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the ovary, which then begins to become excavated at the base; and after some time an ovule makes its appearance, having its apex directed towards the sphacelated line, and placed in contact with it. Mr. Griffith does not say that this sphacelated line is the course of the pollen tubes, nor that it causes the production of the embryo, as in the last case: but, as eventually an embryo is formed in the ovule at the end of a cellular suspensor communicating with the sphacelated line, it may be reasonably supposed that such is the fact. Mr. Griffith states that, in these two cases, the nucleus of the ovule was originally solid, and that the embryo is subsequently produced in it by an excavating process.
By some it has been thought that the molecular locomotive matter found in the interior of pollen grains represented the germs of future embryos, and that the introduction of one such molecule into an ovule was necessary in order to insure the production of an embryo. But it has been shown that the molecules are starch: upon this matter Schleiden has the following remarks:
“ It appears to me, as if the very minute chemical and microscopical researches of Fritsche on the pollen (Petersburg, 1837) have made an end of the so called pollen animalcules ; for it would be contrary to the laws of animal nature, that the lively motions of these apparent infusoria should continue undisturbed after the addition of alcoholic solution of iodine (a poison that immediately kills all infusoria and animal spermatozoa), as Fritsche states to be the case, and which in many instances I have observed.
“In the Enotheræ, however, to which Meyen has particularly referred, I have not been able to see any thing of pollen animalcules (saamenthierchens); and in these cases the contents of the pollen, quoad solida, also for the greatest part consists of starch. I have, at least, in En. Simsiana, grandiflora, and crassipes, throughout, found nothing else in the pollen besides a solution of gum and those easily recognisable small crescent-formed bodies, which Brongniart has described as pollen animalcules. These are, however, decidedly starch, and continue starch even when the pollen tube is already deep in the nucleus of the ovule. In order, however, in this case, to detect the starch, we must employ the aqueous solution of iodine, for the alcoholic solution in the first place would coagulate the gum, and in the second it colours the starch so deeply that, on account of the smallness of the grains, one can no longer judge of their colour, and as they are entirely surrounded with the gum, they may easily be supposed to be dark brown. The curvilinear motions of these so called pollen animalcules, which are said to have been observed by a good many, are very easily explained, since at least many of them, being crescent-shaped, when in motion, appear bent to the left, the right, or appear straight, according to their position to the eye.”
With respect to the sexuality of plants, that at least would appear, from the facts above recited, to be established beyond the reach of controversy ; but lately there has arisen in Germany a school of Botanists, at the head of which are Schleiden and Endlicher, who either deny it, or assert that the nature of the phenomenon connected with it has been misunderstood.
Schleiden states that, “ if the pollen tubes be followed into the ovule, the most delicate process perhaps that occurs in botanical investigations, it will be found that usually only one, rarely a greater number, penetrates the intercellular passages of the nucleus and reaches the embryo-sac, which, being forced forwards, is pressed, indented, and becomes the cylindrical bag which constitutes the embryo in the first stage of its developement, and which consequently consists solely of a cell of parenchyma supported upon the summit of the axis. This bag is therefore formed of a double membrane (except the open radicular end), viz. the indented embryo-sac and the membrane of the pollen tube itself. In Taxus, and especially in Orchis, he has been able to withdraw out of the embryo-sac that portion of the tube which represents the first stage of the embryo, and that indeed at a tolerably advanced period.
“ The tracing of the pollen tube into the interior of the embryo-sac is not so easy in all plants ; because the cells of the nucleus which are arranged around the summit of the embryosac are very firm and opake, so that it and the pollen tube cannot be exhibited quite free. In these cases, however, three circumstances speak for the identity of the embryo with the pollen tube. 1. The constantly equal diameter of the latter, exterior to the embryo-sac, and of the former, just within it. 2. The invariable chemical similarity of their contents, shown by the reaction produced by the application of water, oil of sweet almonds, iodine, sulphuric acid, and alkalies. The general contents of the grain of pollen is starch; and this either proceeds unchanged downwards through the pollen tube, or else passes along, after being changed by a chemico-vital process into a transparent and colourless fluid, which becomes gradually more and more opake, and is coagulable by the application of alcohol: out of this, by an organising process, the cells are produced which fill the end of the pollen tube, extending, in Orchis Morio, far beyond the ovule, and thus forming the parenchyma of the embryo. 3. The identity of the embryo and the pollen tube is farther supported by the fact, that, in such plants as bear several embryoes, there is always precisely the same number of pollen tubes present as we find embryoes developed.
“ The most important result of these facts is, that the sexual classification hitherto adopted in botany is directly false : for, if the ovulum be understood in physiology to represent that material foundation from which the new being becomes immediately developed, and if we term that portion of the organism in which this material commencement is deposited before it becomes developed the female organ, whilst that part which calls into action or promotes the developement of the germ by means of its potential effects is termed the male organ, it is evident that the anther of the plant is nothing but a female ovarium, and each grain of pollen the germ of a new individual. On the other hand, the embryo-sac only works potentially, determining the organisation and developement of the material foundation; and for this reason, therefore, ought to be termed a male principle, were we not to consider, perhaps more correctly (without embarrassing ourselves with lame analogies taken from the animal kingdom), that the embryosac merely conveys new organisable Auids by means of transudation, and thus only serves the office of nourishment.
“In the next place, the process of developement of the embryo, as already described, easily establishes the analogy of Phanerogamous plants and those Cryptogamic plants in which the spores are evident conversions of the cellular tissue of the foliaceous organs or leafy expansions; for the same part furnishes the groundwork of a new plant in both groups, and the only difference existing between the two is this; in Phanerogamæ a previous formative process in the interior of the plant precedes a period of latent vegetation, whilst in Cryptogamæ the spore (the grain of pollen) developes itself as a plant without previous preparation. Difficulties nevertheless occur here in the consideration of Mosses and Hepaticæ, and more particularly in the enigmatical Marsileaceæ. It appears to me, however, that in this last-named family especially, there still remains much to be observed.”
The opinion of Endlicher is to a certain extent that of Schleiden; that is to say, he considers what we call pollen analogous to the spores of Cryptogamic plants, and consequently the anther a female organ, whose contents perform an act similar to that of germination, when they fall upon the stigma; he does not, however, with Schleiden, assign a male influence to the sac of the amnios, but he attributes that property to the stigmatic papillæ, whose moisture lubricates the grains of pollen when they fall upon them.* I know of no one else who maintains this last opinion; but it deserves to be noted that Morren observed a circulating movement (he calls it cyclosis) in the fluid filling the papillæ of Cereus grandiflorus at the period of impregnation.
One of the most curious consequences of the presence of
* See Grundzüge einer neuen Theorie der Pflanzenzeigung. Professor Wydler of Berne, also, insists upon the pollen being the female apparatus, and he denies that plants have two sexes. (Recherches sur l’Ovule, &c., des Scrofulaires.) These speculations have all arisen out of the undoubted fact, that the developement of spores and pollen grains takes place in the same manner, and that there is considerable resemblance in their final structure. This was, I think, first noticed by Mohl (Ueber die Entwicklung der Sporen, fc.), in 1833; Mirbel, in 1835, stated that there was a marvellous resemblance between these parts (Ann. Sc., n.s., iv. 9.); Morren declares that the spore is organised like a grain of pollen (Anat. des Jungermann. p. 10.); and, finally, Wydler admits a great analogy between the formation of pollen and the spores of many foliaceous cryptogamic plants. sexes in plants is, the property the latter consequently possess of producing mules. It is well known, that, in the animal kingdom, if the male and female of two distinct species of the same genus breed together, the result is an offspring intermediate in character between its parents, but uniformly incapable of procreation, unless with one of its parents; while the progeny of varieties of the same species, however dissimilar in habit, feature, or general characters, is in all cases as fertile as the parents themselves. A law very similar to this exists in the vegetable kingdom.
Two distinct species of the same genus will often together produce an offspring intermediate in character between themselves, and capable of performing all its vital functions as perfectly as either parent, with the exception of its being unequal to perpetuating itself permanently by seed; should it not be absolutely sterile, it will become so after a few generations. It may, however, be rendered fertile by the application of the pollen of either of its parents; in which case its offspring assumes the character of the parent by which the pollen was supplied. This power of hybridising appears to be far more common in plants than in animals; for, while only a few animal mules are known, there is scarcely a genus of domesticated plants in which this effect cannot be produced by the assistance of man, in placing the pollen of one species upon the stigma of another. It is, however, in general only between nearly allied species that this intercourse can take place: those which are widely different in structure and constitution not being capable of any artificial union. Thus the different species of Strawberry, of certain tribes of Pelargonium, and of Cucurbitaceæ, intermix with abundant facility, there being a great accordance between them in general structure and constitution; but no one has ever succeeded in compelling the Pear to fertilise the Apple, or the Gooseberry the Currant. And as species that are very dissimilar appear to have some natural impediment which prevents their reciprocal fertilisation, so does this obstacle, of whatever nature it may be, in general present an insuperable bar to the intercourse of different genera. All the stories that are current as to the intermixture of Oranges and Pome