« PreviousContinue »
not therefore surprise us that the buds or ovules of these branches grow only upon their inner side, viz. that side directed towards the axis; for the same is observed in the inflorescence of many plants, for instance, in Æsculus. Lastly, in those plants in which the entire wall of the simple ovarium is occupied with ovules, we find the axis expanded somewhat in the shape of a basin, as may also be seen in the similar modification of the stalk in many Rosaceæ and in Ficus.
“ We find moreover in nature, that in parietal placentæ the edges of the leaves are never laid upon one another throughout their entire length, and so adhere to each other; but they become united from below upwards, by the subsequent growth of a more or less distinctly intermediate substance. This substance is very evident in the Fumariaceæ and Cruciferæ, in which it appears much later than the carpellary leaves, stands exactly within them, and in the latter family forms the spurious partition, by its gradual extension towards the middle, and its subsequent adhesion. The placenta shows itself to be independent of the carpellary leaves, during its growth, most strikingly in the Abietineæ. My investigations of the earliest conditions have shown me that the organ which, since the researches of R. Brown, has been considered as an open ovarium, is only a scale-like expanded placenta ; and that the organ which R. Brown has named bractea is the actual carpellary leaf. This result has been confirmed to me, in a most beautiful manner, by a cone of Pinus alba, which upon the upper half was covered with female, and upon the lower with male, flowers. In the Abietineæ, the placenta, left without the least constraint, developes itself to such an extent, that at length the carpellary leaf itself appears as a mere supplementary part.”
Similar views are entertained by Schykofsky, a Russian botanist, who has written a memoir upon the subject in the Bulletin de la Société Imp. des Naturalistes de Moscou, 1837, No. 5. p. 1. tt. 1. and 2.; but, as it is published in the Russian language, I am unable to state the tendency of his arguments. I may also remark that Schleiden's theory is much in accordance with the ideas of the late Professor Richard.
There is undoubtedly a great deal of force in these arguments, and it is extremely probable that the ovule with its placenta is a developement of the axis, and not of the margin of a carpellary leaf, in numerous instances where its origin has been hitherto supposed to be of the latter kind; especially in Compositæ, Graminaceæ, Polygonaceæ, Plumbaginaceæ, Primulaceæ, &c. But it does not follow that, because the placenta has sometimes, it must always have, such an origin. We know that leaves do produce buds, we also know that the axis produces them; there is therefore no reason why the carpels, as well as the point of the axis which they enclose or surround, should not in like manner produce ovules. In fact we have numerous cases of monsters, especially in Crassulaceæ, Amygdaleæ, and Ranunculaceæ, in which the ovules do most certainly grow on the margins of leaves only partially converted into carpels. Moreover Dr. Grisebach, in his excellent . Genera et Species Gentianearum, has shown that the placenta of that order cannot be an expansion of the axis, because the ovula are originally developed in two or three rows on the face of the carpels, forming a line of minute tumours from the base to half-way up the carpels; “quæ quidem series, parenchymate magis inter ovula quam in dorso carpophylli crescente, demùm ipsius superficiem ferè integram subæqualiter obtegunt.”
Dr. Schleiden disregards the cases on record of leaves producing buds: but he will probably reconsider bis opinions upon that point.
12. Of the Receptacle. The part upon which the carpels are seated is the apex of the peduncle, or the summit of the floral branch, of which the carpels are the termination. Usually this part, which is called the receptacle, is flat, or merely a vanishing point; but in other cases it is very much dilated, and then assumes a variety of curious appearances. This receptacle is called torus, or thalamus as well as receptaculum, and in Greek compounds has the name of clinium.
In Anonaceæ and Magnoliaceæ it elevates itself from the
base of the calyx, and bears the numerous stamens peculiar to these orders: here it is called gonophore (gonophorum) by De Candolle. When it is succulent and much dilated, so as to resemble the receptacle of a Composita, bearing at the same time many ovaries, as in the Strawberry and Raspberry, Richard calls it polyphore : most commonly such a receptacle is sufficiently described by the adjective fleshy. If only a single row of carpels developes upon such a receptacle, as in Ochna, and there is an oblique inclination of the carpels towards the axis of the flower, we have the gymobase (Plate V. fig. 3. a); in the Geranium this part is remarkable for being lengthened into a tapering woody cone to which the styles adhere in the form of a beak; in Nelumbium it is excavated into a number of cavities, in which the ovaries are half-hidden. It may be conjectured that the receptacle is in reality the growing point of the flower bud, and that it is analogous to the spongy head of the spadix in Arum, and to the hard spines of the Blackthorn.
In Caryophylleæ an internode below the receptacle is elongated, and bears on its summit the petals and stamens: De Candolle calls this anthophore (anthophorum).
13. Of the Ovule.
The Ovule (Plate V. fig. 16. to 26.) is a small, semipellucid, pulpy body, borne by the placenta, and gradually changing into a seed. Its internal structure is difficult to determine, both in consequence of its minuteness, and of the extreme delicacy of its parts, which are easily torn and crushed by the dissecting knife. It is doubtless owing to this circumstance chiefly, that the anatomy of the ovule was almost unknown to botanists of the last century, and that it has only begun to be understood within ten or twelve years, during which it has received ample illustration from several skilful observers. Brown, indeed, claims to have pointed out its real nature so long ago as 1814; but the brief and incomplete terms then used by that gentleman, in the midst of a long description of a single species, in the Appendix to Captain Flinders's Voyage, unaccompanied as they were by any explanatory remarks, prove indeed that he knew something of the matter, but by no means entitle him to the credit of having, at that time, made the world acquainted with it. The late Mr. Thomas Smith seems to deserve the honour of having first made any general remarks upon the subject: of what extent they exactly were is not known, as his discoveries, in 1818, were communicated, as it would seem, in conversation only; but it is to be collected from Brown's statement that they were of a highly important nature. Since that period the structure of the ovule has received much attention from Brown, in England; Turpin and Adolphe Brongniart, in France; and Treviranus, in Germany; by all of whom the subject has been greatly illustrated. It is, however, to Mirbel, - who, by collecting the discoveries of others, examining their accuracy, and combining them with numerous observations of his own, has given a full account of the gradual developement and the different modifications of the ovule that we are indebted for by far the best description of that important organ. His two papers read before the Academy of Sciences at Paris, in 1828 and 1829, are a model of careful investigation.
Ovules have been compared to buds, and may be shown to be analogous to them in structure. Of the truth of this there can now be little doubt: for, to say nothing of such plants as Bryophyllum, which habitually form buds on the margins of the leaves; or of Malaxis paludosa, in which the edge of the leaf is frosted by little microscopical points, that are neither exactly ovules nor exactly buds; or of the bracts of Marcgraavia, which Turpin, with much ingenuity, has endeavoured by mere argumentation to prove analogous to the primine of the ovule; it has been shown by Henslow that in the Mignionette the ovules do actually become transformed into leaves, either solitary or rolled together round an axis, of which the nucleus is the termination. (Cambr. Phil. Trans. vol. v. part i.) Engelman, also, mentions and figures instances of similar changes; but he does not say in what plants, nor are his figures satisfactory. He, however, concludes, from the observations of himself and Schimper, that “the ovules are buds of a higher order, their integuments leaves, and their
stalk the axis; all which, in cases of retrograde metamorphosis, are converted into stem and green leaves.” (De Antholysi Prodromus, § 44. 76. t. 5. f. 4, 5.) I should rather say that the evidence goes to prove that the ovule is a leaf-bud in a particular state, that the integuments are scales (i. e. rudimentary leaves) rolled up and united at their touching margins, and that the nucleus is the growing point, to which I have already on so many different occasions directed attention.
In almost all cases the ovule is enclosed within an ovary, as would necessarily happen in consequence of the convolute nature of the carpellary leaves : but if the convolution is imperfect, as in Reseda, the ovules are partially naked; and if it does not exist at all, as in Cycadaceæ and Coniferæ, the ovules are then entirely naked, and, instead of being fertilised by matter conveyed through the stigma and the style, as in other plants, are exposed to the direct influence of the pollen. This was first noticed by Brown; and, although since contradicted, seems to be perfectly true.
When the ovules are attached to the placenta by a kind of cord, that cord is called the funiculus (Plate V. fig. 26.a), and is a prolongation of the placenta.
In the beginning the ovule is a pulpy excrescence (Plate V. fig. 16.), appearing to be perfectly homogeneous,
with no trace of perforation or of envelopes. But, as it advances in growth, it is gradually (Plate V. fig. 17. to 21.) enclosed in two sacs or integuments, which are open only at their apex, where, in both these sacs, a passage exists, called the foramen (Plate V. fig. 21.a); or, in the language of Mirbel, exostome (fig. 25. a) in the outer integument, and endostome (fig. 25.6) in the inner integument. The central part is a fleshy, pointed, pulpy mass, called the nucleus (Plate V. fig. 19, 20. a, 22. b, 23.c, 24. d, 25. e, 27. e).
The outermost of the sacs (Plate V. fig. 22.c, 23. a, 25. c) is called the primine. It is either merely a cellular coating, or it is eventually traversed by veins: these are sometimes very apparent, as in the Orange tribe, and Mirbel seems disposed to think that they often exist in a rudimentary state