The Different Forms of Flowers on Plants of the Same Species — A Reader’s Guide

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Authors: Darwin, Charles, 1809-1882 Digital Source: Project Gutenberg Gutenberg Release Year: 2003 First Publication Year: Not confirmed

Key Word: Plants -- Variation; Fertilization of plants; Heterostylism; Plant physiology; Pollination Target Reader: Readers of public-domain and historical texts

Book Edition: Project Gutenberg digital edition Book Language: en

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Words 106,445

Reading time 463 min

Text sections 11

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Source: Project Gutenberg record #3807

Darwin's meticulous study of heterostyly in plants, examining structural variations in flowers of the same species and their effects on fertility, based on extensive experiments and observations.

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CHAPTER I. 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 palustris.--Androsace vitalliana.

CHAPTER II. HYBRID PRIMULAS.

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 genus Verbascum.

CHAPTER III. 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.

CHAPTER IV. 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.

CHAPTER V. 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.

CHAPTER VI. 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 plants.--Final remarks.

CHAPTER VII. 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.

CHAPTER VIII. CLEISTOGAMIC FLOWERS.

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 cleistogamic flowers.--Vandellia.--Ononis.--Impatiens.--Drosera.--Miscellaneous 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.

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Darwin opens this work with a detailed examination of heterostyled dimorphic plants, focusing on the Primulaceae family. He describes the structural differences between the two forms of Primula veris, noting variations in pistil length, stamen position, and pollen grain size. Through controlled pollination experiments, he demonstrates that legitimate unions between opposite forms yield higher seed sets than illegitimate self- or same-form fertilizations. The text is dense with numerical data, such as average seed counts per capsule, reflecting Darwin's empirical approach.

Structural Variations and Their Functional Roles

Darwin meticulously catalogs the physical differences between floral forms across numerous species. In Primula, he notes that long-styled flowers have stigmas positioned above the anthers, while short-styled forms have the reverse arrangement. He measures pollen grain diameters, finding consistent size differences between the two forms. For instance, in Linum grandiflorum, the long-styled form is utterly sterile with its own pollen, a striking exception. Darwin also observes torsion of pistils in Linum perenne long-styled forms alone, a detail that hints at mechanical adaptations for cross-pollination. These structural descriptions are not mere taxonomy; they are tied directly to functional outcomes in fertility experiments.

Fertility Experiments and Numerical Evidence

Throughout the text, Darwin presents extensive tables and counts of seeds produced under different crossing regimes. For example, in Primula veris, legitimate crosses yield an average of 9.2 seeds per capsule, while illegitimate ones produce fewer. He also reports percentages of flowers that set capsules under netting versus open pollination, as with Oxalis where only 55% of protected flowers yielded capsules. These numbers serve as the backbone of his argument that heterostyly promotes cross-fertilization. Darwin is careful to note exceptions, such as the equal-styled variety of P. Sinensis, which shows no signs of heterostyly in function.

Geographic and Taxonomic Range of Heterostyly

Darwin extends his investigation beyond familiar European plants to include species from Brazil and other regions. He discusses Pontederia, an aquatic monocotyledon, noting that only two of the three expected forms were found in the wild, yet the mid-styled form exists in cultivation and is self-sterile. This case is particularly interesting because heterostyly is rare among monocots, and the flowers are irregular, unlike the symmetrical blooms of most heterostyled plants. Darwin also examines Faramea (Rubiaceae), where pollen grains differ remarkably between forms and stamens exhibit torsion only in the short-styled form. These examples illustrate the diversity of heterostylous adaptations across plant families.

Comparative Analysis and Theoretical Implications

Darwin frequently compares his findings with those of other researchers, such as Hildebrand and Fritz Müller. He notes that in Oxalis, there is no evidence of a trend toward dioecy, contrary to earlier claims. He also discusses hybrid primulas, showing that the oxlip is a natural hybrid between P. veris and P. vulgaris, and that its fertility patterns reflect its mixed ancestry. These comparative analyses strengthen his overarching conclusion that heterostyly is an adaptation to ensure cross-fertilization, not a step toward separate sexes. Darwin’s reasoning is cautious, often stating that further experiments are needed to confirm certain points.

Readers approaching this work should be prepared for a data-rich narrative that moves from detailed descriptions of floral morphology to quantitative fertility experiments. Darwin’s prose is precise, and his arguments are built on cumulative evidence rather than sweeping claims. The book rewards careful attention to the tables and measurements, which reveal the patterns underlying floral diversity. It stands as a model of empirical botanical research.

That damp afternoon, I lost myself in Darwin’s patient counting of primrose pistils and stamens, his quiet awe at such precise, hidden machinery. Closing it, my gaze fell on another volume, waiting unread: Proserpina, Volume 1 Studies of Wayside Flowers, While the Air was Yet Pure Among the Alps and in the Scotland and England Which My Father Knew — Inside the Classic. It felt less like a departure than a quieter room in the same house.