Parthenogenesis (Gr. , virgin, and , birth), a name given to the phenomenon in the organic world, believed by many to occur, though still questioned by others, of a production of successive generations of procreating individuals, originating from a single fertilized ovum, but without any renewal, through such series, of fertilization. Ordinarily careful observations seem at first to result in the rule that, certainly in the animal realm, andprobably in the vegetable, offspring can only arise by means of a union of sexual elements, though this union may be either obvious or concealed. Yet there were those among the earlier writers who held to be possible what they called a lucina sine concubitu. M. Bonnet, about the middle of the 18th century, first gave a scientific standing to this opinion, by discovering that the aphis (plant louse) may produce a numerous offspring, and these be followed by several generations, without the intervention in any known or conceivable way of the masculine fertilizing principle. M. de Quatrefages proposed to name this result agamogenesis, or production without union.
The name at the head of this article was applied to certain cases of this kind by Prof. Owen. Of Siebold's work on this subject a translation appeared in London in 1857. Strictly, the name parthenogenesis is hardly appropriate, since either the producers in these cases are not perfect ordinary females, or the production is not that of perfect ordinary offspring; or both these circumstances may be true. Siebold investigated this unisexual, or at least unusual generation in certain sac-bearing lepidoptera, in the silkworm moth, and in the honey bee. In the first, females only result; in the second, both sexes. Along with Dzier-zon, he obtained in relation to the honey bee the most complete set of observations. The queen bee, impregnated once for all for her five or six years of life, deposits thereafter, at proper periods, the germs of successive swarms or colonies; and the microscope reveals the fact that the eggs destined to become workers (imperfect females) and queens (perfect females) are fertilized, as ordinarily, by contact or penetration of spermatozooids, while those to become drones (males) undergo no such influence; so that the production of these last is agamogenetic.
In further proof, if the queen have her wings crippled from the first, so that she takes no flight, she produces only males, thus ruining the hive; and a like result may follow the pinching or freezing of one side of her body, and also, because the spermatozooids have become exhausted, in her old age. So, rarely, the workers may without fertilization produce eggs, but those of males only. But any of these males, though all directly agamic or fatherless, can become efficient in a return to the ordinary or bisexual mode of reproduction. In his more recent work (Leipsic, 1871), Siebold has continued his observations to the wasps (polistes and vespa) and several other insects, showing that the males in many are developed from unfertilized eggs. According to Von Grimm ("Academy," 1870) parthenogenesis occurs in the pupa state in the dipterous genus chironomus, as "Wagner had previously announced in miastor; this kind of reproduction is called by Von Baer pedogenesis. In this insect the formation of two egg-like reproductive bodies begins in the larva, but the eggs are not extruded till the pupa state is reached; and he thinks these cases may be due to self-fecundation. - Bonnet's experiments with the aphis yield, as intimated above, more curious results.
He carefully isolated a newly hatched aphis by conveying it upon a twig beneath a glass shade dipping into water. Of fourscore offspring produced alive by this insect, one was isolated in like manner, and with similar result; and this was repeated as long as the observations continued, or for nine successive broods. As the young aphides are ready for propagation in about two weeks, it follows that in the course of a summer a single parent may have a progeny of millions, and all without renewed intervention of the male element. Kyber found that when warmth and food were abundantly supplied, this agamic production would go on for two or three years; but these broods, winged or wingless, consist almost wholly of imperfect females, seldom any males. The true females, always wingless, produce only after sexual union, and then eggs, not living offspring. And ordinarily, as the cold of autumn increases and the supply of food fails, the agamic young give place to true males and females, the latter laying eggs which the next spring hatch out again viviparous or imperfect females. Thus there is a cycle of changes; a large but varying number of links of non-paternal, being interposed between any two of paternal generation.
The imperfect females have, in place of ovaries, certain tubular organs, the germs lying in which develop into living insects. Thus the case is only apparently, not really, anomalous; the real individual of the aphides is the perfect male or female only, and union of these must occur for the perpetuation of the race; but under favoring conditions, by a sort of exuberance of vital activity, an intercurrent production by gemmation or budding sets in, terminating finally in a return to the normal individual. According to this view, the drone bees are another instance of production by budding; and still others are said to be found in the daphnice (water fleas), and in some species of butterfly. - In plants, the occurrence of parthenogenesis, the development of an embryo in the ovule, and the production of perfect seed without the agency of the pollen or male element, was maintained in the last century by Spallanzani, who cited hemp and spinach as plants, among others, in which this took place. Since then the subject has been discussed by botanists, including some of the most eminent of the present day, without very decisive results; as experiments by different observers upon plants of the same kind have led to decidedly opposite conclusions, the question of the occurrence of parthenogenesis cannot be regarded as settled.
The great difficulty attending experiments on hermaphrodite or bisexual plants has led observers to use those with separate sexes, and monoecious, or more generally dioecious plants, have been selected. A euphorbiaceous shrub from Australia, ccelebogyne (now alchornea) ili-cifolia, produced in Europe female flowers and perfected seed, while no male plant was known to be in the country; the plant was supposed to be perfectly dioecious, neither male flowers nor stamens being detected, and the production of fertile seeds in this case was regarded as proof that, in this' plant at least, the presence of pollen was not necessary to their formation and development. In 1857 Baillon asserted that he had found a stamen in one of the female flowers of ccelebogyne, but this was denied by De-caisne, who asserted that Baillon had mistaken a glanduliferous bract for a stamen; in 1860 Karsten announced that he had discovered two hermaphrodite flowers upon the plant, in the Berlin botanic garden, between May and August, which was regarded as sufficient to account for the fruiting.
It is said that figs developed in summer contain no male flowers, yet the pistils of these produce seed containing an embryo; but both kinds of flowers in the fig are exceedingly small, and being enclosed within the hollow receptacle, accurate observation is surrounded with difficulties. The experiments of Naudin and Decaisne (Paris) with hemp were conducted with female plants, some in the open air surrounded by a high fence, and others in pots placed in a room in the second story of the house; no male flowers could be discovered on these plants, yet all bore fruit, and the female plants from these seeds, similarly isolated, ripened seeds also. On the other hand, Regel of St. Petersburg, in experimenting upon spinach and mercurialis, which Naudin and Decaisne had cited as giving seeds upon the female plant when isolated, cut back his specimens of these in order to reduce the number of flower clusters, and found that in every instance the female plants thus treated produced more or less male flowers, very much reduced and stunted, but with stamens which produced pollen, though the flowers containing them were so insignificant that they might have been unnoticed had not great care been taken in the search.
Another instance cited by Naudin and Decaisne is bryony, a dioecious plant of the gourd family; the pistillate plants of this, from which access of pollen was carefully shut out, produced fruit in the greatest abundance; in 100 of these fruits 12 had no seeds, 45 had one seed, 29 two seeds, 11 three seeds, two had four, and one had five seeds. These illustrations are sufficient to show the difficulties in determining whether perfect seeds are formed without the influence of pollen upon the ovule. Besides the fact that male flowers may sometimes be developed upon female plants, and thus clandestinely supply pollen, there is another which must be taken into account: in flowers of separated sexes rudiments of the organs of the other sex are often distinctly seen; in the staminate flower, a knob or protuberance stands in the place of the pistil, and in pistillate flowers we have the places of the stamens occupied by glands, or abortive filaments, as if one or the other series of organs had been suppressed to make the flower male or female.
The many well known instances in which a plant produces all three kinds of flowers, staminate, pistillate, and perfeet, show that these suppressed organs maybe developed into activity; and this happening in a single flower, or with a single stamen, might, unobserved, produce sufficient pollen to fertilizo every ovary on the plant. Though the evidence cited to prove that parthenogenesis exists in plants may be of doubtful value, there is no good reason why it may not occur; indeed, analogy with animals, and the methods by which some plants reproduce themselves, indicate that its occurrence is not improbable. In many plants, especially some in high latitudes, small bulbs are produced in place of seeds, and in some abnormal flowers buds have been found occupying the place of the ovules, or prospective seeds; a small bulb, or bulblet, consists of several rudimentary leaves crowded upon a very short stem, and a bud has almost the same structure; the embryo within the seed is more simple than the bulblet and the bud, as it consists of a minute stem and only two leaves, or sometimes only one; that this embryo always requires the presence of pollen for its formation, while the more highly developed bulblet or bud is produced without it, is assuming more than some of our most eminent physiologists will admit.
Until within a comparatively short time ferns and other cryp-togamous plants were considered perfectly asexual, but it is now known that some if not all have organs corresponding in function to stamens and pistils; in ferns, for example, the spore produces a cellular plate, a sort of intermediate plant called prothallus, upon the surfaces of which are produoed organs called archegonia, which when fertilized by the contact of antherozoids, produced by other organs upon the prothallus called antheridia, give birth to a new fern, and the prothallus, having served its purpose, disappears; here then is a regular sexual contact, and it has been supposed to be essential to the production of a now plant among ferns. Not long ago Prof. W. G. Far-low, now of Harvard university, discovered minute fern plantlots issuing from a prothallus upon which no antheridia or archegonia were present; and continuing his observations, he found in the same collection of seedlings about 50 which had been developed from prothalli destitute of both sexual organs, and showing very conclusively that in one fern at least asexual production of plants may take place.