Chapter 38

HETEROGAMY.

This term is here intended to apply to all those cases in which the arrangement of the sexual organs is different from what it is habitually. It is evident that in many instances there is no malformation, no monstrosity, but rather a restoration of organs habitually suppressed, a tendency towards structural completeness rather than the reverse. It must be also understood that the following remarks apply to structural points only, and are not intended to include the question of function. The occurrence of heteromorphic unions renders it necessary to keep in mind that plants hermaphrodite as to structure are by no means necessarily so as to function.

The simplest case of this alteration in the relative position of the sexes is that which occurs in monoecious plants, where the male and female flowers have a definite position, but which in exceptional instances is altered.

=Change in the relative position of male and female flowers= may thus occur in any monoecious plant. Cultivated maize, _Zea Mays_, frequently exhibits alterations of this kind; under ordinary circumstances, the male inflorescence is a compound spike, occupying the extremity of the stem, while the female flowers are borne in simple spikes at a lower level, but specimens may now and then be found where the sexes are mixed in the same inflorescence; the upper branching panicle usually containing male flowers only, under these circumstances, bears female flowers also.[190] In like manner, but less frequently, the female inflorescence occasionally produces male flowers as well.

Among the species of _Carex_ it is a common thing for the terminal spike to consist of male flowers at the top, and female flowers at the base; the converse of this, where the female flowers are at the summit of the spike, is much more uncommon. An illustration of this occurrence is given in the figure (fig. 100). Among the _Coniferæ_ numerous instances have been recorded of the presence of male and female flowers on the same spike, thus Mr. now Professor Alexander Dickson exhibited at the Botanical Society of Edinburgh in July, 1860, some malformed cones of _Abies excelsa_, in which the inferior part of the axis was covered with stamens, whilst the terminal portion produced bracts and scales like an ordinary female cone. The stamens of the lower division were serially continuous with the bracts above. Some of the lower scales of the female portion were in the axils of the uppermost stamens, which last were somewhat modified, the anther cells being diminished, whilst the scale-like crest had become more elongated and pointed, in fact, more or less resembling the ordinary bracts.[191] Mohl, Schleiden, and A. Braun have observed similar cones in _Pinus alba_, and Cramer figures and describes androgynous cones in _Larix microcarpa_. C. A. Meyer ('Bull. Phys. Math.,' t. x, 1850) also describes some catkins of _Alnus fruticosa_ which bore male flowers at the top, and female flowers at the base.

On the subject of this section the reader may consult A. Braun,. 'Das Individ.,' 1853, p. 65. Caspary, 'De Abietin. flor. fem. struct. morphol.' Schleiden. 'Principles,' English edition, p. 299. Mohl, 'Verm. Schrift.,' p. 45. Meyen in 'Wiegm. Archiv.,' 1838, p. 155. Cramer, 'Bildungsabweich,' p. 4, tab. v, figs. 13-17. Parlatore, 'Ann. Sc. Nat.,' ser. iv, vol. xvi, p. 215, tab. 13A. See also under the head of Prolification, Substitutions, &c.

=Change from the monoecious to the dioecious condition.=--This is of less frequent occurrence than might have been anticipated. In the 'Gardeners' Chronicle,' 1847, pp. 541 and 558, several instances are noted of walnut trees bearing female flowers to the exclusion of males. The mulberry tree has also been noticed to produce female blossoms only, while in other plants male flowers only are developed.

It seems probable that the age of the plant may have something to do with this production of flowers of one sex to the exclusion of the other.

=Change from the dioecious to the monoecious condition.--Androgynism.=-- This is of far more common occurrence than the preceding.

In the hop (_Humulus Lupulus_), when monoecious, the female catkins are usually borne on the ends of the branches as shown in the cut (fig. 101), and a similar thing has been noticed in _Urtica dioica_ by Clos, 'Bull. Soc. Bot. France,' vol. 9, p. 7.

Baillon ('Etudes du groupe des Euphorbiacées,' p. 205) mentions the following species of that order as having been seen by him with monoecious inflorescence: _Schismatopera distichophylla_, _Mozinna peltata_, _Hermesia castaneifolia_. Oliver mentions ('Hook. Icon. Plant.,' t. 1044) that in _Leitneria floridana_ the upper scales of the male catkin occasionally subtend an ovary.

It would seem that external conditions have some effect in determining the formation of one sex, as in some species of _Carex_, while in the case of _Salix repens_, Hampe[192] says that when grown partially or for a time under water, those twigs which are thrust up above the surface bear female flowers, while those twigs that blossom after the water is dried up, produce male flowers only.

Carrière[193] says that a plant of _Stauntonia latifolia_ which for some years produced stamens only, now produces flowers of both sexes; it was dioecious, but is now monoecious. The same author alludes to a similar occurrence in _Juniperus Virginiana_. The hops is also said to vary in sexual characteristics from time to time.[194] In addition to the genera, already named, in which this production of flowers of both sexes has been observed may be mentioned _Taxus! Gunnera! Urtica! Mercurialis! Restio! Cannabis! Salix! Humulus!_ as well as others in which the change is less frequent.

Among cryptogams a similar change occurs. As an illustration may be cited _Leucobryum giganteum_, as quoted from Müller in Henfrey's 'Botanical Gazette,' i, p. 100.

As to androgynous willows, in addition to the references given under the head of Substitution of stamens for pistils, see Schlechtendal, 'Flora Berol.,' ii. p. 259. Tausch, 'Bot. Zeit.,' 1833, i. p. 229. Koch, 'Synops. Flor. Germ.,' 740. Host, 'Flor. Aust,.' ii, p. 641 (_S. mirabilis_). See also Hegelmaier, 'Württemberg Naturwissenshaft Jahreshefte,' 1866, p. 30. Other references to less accessible works are given in 'Linnæa,' xiv, p. 372.

=Change from hermaphroditism to unisexuality.=--Many flowers ordinarily hermaphrodite as to structure, become unisexual by the abortion or suppression of their stamens, or of their carpels, as the case may be. This phenomenon is lessened in interest since the demonstration of the fact by Darwin and others, that many plants, structurally hermaphrodite, require for the full and perfect performance of their functions the cooperation of the stamens and pistils, belonging to different individuals of the same species.

Some of the _Ranunculaceæ_ constantly exhibit a tendency towards the dioecious condition, and the rarity with which perfect seeds of _Ranunculus Ficaria_ are formed is to be attributed, in great measure, to the deficiency of pollen in the anthers of these flowers. _Ranunculus auricomus_ also is frequently sterile. Specimens of _Ranunculus bulbosus_ may be met with in which every flower is furnished with carpels, most of which have evidently been fertilised, although there are no perfect stamens in the flowers.

Knight and other vegetable physiologists have been of opinion that a high temperature favours the production of stamens, while a lower degree of heat is considered more favorable to the production of pistils, and in this way the occurrence of "blind" strawberries has been accounted for. Mr. R. Thompson, writing on this subject, speaks of a plantation of Hautbois strawberries which in one season were wholly sterile, and accounts for the circumstance as follows: the plants were taken from the bearing beds the year previous, and were planted in a rich well-manured border, in which they started rapidly into too great luxuriance, the growth being to leaves rather than to fruit. The following season these same plants bore a most abundant crop, hence these plants were accidentally prevented from perfecting their female organs.[195]

Mr. Darwin[196] cites from various sources the following details relating to strawberries which it may be useful to insert in this place, as throwing some light upon the production of unisexual flowers. "Several English varieties, which in this country are free from any such tendency, when cultivated in rich soils under the climate of North America commonly produce plants with separate sexes. Thus, a whole acre of Keen's seedlings in the United States has been observed to be almost sterile in the absence of male flowers; but the more general rule is, that the male plants over-run the females.... The most successful cultivators in Ohio plant, for every seven rows of pistillate flowers, one row of hermaphrodites, which afford pollen for both kinds; but the hermaphrodites, owing to their expenditure in the production of pollen, bear less fruit than the female plants."

_Stratiotes aloides_ has been said to produce its carpels with greater abundance towards the northern limits of its geographical distribution, and its stamens, on the other hand, are stated to be more frequently developed in more southern districts.

_Honckenya peploides_ affords another illustration of the sexual arrangements in the flower being altered as it would seem by climatal conditions. Thus, in the United States, according to Professor Asa Gray, the flowers are frequently hermaphrodite, while in this country they are usually sub-dioecious.[197]

Treviranus[198] says that the flowers of _Hippuris_ and _Callitriche_ are apt to be hermaphrodite in summer, but female only at a later period.

For further remarks on this subject, see sections relating to suppression of stamens and pistils.

=Change from unisexuality to hermaphroditism.=--This occurrence depends on one of two causes, either organs are developed (stamens or pistils as the case may be), which are habitually absent in the particular flower; or some of the stamens may be more or less completely converted into or replaced by pistils, or _vice versâ_.

The first condition is the opposite of suppression; it is, as it were, a restoration of symmetry, and might be included under the head of regular peloria, inasmuch as certain organs which habitually undergo suppression at a certain stage in their development, by exception, go on growing, and produce a perfect, instead of an imperfect flower. In teratological records it is not always stated clearly to which of the two above-named causes the unusual hermaphroditism belongs, though it is generally easy to ascertain this point. Very many, perhaps all, diclinous flowers may, under certain conditions, become perfect, at least structurally. I have myself seen hermaphrodite flowers in _Cucurbita_,[199] _Mercurialis_, _Cannabis_, _Zea Mays_, and _Aucuba japonica_, as well as in many _Restiaceæ_, notably _Cannamois virgata_ and _Lepyrodia hermaphrodita_. _Spinacia oleracea_, _Rhodiola rosea_, _Cachrys taurica_, and _Empetrum nigrum_ are also occasionally hermaphrodite.

Gubler[200] alludes to a similar occurrence in _Pistacia Lentiscus_, wherein, however, he adds that there was a deficiency of pollen in the flowers.

Schnizlein[201] observed hermaphrodite flowers in the beech, _Fagus sylvatica_, the ovaries being smaller than usual, and the stamens epigynous.

Baillon[202] enumerates the following _Euphorbiaceæ_ as having exceptionally produced hermaphrodite flowers, _Crozophora tinctoria_, _Suregada_ sp., _Phyllanthus longifolius_, _Breynia_ sp., _Philyra brasiliensis_, _Ricinus communis_, _Conceveiba macrophylla_, _Cluytia semperflorens_, _Wall_, non _Roxb_. _Mercurialis annua_ and _Cleistanthus polystachyus_.

In some of these cases the hermaphroditism is due to the development of anthers on the usually barren staminodes, though, in other cases, the stamens would seem to be separate, independent formations, as they do not occupy the same relative position that the ordinary stamens would do if developed.[203]

Robert Brown[204] observed stamens within the utricle of _Carex acuta_, and Gay is stated by Moquin ('El. Ter. Veg.,' p. 343) to have observed a similar occurrence in _Carex glauca_.

Paasch[205] observed a similar occurrence in _C. cæspitosa_, and Schauer, in _C. paludosa_,[206] though in the latter instance the case seems to have been one of transformation or substitution rather than one of hermaphroditism.

The second cause of this pseudo-hermaphroditism is due either to the more or less perfect mutation of male and female organs, or it may be to the complete absence of one and its replacement by another, as when out of many stamens, one or more are deficient, and their places occupied by carpels. This happens very frequently in willows and poplars, and has been seen in the beech.[207]

In _Begonia frigida_[208] the anomaly is increased by the position of the ovaries above, the perianth, a position due, not to any solution or detachment of the latter from the former, but simply to the presence of ovaries where, under ordinary circumstances, stamens only are formed, as happened also in a garden variety of a _Fuchsia_, wherein, however, the change was less perfect than in the _Begonia_, and in which, as the flower is naturally hermaphrodite, the alteration is of the less importance.

In hermaphrodite flowers of _Carica Papaya_ (fig. 103) there is a single row of five stamens instead of two rows of five each as in the normal male flowers, the position of the second or inner row of stamens being occupied by five carpels, which, however, are not adherent to the corolla as the stamens are, thus, supposing the arrangement of parts in the normal male flowers to be as follows:

--------------------------- s s s s s --------------------------- | p p p p p | st st st st st | st st st st st |

That of the hermaphrodite blossoms would be, in brief, as follows:

| 5 s |------------ | 5 p | 5 st | 5 c |

One of the most curious cases of this kind recorded is one mentioned by Mr. Berkeley,[209] wherein a large white-seeded gourd presented a majority of flowers in which the pollen was replaced by ovules. It would seem probable from the appearances presented by the figure that these ovules were, some of them, polliniferous, like those of the _Passiflora_, &c., described at p. 185, but nothing is stated on the subject.

See also section on Regular Peloria, Substitution, Pistillody of the stamens, &c.

FOOTNOTES:

[190] See also Clos., 'Mem. Acad. Toulouse,' sixth ser., t. iii, pp. 294-305. Scott, 'Trans. Bot. Soc. Edinburgh,' t. viii, p. 60. Wigand, 'Flora,' 1856, p. 707.

[191] Professor Dickson concludes from the examination of these structures that the male cone, consisting of simple stamens developed on one common axis, must be regarded as a simple male flower, while the axillary scales of the female cone are by him compared with the flattened shoots of _Ruscus_.

[192] 'Linnæa,' xiv, 367.

[193] Rev. Hortic.,' January, 1867.

[194] See Royle, 'Man. Materia Medica,' ed. 1, p. 567.

[195] Thomson, 'Gardener's Assistant,' p. 577.

[196] 'Variation of Animals and Plants,' i, 353.

[197] Babington, 'Ann. Nat. Hist.,' vol. ix, 1852, p. 156.

[198] 'Phys. der Gewächse,' ii, p. 323.

[199] See also Schlechtendal, 'Linnæa,' viii, p. 623, and Lindley, 'Veg. Kingd.,' p. 315.

[200] 'Bull. Soc. Bot. France,' vol. ix, p. 81.

[201] Cited in Henfrey, 'Bot. Gazette.' 3, p. 11.

[202] Baillon. 'Etudes du Groupe des Euphorbiacées,' p. 205, tab. xv, fig. 19, tab. xix, fig. 31.

[203] See also Guillemin, 'Mém. Soc. Nat. Hist. Paris,' I, p. 16; hermaphrodite flowers in _Euphorbia esula_.

[204] 'Prod. Flor. N. Holl.,' p. 242.

[205] 'Bot. Zeit.,' 1837, p. 335.

[206] 'Pflanz, Terat.,' von Moquin-Tandon, p. 208.

[207] Schnizlein, loc. cit.

[208] 'Bot. Mag.,' tab. 5160, fig. 4. See also 'Gard. Chron.,' 1860, pp. 146, 170; 1861, p. 1092.

[209] 'Gard. Chron.,' 1851, p. 499.