This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
The seed is the grand provision for continuing and multiplying vegetable species, and presents a considerable analogy in the vascular classes of the vegetable kingdom to an egg in the oviparous classes of the animal. Every seed, if properly fertilised, contains the embryo or rudiment of a future plant, and comprises an ample and most beautiful provision for its protection during dormancy, and for arousing and feeding it at the time of germination.
In some cases the embryo occupies the whole of the interior; and in some cases it is so small as to be minute; and again it is altogether absent. In the two former cases the seed is termed albuminous : the Bean or Pea when the skin is taken off presents a good example of nothing but the embryo. The embryo of the seed of a Palm when cut vertically is very small and white. The seed may also be described as the ovule (see No. VII.) arrived at maturity, and consists of integuments, - collectively termed testa, which consist of membranes resulting from the sacs of the ovule, and sometimes expanded into wings, that are probably intended to render seeds buoyant, and are frequently spongy, and sometimes consist of spiral cells, - and embryo - which is the organised body that lies within the seed, for the purpose of protecting and nourishing which the seed was created - and was originally included within the sac of the amnios which contains a fluid in which the embryo is developed, and which is usually absorbed or obliterated during the advances of the embryo to maturity; being exceptional in the case of Vitellus, etc, where it remains surrounding the ripe embryo.
The embryo consists of (1) cotyledons, (2) radicle, (3) plumule, and (4) collar. The cotyledons represent undeveloped leaves, - a very good example is found in the Broad Bean, whose cotyledons, after performing the required functions under the soil, are afterwards formed into the first leaves or cotyledon leaves. The radicle is (the rudiment of) the descending axis or root, which throws out fibrils and spongioles, to absorb moisture and other nourishment, and to preserve the plant's equilibrium. The plumule is the ascending axis, which gives out the stems, leaves, flowers, etc. The collar is the line of separation between the radicle and the cotyledons, and the space between the collar and base of the cotyledons is called cauliculus.
The direction of the embryo, with respect to the seed, will depend upon the relation that the integuments, (1) the raphe (i.e., the cord of fibro-vascular tissue which connects the base of the nucleus of an ovule with a placenta); (2) the chalaza (the part of the seed where the nucleus joins the integuments, and is invariably opposite the end of the cotyledons); (3) the hilum; and (4) the micropyle (the aperture in the skin of a seed which was once the foramen of the ovule) - bear to each other. If the nucleus be erect, the embryo will be inverted, or antitropal: Ex. Nettle. If the nucleus be inverted, the embryo will be erect, or orthotropal: Ex. Apple. If the micropyle is at neither end of the seed, the embryo will be neither erect nor inverted, but will be in a more or less oblique direction with respect to the seed; and it is said to be heterotropal: Ex. Primrose.
These must not be confounded with similar terms applied to the ovule, and consequently to the seed itself. In general seeds are, like ovules, enclosed within a covering, arising from a carpellary leaf; but all gymnosperms are an exception to this. Moreover, some ovules rupture the ovary soon after they begin to advance towards the state of the seed, and thus become naked seeds : Ex. Leontics. Others are imperfectly protected by the ovary, the carpels not being perfectly closed up : Ex. Reseda.
A very easy and simple way of examining the growth of the plumule, radicle, etc., is to grow some of the large Broad Beans on damp cloth, or else in a little earth, when the successive growth of each can be very plainly seen, the radicle making the first appearance, followed by the plumule. W. Roberts.
(To be continued).