« PreviousContinue »
them is not observable: but in Taxus, Juniperus, Cupressus, Thuja, the outer membrane has, as Mohl states, so little extensibility, that it is torn irregularly, and the inner membrane protrudes beyond the crevices, and, swelling more and more, generally disengages itself from the extine. It sometimes happens that the inner membrane protrudes beyond the outer shell, in the form of a short sac or tube: this phenomenon may be produced artificially at will by placing the pollen-grain in weak nitric or sulphuric acid; but it is quite a distinct emission from that of the pollen-tubes hereafter to be noticed.
A third membrane, intermediate between the extine and intine, was first noticed by Mohl in the pollen of Taxus, Juniperus, Cupressus, and Thuja. Fritzsche calls it the Exintine, and finds it not only in these plants, but also in Pinus, Cucurbita Pepo, and Tigridia Pavonia, and considers it probably a
structure. The same minute observer speaks of four coatings to the pollen of Clarkia elegans, calling the fourth, which is next the extine, the Intexine ; he also finds the same structure in other Onagraceæ.
Mohl names Asclepiadaceous plants as those only in which pollen has but one tunic; but Fritzsche asserts that these plants have both an extine and intine, and he figures them in Asclepias syriaca; he adds, that in Caulinia fragilis, Zannichellia pedunculata, Zostera marina, and Naias minor, the pollen has really nothing but the intine present.
There are few forms of pollen in which the extine presents the appearance of a vesicle completely closed. In many cases the grains are marked by a longitudinal furrow on one side, and look when dry like a grain of wheat. Mr. Griffith has shown, as is above stated, that this appearance is caused by a fissure in the extine, and if such pollen is put into water the fissure becomes less visible. The presence of one or more such clefts is of common occurrence; and have been supposed to be openings through the extine down to the intine; Mohl, however, considers them not to be really openings, but only extremely thin spaces in the extine. Fritzsche calls them pores, and regards them as certainly openings. Instead of slits many kinds of pollen have circular holes, varying in number, in different species, from one to an indefinite quantity in Alcea rosea. Under these holes Fritzsche finds small, plano-convex, lenticular bodies (zwischenkörpern), lying between the extine and the intine, with their convexity reposing upon the latter; he represents them as particularly visible and large in some Malvaceæ.
The colour of pollen is chiefly yellow. In Epilobium angustifolium and many Polemoniaceæ it is blue; in Verbascum it is red; and it occasionally assumes almost every other colour, except green. According to Fourcroy and Vauquelin, the pollen of the Date tree consists of malic acid, phosphate of magnesia and lime, and also of an insoluble animal matter intermediate between gluten and albumen. Macaire Prinsep has ascertained that the pollen of the Cedar contains malate of potass, sulphate of potass, phosphate of lime, silica, sugar, gum, yellow resin, and a substance which, by its characters, approximated to starch. Being analysed as a whole, it gave, per cent., 40 carbon, 11:7 hydrogen, and 48:3 oxygen, but no nitrogen. — Bibl. Univers. 1830. 45.
The matter contained in the granules is called the fovilla. Under common magnifiers it appears like a turbid fluid; under glasses of greater power it has been found to consist of a multitude of particles moving on their axes with activity, of such excessive minuteness as to be invisible unless viewed with a magnifying power equal to 300 diameters, and measuring from the 4000th or 5000th to the 20,000th or 30,000th of an inch in length. This motion was first distinctly noticed by Gleichen; but it seems to have escaped the recollection of succeeding botanists until the fact was confirmed by Amici, who some time before 1824 saw and described a distinct, active, molecular motion in the pollen of Portulaca oleracea. In 1825 the existence of this motion was confirmed by Guillemin, who ascertained its presence in other species. In June 1827, I was shown the motion by Dr. Brown, who subsequently published some valuable observations upon the subject, without, however, noticing those of either Amici or Guillemin. The most important addition that was made by Brown to the knowledge that previously existed, consisted in the discovery of the presence of two kinds of active particles in pollen; of which, one is spheroidal, extremely minute, and not dis
tinguishable from the moving, ultimate, organic molecules common to all parts of a vegetable; the other, much larger, often oblong, and unlike any other kind of particle hitherto detected in plants. Clarkia pulchella, and some other Onagraceous plants, show this difference, as well as the motion, in a very conspicuous manner. In consequence of their manifest motion it has been conjectured that the larger particles of the fovilla were the incipients of the embryo, and that it is by the introduction of one or more of these into the ovule that the act of impregnation is accomplished by the deposit of a rudimentary embryo in the ovule. But both Fritzsche and Mohl agree in considering many of the smaller 'particles of the fovilla as minute drops of oil : the molecular motion has been ascribed to currents in the fluid, in which the fovilla is suspended, and which, according to Fraunhofer, no precautions can possibly prevent; and, what is more important, the larger particles become blue upon the application of iodine, without however losing their property of motion, as Fritzsche has shown: they are therefore starch.
When the pollen falls upon the stigma it emits a fine transparent tube, which is a prolongation of the intine, and down which the fovilla passes until the grain is emptied. The pollen-tube thus formed was first observed by Amici, and is now known to be constantly produced at the period of impregnation. Of the important offices these tubes have to perform an account will be given in Book II. Chap. vi.
For further information concerning pollen the reader is referred to the following works:
1. Fritzsche, De Plantarum Polline : Berolini, 1833. This ingenious observer found that several modes of examining pollen are preferable to those usually employed: in particular he recommends the employment of sulphuric acid, in the proportion of two parts of concentrated acid to three parts of water, for the purpose of viewing the pollen by transmitted light; by this means it is rendered transparent, and the spontaneous emission of pollen-tubes is effected. In cases of very opaque pollen he employs oil instead of diluted acid, and he finds it renders an object more transparent than the acid itself; and in other cases, where the coat of the pollen is either too much or too little transparent to show the apertures in its sides, he finds a solution of iodine in weak spirits of wine extremely useful.—2. Mohl, Sur la Structure et les Formes des Grains de Pollen : translated from the German in the Annales des Sciences, n. s., III. 148.- And, 3., to Fritzsche, Ueber der Pollen : 4to, St. Petersburgh, 1837, with thirteen coloured plates. In this last excellent work all the refinements of optical instruments and chemical manipulation have been employed in the investigation of the subject.
10. Of the Disk.
By this term are meant certain bodies or projections, situated between the base of the stamens and the base of the ovary, but forming part with neither ; they are referred by the school of Linnæus, along with other things, to nectarium : Link calls them sarcoma and perigynium ; and Turpin, phycostemones. The most common form is that of a fleshy ring, either entire or variously lobed, surrounding the base of the ovary (Plate V. fig. 4. e, 8. d.) as in Lamium, Cobæa, Gratiola, Orobanche, &c.; in Gesneraceæ and Proteaceæ the disk consists of fleshy bodies of a conical figure, which are usually called glandula hypogynæ. It occasionally assumes the appearance of a cup, named by De Candolle, in Pæonias and Aconites, lepisma, a bad term, for which it is better to say discus cyathiformis. In flowers with an inferior ovary (Plate V. fig. 9. c, 7. c) the disk necessarily ceases to be hypogynous, and generally also to appear in the form of scales. In Compositæ it is a fleshy solid body, interposed between the top of the ovary and the base of the style ; and has given rise, when much enlarged, to the unfounded belief in the existence of a superior ovary in that order, as in Tarchonanthus. In Apiaceæ it is dilated, and covers the whole summit of the ovary, adhering firmly to the base of the styles; by Hoffmann it is then called stylopodium, a word which is seldom used.
It is an opinion which daily gains ground, that the disk is really only a rudimentary state of the stamens; and it is thought that proofs of the correctness of this hypothesis are to be found in the frequent separation of the cyathiform disk
into bodies alternating with the true stamens, as in Gesnera; in its resemblance in Parnassia to bundles of polyadelphous stamens; and particularly in the fact noticed by Brown, that an anther is occasionally produced upon the highly developed disk of Pæonia Moutan. To which may be added the observation of Dunal, that half the disk of Cistus vaginatus occasionally turns into stamens. (Considerations, &c.,
Like the petals, sepals, and stamens, the disk always originates below the pistil; but it often contracts an adhesion with the sides of the calyx, when it becomes perigynous, as in Amygdalus; or with both the calyx and the sides of an inferior ovary, when it becomes epigynous, as in umbelliferous plants.
11. Of the Pistil.
The last organ to enumerate in the flower is that which constitutes the female system, or gynæceum of Röper, and which is usually called the pistil. In all cases it occupies the centre of the flower, terminating the axis of growth of the peduncle : and is consequently the part around which every other organ, without exception, is arranged.
It is distinguished into three parts; viz. the ovary (Plate V. fig. 7. a), the style (fig. 7. f), and the stigma (fig. 7. g).
The ovary, called germen by Linnæus, is a hollow case placed at the base of the pistil, enclosing the ovules, and always containing one or more cells or cavities. It is the part which ultimately becomes the fruit; and consequently, whatever may be the structure of the ovary, such must necessarily be that of the fruit: allowance being made, as will hereafter be explained, for changes that may occur during the progress of the ovary to maturity.
Notwithstanding what has been stated of the pistil constantly occupying the centre of the flower, and being the part around which all the other parts are arranged, an apparent exception exists in those flowers the calyx of which is said to