(*Footnote. Prodr. Flor. Nov. Holl. 1 page 306.)
The same explanation of structure applies to the seeds of Piperaceae and Saururus; and other instances occur of the persistence either of the membrane or of the substance of the amnios in the ripe seed.
It may be concluded from the whole account which I have given of the structure of the ovulum, that the more important changes consequent to real, or even to spurious fecundation, must take place within the nucleus: and that the alb.u.men, properly so called, may be formed either by a deposition or secretion of granular matter in the utriculi of the amnios, or in those of the nucleus itself, or lastly, that two substances having these distinct origins, and very different textures, may co-exist in the ripe seed, as is probably the case in Scitamineae.
On the subject of the ovulum, as contained in an ovarium, I shall at present make but one other remark, which forms a necessary introduction to the observations that follow.
ON THE STRUCTURE OF THE FEMALE FLOWER IN CYCADEAE AND CONIFERAE.
That the apex of the nucleus is the point of the ovulum where impregnation takes place, is at least highly probable, both from the constancy in the appearance of the embryo at that point, and from the very general inversion of the nucleus; for by this inversion its apex is brought nearly, or absolutely, into contact with that part of the parietes of the ovarium, by which the influence of the pollen may be supposed to be communicated. In several of those families of plants, however, in which the nucleus is not inverted, and the placentae are polyspermous, as Cistineae,* it is difficult to comprehend in what manner this influence can reach its apex externally, except on the supposition, not hastily to be admitted, of an impregnating aura filling the cavity of the ovarium; or by the complete separation of the fecundating tubes from the placentae, which, however, in such cases I have never been able to detect.
(*Footnote. This structure of ovulum, indicated by that of the seed, as characterizing and defining the limits of Cistineae (namely, Cistus, Helianthemum, Hudsonia and Lechea) I communicated to Dr. Hooker, by whom it is noticed in his Flora Scotica (page 284) published in 1821; where, however, an observation is added respecting Gaertner"s description of Cistus and Helianthemum, for which I am not accountable.)
It would entirely remove the doubts that may exist respecting the point of impregnation, if cases could be produced where the ovarium was either altogether wanting, or so imperfectly formed, that the ovulum itself became directly exposed to the action of the pollen, or its fovilla; its apex, as well as the orifice of its immediate covering, being modified and developed to adapt them to this economy.
But such, I believe, is the real explanation of the structure of Cycadeae, of Coniferae, of Ephedra, and even of Gnetum, of which Thoa of Aublet is a species.
To this view the most formidable objection would be removed, were it admitted, in conformity with the preceding observations, that the apex of the nucleus, or supposed point of impregnation, has no organic connexion with the parietes of the ovarium. In support of it, also, as far as regards the direct action of the pollen on the ovulum, numerous instances of a.n.a.logous economy in the animal kingdom may be adduced.
The similarity of the female flower in Cycadeae and Coniferae to the ovulum of other phaenogamous plants, as I have described it, is indeed sufficiently obvious to render the opinion here advanced not altogether improbable. But the proof of its correctness must chiefly rest on a resemblance, in every essential point, being established, between the inner body in the supposed female flower in these tribes, and the nucleus of the ovulum in ordinary structures; not only in the early stage, but also in the whole series of changes consequent to fecundation. Now as far as I have yet examined, there is nearly a complete agreement in all these respects. I am not entirely satisfied, however, with the observations I have hitherto been able to make on a subject naturally difficult, and to which I have not till lately attended with my present view.
The facts most likely to be produced as arguments against this view of the structure of Coniferae, are the unequal and apparently secreting surface of the apex of the supposed nucleus in most cases; its occasional projection beyond the orifice of the outer coat; its cohesion with that coat by a considerable portion of its surface, and the not unfrequent division of the orifice of the coat. Yet most of these peculiarities of structure might perhaps be adduced in support of the opinion advanced, being apparent adaptations to the supposed economy.
There is one fact that will hardly be brought forward as an objection, and which yet seems to me to present a difficulty, to this opinion; namely, the greater simplicity in Cycadeae, and in the princ.i.p.al part of Coniferae, of the supposed ovulum which consists of a nucleus and one coat only, compared with the organ as generally existing when enclosed in an ovarium. The want of uniformity in this respect may even be stated as another difficulty, for in some genera of Coniferae the ovulum appears to be complete.
In Ephedra, indeed, where the nucleus is provided with two envelopes, the outer may, perhaps, be supposed rather a.n.a.logous to the calyx, or involucrum of the male flower, than as belonging to the ovulum; but in Gnetum, where three envelopes exist, two of these may, with great probability, be regarded as coats of the nucleus; while in Podocarpus and Dacrydium, the outer cupula, as I formerly termed it,* may also, perhaps, be viewed as the testa of the ovulum. To this view, as far as relates to Dacrydium, the longitudinal fissure of the outer coat in the early stage, and its state in the ripe fruit, in which it forms only a partial covering, may be objected.** But these objections are, in a great measure, removed by the a.n.a.logous structure already described in Banksia and Dryandra.
(*Footnote. Flinders Voyage volume 2 page 573.)
(**Footnote. Id. loc. cit.)
The plurality of embryos sometimes occurring in Coniferae, and which, in Cycadeae, seems even to be the natural structure, may also, perhaps, be supposed to form an objection to the present opinion, though to me it appears rather an argument in its favour.
Upon the whole, the objections to which the view here taken of the structure of these two families is still liable, seem to me, as far as I am aware of them, much less important than those that may be brought against the other opinions that have been advanced, and still divide botanists on this subject.
According to the earliest of these opinions, the female flower of Cycadeae and Coniferae is a monospermous pistillum, having no proper floral envelope.
To this structure, however, Pinus itself was long considered by many botanists as presenting an exception.
Linnaeus has expressed himself so obscurely in the natural character which he has given of this genus, that I find it difficult to determine what his opinion of its structure really was. I am inclined, however, to believe it to have been much nearer the truth than is generally supposed; judging of it from a comparison of his essential with his artificial generic character, and from an observation recorded in his Praelectiones, published by Giseke.*
(*Footnote. Praelect. in Ord. Nat. page 589.)
But the first clear account that I have met with, of the real structure of Pinus, as far as regards the direction, or base and apex of the female flowers, is given, in 1767, by Trew, who describes them in the following manner: "Singula semina vel potius germina stigmati tanquam organo feminino gaudent,"* and his figure of the female flower of the Larch, in which the stigmata project beyond the base of the scale, removes all doubt respecting his meaning.
(*Footnote. Nov. Act. Acad. Nat. Curios. 3 page 453 table 13 figure 23.)
In 1789, M. de Jussieu, in the character of his genus Abies,* gives a similar account of structure, though somewhat less clearly as well as less decidedly expressed. In the observations that follow, he suggests, as not improbable, a very different view, founded on the supposed a.n.a.logy with Araucaria, whose structure was then misunderstood; namely, that the inner scale of the female amentum is a bilocular ovarium, of which the outer scale is the style. But this, according to Sir James Smith,** was also Linnaeus" opinion; and it is the view adopted in Mr. Lambert"s splendid monograph of the genus published in 1803.
(*Footnote. Gen. Pl. page 414.)
(**Footnote. Rees Cyclop. art. Pinus.)
In the same year in which Mr. Lambert"s work appeared, Schkuhr*
describes, and very distinctly figures, the female flower of Pinus, exactly as it was understood by Trew, whose opinion was probably unknown to him.
(*Footnote. Botan. Handb. 3 page 276 table 308.)
In 1807, a memoir on this subject, by Mr. Salisbury, was published,* in which an account of structure is given, in no important particular different from that of Trew and Schkuhr, with whose observations he appears to have been unacquainted.
(*Footnote. Linnean Society Transactions 8 page 308.)
M. Mirbel, in 1809,* held the same opinion, both with respect to Pinus and to the whole natural family. But in 1812, in conjunction with M.
Schoubert,** he proposed a very different view of the structure of Cycadeae and Coniferae, stating, that in their female flowers there is not only a minute cohering perianthium present, but an external additional envelope, to which he has given the name of cupula.
(*Footnote. Ann. du Mus. d"Hist. Nat. tome 15 page 473.)
(**Footnote. Nouv. Bulletin des Sc. tome 3 pages 73, 85 et 121.)
In 1814 I adopted this view, as far, at least, as regards the manner of impregnation, and stated some facts in support of it.* But on reconsidering the subject, in connexion with what I had ascertained respecting the vegetable ovulum, I soon after altogether abandoned this opinion, without, however, venturing explicitly to state that now advanced, and which had then suggested itself.**
(*Footnote. Flinders Voyage 2 572.)
(**Footnote. Tuckey Congo page 454 et Linnean Society Transactions volume 13 page 213.)
It is well known that the late M. Richard had prepared a very valuable memoir on these two families of plants; and he appears, from some observations lately published by his son, M. Achille Richard,* to have formed an opinion respecting their structure somewhat different from that of M. Mirbel, whose cupula is, according to him, the perianthium, more or less cohering with the included pistillum. He was probably led to this view, on ascertaining, which I had also done, that the common account of the structure of Ephedra was incorrect,** its supposed style being in reality the elongated tubular apex of a membranous envelope, and the included body being evidently a.n.a.logous to that in other genera of Coniferae.
(*Footnote. Dict. Cla.s.s. d" Hist. Nat. tome 4 page 395 et tome 5 page 216.)
(**Footnote. Dict. Cla.s.s. d"Hist. Nat. tome 6 page 208.)
To the earliest of the opinions here quoted, that which considers the female flower of Coniferae and Cycadeae as a naked pistillum, there are two princ.i.p.al objections. The first of these arises from the perforation of the pistillum, and the exposure of that point of the ovulum where the embryo is formed to the direct action of the pollen; the second from the too great simplicity of structure of the supposed ovulum, which, I have shown, accords better with that of the nucleus as existing in ordinary cases.
To the opinions of MM. Richard and Mirbel, the first objection does not apply, but the second acquires such additional weight, as to render those opinions much less probable, it seems to me, than that which I have endeavoured to support.
In supposing the correctness of this opinion to be admitted, a question connected with it, and of some importance, would still remain, namely, whether in Cycadeae and Coniferae the ovula are produced on an ovarium of reduced functions and altered appearance, or on a rachis or receptacle.
In other words, in employing the language of an hypothesis, which, with some alterations, I have elsewhere attempted to explain and defend, respecting the formation of the s.e.xual organs in Phaenogamous plants,*
whether the ovula in these two families originate in a modified leaf, or proceed directly from the stem.
(*Footnote. Linnean Society Transactions volume 13 page 211.)
Were I to adopt the former supposition, or that best agreeing with the hypothesis in question, I should certainly apply it, in the first place, to Cycas, in which the female spadix bears so striking a resemblance to a partially altered frond or leaf, producing marginal ovula in one part, and in another being divided into segments, in some cases nearly resembling those of the ordinary frond.
But the a.n.a.logy of the female spadix of Cycas to that of Zamia is sufficiently obvious; and from the spadix of Zamia to the fruit-bearing squama of Coniferae, strictly so called, namely, of Agathis or Dammara, Cunninghamia, Pinus, and even Araucaria, the transition is not difficult.
This view is applicable, though less manifestly, also to Cupressinae; and might even be extended to Podocarpus and Dacrydium. But the structure of these two genera admits likewise of another explanation, to which I have already adverted.
If, however, the ovula in Cycadeae and Coniferae be really produced on the surface of an ovarium, it might, perhaps, though not necessarily, be expected that their male flowers should differ from those of all other phaenogamous plants, and in this difference exhibit some a.n.a.logy to the structure of the female flower. But in Cycadeae, at least, and especially in Zamia, the resemblance between the male and female spadices is so great, that if the female be a.n.a.logous to an ovarium, the partial male spadix must be considered as a single anthera, producing on its surface either naked grains of pollen, or pollen subdivided into ma.s.ses, each furnished with its proper membrane.
Both these views may at present, perhaps, appear equally paradoxical; yet the former was entertained by Linnaeus, who expresses himself on the subject in the following terms, Pulvis floridus in Cycade minime pro Antheris agnoscendus est sed pro nudo polline, quod unusquisque qui unquam pollen antherarum in plantis examinavit fatebitur.* That this opinion, so confidently held by Linnaeus, was never adopted by any other botanist, seems in part to have arisen from his having extended it to dorsiferous Ferns. Limited to Cycadeae, however, it does not appear to me so very improbable, as to deserve to be rejected without examination. It receives, at least, some support from the separation, in several cases, especially in the American Zamiae, of the grains into two distinct, and sometimes nearly marginal, ma.s.ses, representing, as it may be supposed, the lobes of an anthera; and also from their approximation in definite numbers, generally in fours, a.n.a.logous to the quaternary union of the grains of pollen, not unfrequent in the antherae of several other families of plants. The great size of the supposed grains of pollen, with the thickening and regular bursting of their membrane, may be said to be circ.u.mstances obviously connected with their production and persistence on the surface of an anthera, distant from the female flower; and with this economy, a corresponding enlargement of the contained particles or fovilla might also be expected. On examining these particles, however, I find them not only equal in size to the grains of pollen of many antherae, but, being elliptical and marked on one side with a longitudinal furrow, they have that form which is one of the most common in the simple pollen of phaenogamous plants. To suppose, therefore, merely on the grounds already stated, that these particles are a.n.a.logous to the fovilla, and the containing organs to the grains of pollen in antherae of the usual structure, would be entirely gratuitous. It is, at the same time, deserving of remark, that were this view adopted on more satisfactory grounds, a corresponding development might then be said to exist in the essential parts of the male and female organs. The increased development in the ovulum would not consist so much in the unusual form and thickening of the coat, a part of secondary importance, and whose nature is disputed, as in the state of the nucleus of the seed, respecting which there is no difference of opinion; and where the plurality of embryos, or at least the existence and regular arrangement of the cells in which they are formed, is the uniform structure in the family.