Book I. 



INTERNAL STRUCTURE OF PLANTS. 



143 



are two in number, an exterior integument and an interior integument ; which are sometimes, liowcver, 

 enveloped by the additional integument constituting an appendage of the seed, under tlie title of the 

 pellicle or seminal epidermis. The exterior integument, or testa, is the original cuticle oT the nucleus, not 

 detachable in the early stages of its growth, but detachable at the period of the m»aturity of the fruit, 

 when it is generally of a membranaceous or leathery texture ; though sometimes soft and fleshy, and 

 sometimes crustaceous and bony. It may be very easily distinguished in the transverse or longitudinal 

 section of the garden-bean or any other large seed, and may be also easily detached by the aid of a little 

 manipulation. The interior integument, or sub-testa, lines the exterior integument, or testa, and immedi- 

 ately envelopes the nucleus, deriving its origin from the interior portion of the umbilical cord, which, after 

 perforating the testa, disperses into a multiplicity of ramifications connected by a fine membrane, and 

 forms the interior integument. Like the testa, to which indeed it adheres, it may be easily distinguished 

 in the garden-bean {fig. 47.), or in a ripe walnut ; in which last it is a fine transparent and net-like mem- 

 brane. 



G08. The nucleus is that part of the seed which is 

 contained within the proper integuments, consisting 

 of the albumen with the vitellus, when present, and 

 embryo. The albumen is an organ resembling in its 

 consistence the white of an egg, and forming, in most 

 cases, the exterior portion of the nucleus, but always 

 separable from the interior or remaining portion. 

 The vitellus is an organ of a fleshy but firm contex- 

 ture, situated, when present, between the albumen 

 and embryo ; to the former of which it is attached 

 only by. adhesion, but to the latter by incorporation 

 of substance, so as to be inseparable from it, except 

 by force. The embryo (Jig. 47. a) which is the last and 

 most essential part of the seed and final object of the 

 fructification, as being the germ of the future plant, 

 is a small and often very minute organ, enclosed 

 within the albumen and occupying the centre of the seed. The cotyledon or seed-lobe (A), is that portion 

 of the embryo, that encloses and protects the plantlet, and springs up during the process of germination 

 into what is usually denominated the seminal leaf, if the lobe is solitary ; or seminal leaves, if there are 

 more lobes than one. In the former case the seed is said to be monocotyledonous ; in the latter case, it 

 is said to be dicotyledonous. Dicotyledonous seeds, which constitute by far the majority of seeds, are 

 well exemplified in the garden-bean. As there are some seeds who&e cotyledon consists of one lobe only, 

 falling short of the general number, so there are also a few whose cotyledon is divisible into several lobes, 

 exceeding the general number. They have been denominated polycotyledonous seeds, and are exempli- 

 fied in the case of lepidium sativum or common garden-cress, in which the lobes are six in number; as in 

 that also of the different species of the genus pinus, in which they vary from three to twelve. But 

 although by far the greater number of seeds are furnished with two cotyledons, or with a cotyledon divi- 

 sible or not divisible into several lobes, there is also a considerable proportion in which the cotyledon is 

 altogether wanting, or at least believed to be wanting by botanists in general. These, according to 

 Gasrtner, are exemplified in the fuci, ferns, and fungi, the embryo being merely a germinating cicatrice 

 imbedded in the surface of a vitellus which forms the mass of the seed. But Hedwig, to whose opinions 

 on this subject much deference is also due, maintains that the seeds of the plants in question are furnished 

 with cotyledons as well as those of other plants, and that no seed whatever is without them. This is a 

 case, however, in which the general opinion of botanists is against him, as may be seen from the many 

 systems founded upon the presence, or absence, or number of the cotyledons, and exemplified, as we have 

 seen, in that of the great and justly celebrated Jussieu, whose primary divisions are those of acotyledonous, 

 monocotyledonous, and dicotyledonous plants, the polycotyledonous being thought to be too few in num- 

 ber to constitute a separate division. It should be recollected, however, that the above divisions were 

 instituted at a time when the subject had not yet undergone any thing like a rigorous scrutiny, that 

 •already many changes have been found necessary, and that future investigations will in all probability 

 point out the necessity of more. In watching the germination of fern-seed, Mirbel observed some sub- 

 stances which he regards as cotyledons, and so far supports the position of Hedwig. The plantlet, or future 

 plant in miniature, is the interior and essential portion of the embryo, and seat of vegetable life. In some 

 seeds it is so minute as to be scarcely perceptible ; while in others it is so large as to be divisible into dis- 

 tinct parts, as in the garden-bean. 



609. The pericarp, which in different species of fruit assumes so many varieties of contexture, acquires its 

 several aspects, not so much from a diversity of substance as of modification. The valves of the capsule, 

 but particularly the partitions by which it is divided into cells, are composed of a thin and skinny mem- 

 brane, or of an epidermis covering a pulp more or less indurated, and interspersed with longitudinal fibres. 

 The capsule of the mosses is composed of a double and net-like membrane, enclosed within a fine epider- 

 mis. The pome is composed of a fine but double epidermis, or, according to Knight, of two skins, enclosing 

 a soft and fleshy pulp, with bundles of longitudinal fibres passing through it, contiguous to, and in the 

 direction of, its longitudinal axis. The valves of the legume are composed of an epidermis enclosing a 

 firm but fleshy pulp, lined for the most part with a skinny membrane, and of bundles of longitudinal fibres, 

 forming the seam. The nutshell, whether hard or bony, or flexible and leathery, is composed of a pulp 

 more or less highly indurated, interspersed with longitudinal fibres, and covered with an epidermis. 'I'he 

 drupe is composed of an epidermis enclosing a fleshy pulp, which is sometimes so interwoven with a mul- 

 tiplicity of longitudinal fibres as to seem to consist wholly of threads, as in the cocoa-nut. The be7-ry is 

 composed of a very fine epidermis enclosing a soft and juicy pulp. The scales of the strobile are composed 

 of a tough and leathery epidermis, enclosing a spongy but often highly indurated pulp interspersed with 

 longitudinal fibres that pervade also the axis. 



610. The flower-stalk, or peduncle supporting the flower, which is a prolongation of the stem or brancli, or 

 rather a partial stem attached to it, if carefully dissected with the assistance of a good glass, will be found 

 to consist of the following several parts : — 1st, An epidermis, or external envelope ; 2dly, A parenchyma, 

 or soft and pulpy mass ; 3dly, Bundles of longitudinal threads or fibres, originating in the stem or branch, 

 and passing throughout the whole extent of the parenchyma. The several organs of the flower are merely 

 prolongations of the component parts of the flower-stalk, though each organ does not always contain the 

 whole of such component parts, or at least not under the same modifications. The epidermis, however, 

 and parenchyma are common to them all j but the longitudinal threads or fibres are seldom if ever to be 

 found except in the calyx or coroUa. 



611. The leaf -stalk, or petiole supporting the leaf, which is a prolongation of the branch or stem, or rather 

 a partial stem attached to'it, exhibits upon dissection the same sort of structure as the peduncle, namely, an 

 epidermis, a pulp or parenchyma, and bundles of longitudinal threads or fibres. 



612. Gems. There exist among the different tribes of vegetables four distinct species of gems, two peculiar 

 to perfect plants, the bud and bulb, and two peculiar to imperfect plants, the propago and gongylus ; the 

 latter being denominated simple gems, because furnished with a single envelope only ; and the former being 

 denominated compound gems, because furnished with more than a single envelope. 



