper subsequently
published in the ‘Annals and Magazine of Natural History’ it has been
shown that although certain kinds of orchids possess a nectary, no
nectar is actually secreted by it; but that insects penetrate the inner
walls and suck the fluid contained in the intercellular spaces. I
further suggested, in the case of some other orchids which do not
secrete nectar, that insects gnawed the labellum; and this suggestion
has since been proved true. Hermann Muller and Delpino have now shown
that some other plants have thickened petals which are sucked or gnawed
by insects, their fertilisation being thus aided. All the known facts on
this head have been collected by Delpino in his ‘Ult. Osserv.’ part 2
fasc. 2 1875 pages 59-63.)
The amount of pollen produced by anemophilous plants, and the distance
to which it is often transported by the wind, are both surprisingly
great. Mr. Hassall found that the weight of pollen produced by a single
plant of the Bulrush (Typha) was 144 grains. Bucketfuls of pollen,
chiefly of Coniferae and Gramineae, have been swept off the decks of
vessels near the North American shore; and Mr. Riley has seen the ground
near St. Louis, in Missouri, covered with pollen, as if sprinkled with
sulphur; and there was good reason to believe that this had been
transported from the pine-forests at least 400 miles to the south.
Kerner has seen the snow-fields on the higher Alps similarly dusted; and
Mr. Blackley found numerous pollen-grains, in one instance 1200,
adhering to sticky slides, which were sent up to a height of from 500 to
1000 feet by means of a kite, and then uncovered by a special mechanism.
It is remarkable that in these experiments there were on an average
nineteen times as many pollen-grains in the atmosphere at the higher
than at the lower levels. (10/54. For Mr. Hassall’s observations see
‘Annals and Magazine of Natural History’ volume 8 1842 page 108. In the
‘North American Journal of Science’ January 1842, there is an account of
the pollen swept off the decks of a vessel. Riley ‘Fifth Report on the
Noxious Insects of Missouri’ 1873 page 86. Kerner ‘Die Schutzmittel des
Pollens’ 1873 page 6. This author has also seen a lake in the Tyrol so
covered with pollen, that the water no longer appeared blue. Mr.
Blackley ‘Experimental Researches on Hay-fever’ 1873 pages 132,
141-152.) Considering these facts, it is not so surprising as it at
first appears that all, or nearly all, the stigmas of anemophilous
plants should receive pollen brought to them by mere chance by the wind.
During the early part of summer every object is thus dusted with pollen;
for instance, I examined for another purpose the labella of a large
number of flowers of the Fly Ophrys (which is rarely visited by
insects), and found on all very many pollen-grains of other plants,
which had been caught by their velvety surfaces.
The extraordinary quantity and lightness of the pollen of anemophilous
plants are no doubt both necessary, as their pollen has generally to be
carried to the stigmas of other and often distant flowers; for, as we
shall soon see, most anemophilous plants have their sexes separated. The
fertilisation of these plants is generally aided by the stigmas being of
large size or plumose; and in the case of the Coniferae, by the naked
ovules secreting a drop of fluid, as shown by Delpino. Although the
number of anemophilous species is small, as the author just quoted
remarks, the number of individuals is large in comparison with that of
entomophilous species. This holds good especially in cold and temperate
regions, where insects are not so numerous as under a warmer climate,
and where consequently entomophilous plants are less favourably
situated. We see this in our forests of Coniferae and other trees, such
as oaks, beeches, birches, ashes, etc.; and in the Gramineae,
Cyperaceae, and Juncaceae, which clothe our meadows and swamps; all
these trees and plants being fertilised by the wind. As a large quantity
of pollen is wasted by anemophilous plants, it is surprising that so
many vigorous species of this kind abounding with individuals should
still exist in any part of the world; for if they had been rendered
entomophilous, their pollen would have been transported by the aid of
the senses and appetites of insects with incomparably greater safety
than by the wind. That such a conversion is possible can hardly be
doubted, from the remarks lately made on the existence of intermediate
forms; and apparently it has been effected in the group of willows, as
we may infer from the nature of their nearest allies. (10/55. Hermann
Muller ‘Die Befruchtung’ etc. page 149.)
It seems at first sight a still more surprising fact that plants, after
having been once rendered entomophilous, should ever again have become
anemophilous; but this has occasionally though rarely occurred, for
instance, with the common Poterium sanguisorba, as may be inferred from
its belonging to the Rosaceae. Such cases are, however, intelligible, as
almost all plants require to be occasionally intercrossed; and if any
entomiphilous species ceased to be visited by insects, it would probably
perish unless it were rendered anemophilous. A plant would be neglected
by insects if nectar failed to be secreted, unless indeed a large supply
of attractive pollen was present; and from what we have seen of the
excretion of saccharine fluid from leaves and glands being largely
governed in several cases by climatic influences, and from some few
flowers which do not now secrete nectar still retaining coloured
guiding-marks, the failure of the secretion cannot be considered as a
very improbable event. The same result would follow to a certainty, if
winged insects ceased to exist in any district, or became very rare. Now
there is only a single plant in the great order of the Cruciferae,
namely, Pringlea, which is anemophilous, and this plant is an inhabitant
of Kerguelen Land, where there are hardly any winged insects, owing
probably, as was suggested by me in the case of Madeira, to the risk
which they run of being blown out to sea and destroyed. (10/56. The
Reverend A.E. Eaton in ‘Proceedings of the Royal Society’ volume 23 1875
page 351.)
A remarkable fact with respect to anemophilous plants is that they are
often diclinous, that is, they are either monoecious with their sexes
separated on the same plant, or dioecious with their sexes on distinct
plants. In the class Monoecia of Linnaeus, Delpino shows that the
species of twenty-eight genera are anemophilous, and of seventeen genera
entomophilous. (10/57. ‘Studi sopra un Lignaggio anemofilo delle
Compositae’ 1871.) The larger proportion of entomophilous genera in this
latter class is probably the indirect result of insects having the power
of carrying pollen to another and sometimes distant plant much more
securely than the wind. In the above two classes taken together there
are thirty-eight anemophilous and thirty-six entomophilous genera;
whereas in the great mass of hermaphrodite plants the proportion of
anemophilous to entomophilous genera is extremely small. The cause of
this remarkable difference may be attributed to anemophilous plants
having retained in a greater degree than the entomophilous a primordial
condition, in which the sexes were separated and their mutual
fertilisation effected by means of the wind. That the earliest and
lowest members of the vegetable kingdom had their sexes separated, as is
still the case to a large extent, is the opinion of a high authority,
Nageli. (10/58. ‘Entstehung und Begriff der Naturhist. Art’ 1865 page
22.) It is indeed difficult to avoid this conclusion, if we admit the
view, which seems highly probable, that the conjugation of the Algae and
of some of the simplest animals is the first step towards sexual
reproduction; and if we further bear in mind that a greater and greater
degree of differentiation between the cells which conjugate can be
traced, thus leading apparently to the development of the two sexual
forms. (10/59. See the interesting discussion on this whole subject by
O. Butschli in his ‘Studien uber die ersten Entwickelungsvorgange der
Eizelle; etc. 1876 pages 207-219. Also Engelmann “Ueber Entwickelung von
Infusorien” ‘Morphol. Jahrbuch’ B. 1 page 573. Also Dr. A. Dodel “Die
Kraushaar-Algae” ‘Pringsheims Jahrbuch f. Wiss. Bot.’ B. 10.) We have
also seen that as plants became more highly developed and affixed to the
ground, they would be compelled to be anemophilous in order to
intercross. Therefore all plants which have not since been greatly
modified, would tend still to be both diclinous and anemophilous; and we
can thus understand the connection between these two states, although
they appear at first sight quite disconnected. If this view is correct,
plants must have been rendered hermaphrodites at a later though still
very early period, and entomophilous at a yet later period, namely,
after the development of winged insects. So that the relationship
between hermaphroditism and fertilisation by means of insects is
likewise to a certain extent intelligible.
Why the descendants of plants which were originally dioecious, and which
therefore profited by always intercrossing with another individual,
should have been converted into hermaphrodites, may perhaps be explained
by the risk which they ran, especially as long as they were
anemophilous, of not being always fertilised, and consequently of not
leaving offspring. This latter evil, the greatest of all to any
organism, would have been much lessened by their becoming
hermaphrodites, though with the contingent disadvantage of frequent
self-fertilisation. By what graduated steps an hermaphrodite condition
was acquired we do not know. But we can see that if a lowly organised
form, in which the two sexes were represented by somewhat different
individuals, were to increase by budding either before or after
conjugation, the two incipient sexes would be capable of appearing by
buds on the same stock, as occasionally occurs with various characters
at the present day. The organism would then be in a monoecious
condition, and this is probably the first step towards hermaphroditism;
for if very simple male and female flowers on the same stock, each
consisting of a single stamen or pistil, were brought close together and
surrounded by a common envelope, in nearly the same manner as with the
florets of the Compositae, we should have an hermaphrodite flower.
There seems to be no limit to the changes which organisms undergo under
changing conditions of life; and some hermaphrodite plants, descended as
we must believe from aboriginally diclinous plants, have had their sexes
again separated. That this has occurred, we may infer from the presence
of rudimentary stamens in the flowers of some individuals, and of
rudimentary pistils in the flowers of other individuals, for example in
Lychnis dioica. But a conversion of this kind will not have occurred
unless cross-fertilisation was already assured, generally by the agency
of insects; but why the production of male and female flowers on
distinct plants should have been advantageous to the species,
cross-fertilisation having been previously assured, is far from obvious.
A plant might indeed produce twice as many seeds as were necessary to
keep up its numbers under new or changed conditions of life; and if it
did not vary by bearing fewer flowers, and did vary in the state of its
reproductive organs (as often occurs under cultivation), a wasteful
expenditure of seeds and pollen would be saved by the flowers becoming
diclinous.
A related point is worth notice. I remarked in my Origin of Species that
in Britain a much larger proportion of trees and bushes than of
herbaceous plants have their sexes separated; and so it is, according to
Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in
the ‘London Catalogue of British Plants’ that there are thirty-two
indigenous trees and bushes in Great Britain, classed under nine
families; but to err on the safe side, I have counted only six species
of willows. Of the thirty-two trees and bushes, nineteen, or more than
half, have their sexes separated; and this is an enormous proportion
compared with other British plants. New Zealand abounds with diclinous
plants and trees; and Dr. Hooker calculates that out of about 756
phanerogamic plants inhabiting the islands, no less than 108 are trees,
belonging to thirty-five families. Of these 108 trees, fifty-two, or
very nearly half, have their sexes more or less separated. Of bushes
there are 149, of which sixty-one have their sexes in the same state;
whilst of the remaining 500 herbaceous plants only 121, or less than a
fourth, have their sexes separated. Lastly, Professor Asa Gray informs
me that in the United States there are 132 native trees (belonging to
twenty-five families) of which ninety-five (belonging to seventeen
families) “have their sexes more or less separated, for the greater part
decidedly separated.”) It is, however, doubtful how far this rule holds
good generally, and it certainly does not do so in Australia. But I have
been assured that the flowers of the prevailing Australian trees,
namely, the Myrtaceae, swarm with insects, and if they are dichogamous
they would be practically diclinous. (10/61. With respect to the
Proteaceae of Australia, Mr. Bentham ‘Journal of the Linnean Society
Botany’ volume 13 1871 pages 58, 64, remarks on the various contrivances
by which the stigma in the several genera is screened from the action of
the pollen from the same flower. For instance, in Synaphea “the stigma
is held by the eunuch (i.e., one of the stamens which is barren) safe
from all pollution from her brother anthers, and is preserved intact for
any pollen that may be inserted by insects and other agencies.”) As far
as anemophilous plants are concerned, we know that they are apt to have
their sexes separated, and we can see that it would be an unfavourable
circumstance for them to bear their flowers very close to the ground, as
their pollen is liable to be blown high up in the air (10/62. Kerner
‘Schutzmittel des Pollens’ 1873 page 4.); but as the culms of grasses
give sufficient elevation, we cannot thus account for so many trees and
bushes being diclinous. We may infer from our previous discussion that a
tree bearing numerous hermaphrodite flowers would rarely intercross with
another tree, except by means of the pollen of a distinct individual
being prepotent over the plants’ own pollen. Now the separation of the
sexes, whether the plant were anemophilous are entomophilous, would most
effectually bar self-fertilisation, and this may be the cause of so many
trees and bushes being diclinous. Or to put the case in another way, a
plant would be better fitted for development into a tree, if the sexes
were separated, than if it were hermaphrodite; for in the former case
its numerous flowers would be less liable to continued
self-fertilisation. But it should also be observed that the long life of
a tree or bush permits of the separation of the sexes, with much less
risk of evil from impregnation occasionally failing and seeds not being
produced, than in the case of short-lived plants. Hence it probably is,
as Lecoq has remarked, that annual plants are rarely dioecious.
Finally, we have seen reason to believe that the higher plants are
descended from extremely low forms which conjugated, and that the
conjugating individuals differed somewhat from one another,--the one
representing the male and the other the female--so that plants were
aboriginally dioecious. At a very early period such lowly organised
dioecious plants probably gave rise by budding to monoecious plants with
the two sexes borne by the same individual; and by a still closer union
of the sexes to hermaphrodite plants, which are now much the commonest
form. (10/63. There is a considerable amount of evidence that all the
higher animals are the descendants of hermaphrodites; and it is a
curious problem whether such hermaphroditism may not have been the
result of the conjugation of two slightly different individuals, which
represented the two incipient sexes. On this view, the higher animals
may now owe their bilateral structure, with all their organs double at
an early embryonic period, to the fusion or conjugation of two
primordial individuals.) As soon as plants became affixed to the ground,
their pollen must have been carried by some means from flower to flower,
at first almost certainly by the wind, then by pollen-devouring, and
afterwards by nectar-seeking insects. During subsequent ages some few
entomophilous plants have been again rendered anemophilous, and some
hermaphrodite plants have had their sexes again separated; and we can
vaguely see the advantages of such recurrent changes under certain
conditions.
Dioecious plants, however fertilised, have a great advantage over other
plants in their cross-fertilisation being assured. But this advantage is
gained in the case of anemophilous species at the expense of the
production of an enormous superfluity of pollen, with some risk to them
and to entomophilous species of their fertilisation occasionally
failing. Half the individuals, moreover, namely, the males, produce no
seed, and this might possibly be a disadvantage. Delpino remarks that
dioecious plants cannot spread so easily as monoecious and hermaphrodite
species, for a single individual which happened to reach some new site
could not propagate its kind; but it may be doubted whether this is a
serious evil. Monoecious plants can hardly fail to be to a large extent
dioecious in function, owing to the lightness of their pollen and to the
wind blowing laterally, with the great additional advantage of
occasionally or often producing some self-fertilised seeds. When they
are also dichogamous, they are necessarily dioecious in function.
Lastly, hermaphrodite plants can generally produce at least some
self-fertilised seeds, and they are at the same time capable, through
the various means specified in this chapter, of cross-fertilisation.
When their structure absolutely prevents self-fertilisation, they are in
the same relative position to one another as monoecious and dioecious
plants, with what may be an advantage, namely, that every flower is
capable of yielding seeds.