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THE DIFFERENT FORMS OF FLOWERS ON PLANTS OF THE SAME SPECIES
by CHARLES DARWIN, M.A., F.R.S.
PROFESSOR ASA GRAY
THIS VOLUME IS DEDICATED BY THE AUTHOR
AS A SMALL TRIBUTE OF RESPECT AND AFFECTION.
HETEROSTYLED DIMORPHIC PLANTS: PRIMULACEAE.
Primula veris or the cowslip.--Differences in structure between the two forms.--
Their degrees of fertility when legitimately and illegitimately united.--P.
elatior, vulgaris, Sinensis, auricula, etc.--Summary on the fertility of the
heterostyled species of Primula.--Homostyled species of Primula.--Hottonia
The oxlip a hybrid naturally produced between Primula veris and vulgaris.--The
differences in structure and function between the two parent-species.--Effects
of crossing long-styled and short-styled oxlips with one another and with the
two forms of both parent-species.--Character of the offspring from oxlips
artificially self-fertilised and cross-fertilised in a state of nature.--Primula
elatior shown to be a distinct species.--Hybrids between other heterostyled
species of Primula.--Supplementary note on spontaneously produced hybrids in the
HETEROSTYLED DIMORPHIC PLANTS--continued.
Linum grandiflorum, long-styled form utterly sterile with own-form pollen.--
Linum perenne, torsion of the pistils in the long-styled form alone.--Homostyled
species of Linum.--Pulmonaria officinalis, singular difference in self-fertility
between the English and German long-styled plants.--Pulmonaria angustifolia
shown to be a distinct species, long-styled form completely self-sterile.--
Polygonum fagopyrum.--Various other heterostyled genera.--Rubiaceae.--Mitchella
repens, fertility of the flowers in pairs.--Houstonia.--Faramea, remarkable
difference in the pollen-grains of the two forms; torsion of the stamens in the
short-styled form alone; development not as yet perfect.--The heterostyled
structure in the several Rubiaceous genera not due to descent in common.
HETEROSTYLED TRIMORPHIC PLANTS.
Lythrum salicaria.--Description of the three forms.--Their power and complex
manner of fertilising one another.--Eighteen different unions possible.--Mid-
styled form eminently feminine in nature.--Lythrum Graefferi likewise
trimorphic.--L. hymifolia dimorphic.--L. hyssopifolia homostyled.--Nesaea
verticillata trimorphic.--Lagerstroemia, nature doubtful.--Oxalis, trimorphic
species of.--O. Valdiviana.--O. Regnelli, the illegitimate unions quite barren.-
-O. speciosa.--O. sensitiva.--Homostyled species of Oxalis.--Pontederia, the one
monocotyledonous genus known to include heterostyled species.
ILLEGITIMATE OFFSPRING OF HETEROSTYLED PLANTS.
Illegitimate offspring from all three forms of Lythrum salicaria.--Their dwarfed
stature and sterility, some utterly barren, some fertile.--Oxalis, transmission
of form to the legitimate and illegitimate seedlings.--Primula Sinensis,
illegitimate offspring in some degree dwarfed and infertile.--Equal-styled
varieties of P. Sinensis, auricula, farinosa, and elatior.--P. vulgaris, red-
flowered variety, illegitimate seedlings sterile.--P. veris, illegitimate plants
raised during several successive generations, their dwarfed stature and
sterility.--Equal-styled varieties of P. veris.--Transmission of form by
Pulmonaria and Polygonum.--Concluding remarks.--Close parallelism between
illegitimate fertilisation and hybridism.
CONCLUDING REMARKS ON HETEROSTYLED PLANTS.
The essential character of heterostyled plants.--Summary of the differences in
fertility between legitimately and illegitimately fertilised plants.--Diameter
of the pollen-grains, size of anthers and structure of stigma in the different
forms.--Affinities of the genera which include heterostyled species.--Nature of
the advantages derived from heterostylism.--The means by which plants became
heterostyled.--Transmission of form.--Equal-styled varieties of heterostyled
POLYGAMOUS, DIOECIOUS, AND GYNO-DIOECIOUS PLANTS.
The conversion in various ways of hermaphrodite into dioecious plants.--
Heterostyled plants rendered dioecious.--Rubiaceae.--Verbenaceae.--Polygamous
and sub-dioecious plants.--Euonymus.--Fragaria.--The two sub-forms of both sexes
of Rhamnus and Epigaea.--Ilex.--Gyno-dioecious plants.--Thymus, difference in
fertility of the hermaphrodite and female individuals.--Satureia.--Manner in
which the two forms probably originated.--Scabiosa and other gyno-dioecious
plants.--Difference in the size of the corolla in the forms of polygamous,
dioecious, and gyno-dioecious plants.
General character of cleistogamic flowers.--List of the genera producing such
flowers, and their distribution in the vegetable series.--Viola, description of
the cleistogamic flowers in the several species; their fertility compared with
that of the perfect flowers.--Oxalis acetosella.--O. sensitiva, three forms of
observations on various other cleistogamic plants.--Anemophilous species
producing cleistogamic flowers.--Leersia, perfect flowers rarely developed.--
Summary and concluding remarks on the origin of cleistogamic flowers.--The chief
conclusions which may be drawn from the observations in this volume.
THE DIFFERENT FORMS OF FLOWERS ON PLANTS OF THE SAME SPECIES.
The subject of the present volume, namely the differently formed flowers
normally produced by certain kinds of plants, either on the same stock or on
distinct stocks, ought to have been treated by a professed botanist, to which
distinction I can lay no claim. As far as the sexual relations of flowers are
concerned, Linnaeus long ago divided them into hermaphrodite, monoecious,
dioecious, and polygamous species. This fundamental distinction, with the aid of
several subdivisions in each of the four classes, will serve my purpose; but the
classification is artificial, and the groups often pass into one another.
The hermaphrodite class contains two interesting sub-groups, namely,
heterostyled and cleistogamic plants; but there are several other less important
subdivisions, presently to be given, in which flowers differing in various ways
from one another are produced by the same species.
Some plants were described by me several years ago, in a series of papers read
before the Linnean Society, the individuals of which exist under two or three
forms, differing in the length of their pistils and stamens and in other
respects. (Introduction/1. "On the Two Forms or Dimorphic Condition in the
Species of Primula, and on their remarkable Sexual Relations" 'Journal of the
Proceedings of the Linnean Society' volume 6 1862 page 77. "On the Existence of
Two Forms, and on their Reciprocal Sexual Relation, in several Species of the
Genus Linum" Ibid volume 7 1863 page 69. "On the Sexual Relations of the Three
Forms of Lythrum salicaria" Ibid volume 8 1864 page 169. "On the Character and
Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic
and Trimorphic Plants" Ibid volume 10 1868 page 393. "On the Specific
Differences between Primula veris, Brit. Fl. (var. officinalis, Linn.), P.
vulgaris, Brit. Fl. (var. acaulis, Linn.), and P. elatior, Jacq.; and on the
Hybrid Nature of the Common oxlip. With Supplementary Remarks on Naturally
Produced Hybrids in the Genus Verbascum" Ibid volume 10 1868 page 437.) They
were called by me dimorphic and trimorphic, but have since been better named by
Hildebrand, heterostyled. (Introduction/2. The term "heterostyled" does not
express all the differences between the forms; but this is a failure common in
many cases. As the term has been adopted by writers in various countries, I am
unwilling to change it for that of heterogone or heterogonous, though this has
been proposed by so high an authority as Professor Asa Gray: see the 'American
Naturalist' January 1877 page 42.) As I have many still unpublished observations
with respect to these plants, it has seemed to me advisable to republish my
former papers in a connected and corrected form, together with the new matter.
It will be shown that these heterostyled plants are adapted for reciprocal
fertilisation; so that the two or three forms, though all are hermaphrodites,