the dissepiment d will be opposite the placentæ m, 1, formed by the cohesion of the contiguous margins of the carpella A and B. 5. A single carpellum can have no dissepiment whatever. 130 6. The dissepiment will always alternate with the stigma ; — for the stigma is the extremity of the mid-rib of the carpellary leaf, or of the dorsal suture of the carpellum; and the sides of either of these (which form dissepiments) will be right and left of the stigma, or in the same position with regard to the latter organ as the sides of the lamina of a leaf to its apex. Let the triangle a, b, c represent a transverse section of a three-celled ovarium, of which d, e, f are the dissepiments. The stigmata would occupy a position equal to that of the spaces s, s, s, and would consequently be alternate with d, e, f, the dissepiments: they could not possibly be placed opposite d, e, f, upon any principle of structure with which we are acquainted. This law proves, that neither the membrane which separates the two cells of a Cruciferous siliqua, nor the vertical plate that divides the ovarium of Astragalus into two equal portions, are dissepiments; both are expansions of the placenta, or some other part, in different degrees. Such is the structure of an ovarium in its most common state; certain deviations from it remain to be explained. We have seen that when carpella become syncarpous, they form a pistillum, the ovarium of which has as many cells and dissepiments as there are carpella employed in its construction. But sometimes the united sides of the carpella do not project so far into the cavity of the ovarium as to meet in the axis, as in the Poppy; and then an ovarium is the result, which, although composed of many carpella, is nevertheless onecelled (fig. 133.) In such case the dissepiments project a short distance only beyond the inner lining, or paries, of the ovarium, and, bearing on their edges the placenta, the latter are said to be parietal. In other plants, such as Corydalis, Viola, and Orchis, the carpella are not folded together at all, but are spread open and united by their edges (fig. 132.): in that case the placenta do not project at all into the cavity of the ovarium, but are still more strictly parietal than the last. Another class of anomalies of a still more remarkable character, is that in which the dissepiments are obliterated, while the placentæ remain a distinct mass in the centre of the ovarium, as in Lychnis; forming what is called a free central placenta (fig. 131.). But, if we examine these plants at a very early period of their formation, long before the flowers expand, the explanation of the anomaly will be obvious. Such plants are, at that time, constructed upon the ordinary plan, with their dissepiments meeting in the centre and forming there a fungous placenta; but subsequently the shell of the ovarium grows more rapidly than the dissepiments, and breaks away from them; while the excessive growth of the placenta afterwards destroys almost all trace of them: their previous presence is only to be detected by lines upon the shell of the ovarium, or by a separation of the mass of ovula into distinct parcels upon the placenta. All partitions whose position is at variance with the foregoing laws are spurious. Such spurious dissepiments are caused by many circumstances, the chief of which are the following:- they are caused by expansions of the placenta, as in Cruciferæ, when they form a partition stretching from one side to the other of the fruit; or they are mere dilatations of the lining of the pericarpium, as in Cathartocarpus Fistula, in which they are horizontal; or they are internal expansions of the dorsal or ventral suture, as in Amelanchier, Astragalus, and Thespesia, in which they are distinguishable from their dissepiments by not bearing the placentæ, and by being opposite the stigma, or by projecting beyond the placentæ; or, finally, they are caused by the sides of the ovarium projecting into the cavity, uniting and forming many supernumerary cells, as in Diplophractum. 11. Of the Receptacle. The part upon which the carpella are seated is the apex of the peduncle, or the summit of the floral branch, of which the carpella 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 Annonaceæ and Magnoliaceae 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. In Caryophylleæ the receptacle is elongated, and bears on its summit the petals and stamens: M. De Candolle calls this form Anthophore (Anthophorum). When the receptacle bears only the ovarium, and is not a support to either corolla or stamens (Plate V. fig. 1. a.) it is called Carpophore, or Gynophore (Carpophorum, Gynophorum): this may either be a simple rounded stalk to the fruit, and of the same texture with it, as in Capparis, Phaca, and others,—when Richard calls it Basigynium or Podog ynium, and Ehrhart Thecaphore, or it may be succulent and much dilated, so as to resemble the receptacle of a Composita, bearing at the same time many ovaria, as in the Strawberry and Raspberry, when Richard calls it Polyphorum: most commonly such a receptacle is sufficiently described by the adjective fleshy. In Geranium it is remarkable for being lengthened into a tapering cone, to which the styles adhere, in the form of a beak; and in Nelumbium it is excavated into a number of cavitities, in which the ovaria are half hidden. 12. Of the Ovulum. The ovulum (Plate V. fig. 16. to 26.) is a small, semipellucid, pulpy body, borne by the placenta, and gradually chang ing into a seed. Its internal structure is exceedingly difficult to determine, either in consequence of its minuteness, or of the extreme delicacy of its parts, which are easily torn and crushed by the dissecting knife. It is doubtless to this circumstance chiefly, that the anatomy of the ovulum 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. Mr. 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' Voyage, unaccompanied as they were by any explanatory remarks, prove indeed that he knew something of the subject, 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 credit 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 Mr. Brown's statement that they were of a highly important nature. Since that period the structure of the ovulum has received much attention from Messrs. 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 the learned M. Mirbel,—who, by collecting the discoveries of others, examining their accuracy, snd combining them with numerous admirable observations of his own, has given a full account of the gradual developement and the different modifications of the ovulum-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 perfect model of candour and patient investigation, and form the basis of what is here about to be recorded on the subject. I regret, however that the space which can now be devoted to the explanation of the structure of the ovulum is by no means such as its intricacy and interest demand. As the ovula are the production of the placentæ, they necessarily originate in the margins of the carpellary leaf; and hence they have not only been compared to the buds found upon the margins of some true leaves, in a theoretical point of view, but it also has been attempted to be shown that they are analogous to them in structure. Of the truth of the former there can be little doubt; for, to say nothing of such plants as Bryophyllum, which habitually form buds on the margins of the leaves, the case of Malaxis paludosa, first recorded by Professor Henslow, in which the edge of the leaf is frosted by little microscopical points, which are neither exactly ovula nor exactly buds, seems a sufficient proof: but that in structure they are analogous to buds is by no means well made out. M. Turpin indeed has attempted, with his usual ingenuity, to demonstrate an analogy between the bractea of Marcgraavia and the outer integument or primine of the ovulum; but his statement cannot be regarded, as a sufficient evidence, of the fact. It is, therefore, only safe, at present, to regard the ovulum as analogous to the marginal buds of leaves in nature and position, but not in structure. In almost all cases the ovulum is enclosed within an ovarium, as would necessarily happen in consequence of the convolute nature of the carpellary leaves; but if the convolution is imperfect, as in Reseda, the ovula are partially naked; and if it does not exist at all, as in Cycadeæ and Coniferæ, the ovula are then entirely naked; and, instead of being fertilised by molecules 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 Mr. Brown; and, although since contradicted, is no doubt perfectly true. When the ovula are attached to the placenta by a kind of cord, that cord is called the funiculus (Plate 5. fig. 26, a.), and is a mere prolongation of the placenta. In the beginning, the ovulum is a pulpy excrescence (Plate 5. 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 5. 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 |