On the outer side of the wood is the inner bark or liber (fig. 36 c); and outside this is the spongy outer bark (d), covered by its epidermis.
These structures are of different composition, as may be best seen in longitudinal sections. The pith and the medullary rays consist of cellular tissue, the cells being mostly rounded in the former, and more closely pressed together and squarish in the latter. The medullary sheath consists of vascular tissue; and the wood, of wood-cells traversed longitudinally by bundles of vascular tissue and ducts, the latter being larger and more distinct towards its outer boundary. The liber is composed of woody fibre, and the outer bark of cellular tissue.
The new woody matter being deposited outside the old, between the bark and the previously formed layer, gives origin to the term exogen (ἔξω, outside, γεννἁω, to produce). These structures may be examined in the section of a branch of the lime-tree or lilac.
In the Monocotyledons or Endogens ([Pl. I.] fig. 37), there is no distinct bark, nor pith, nor medullary rays—the entire stem consisting of cellular tissue with isolated bundles of fibro-vascular tissue scattered through it. Moreover the new substance is added to the centre of the stem, or within the old; hence the term endogen (ἔνδον, within, γεννἁω). A section of a piece of cane will exhibit this structure.
To examine the structure of stems, sections must be made in various directions. The relative position of the component parts of a stem are best seen in a transverse section; but the structure of the tissues is most evident in longitudinal sections, and under the higher powers. The annual rings of the Exogens are best observed in transversely sawn-off pieces of perfectly dry stems, which have been polished with sandpaper, and varnished with spirit varnish.
Roots.—The structure of roots is very similar to that of stems; there is, however, no distinct pith, nor are there stomata on the epidermis; and the vessels are replaced by ducts. The very fine rootlets or radicles of water-plants often show the rotation of the protoplasm very distinctly.
Flowers.—The various parts of flowers, being each a modified leaf, present the same general structure as the latter. As the reader may not be acquainted with the names of these parts or organs in the higher plants, and as we shall have to compare them with their representatives in the lower forms of vegetable life, it will be well briefly to indicate them. A common and beautiful yet despised flower ([Pl. I.] fig. 32) may serve for illustration; this is chickweed (Stellária média), which can be found everywhere. The outermost circle of flower-leaves, which forms a kind of cup to the rest of the flower (a), is the calyx; the separate leaves being called the sepals. The row within this, in most flowers consisting of brilliantly coloured pieces, forms the corolla (b); the individual pieces being the petals. When the two kinds are equally coloured, or not distinguishable, the whole is called the perianth, as in a tulip. When the segments of the perianth are dry and chaffy, as in the flowers of grasses, the outermost are said to constitute the glumes, and the innermost the paleæ. Within the ring of petals are certain thread-like organs called stamens (c); and these consist of a filament (fig. 39 a), surmounted at the top or apex by the anther (fig. 39 b), which is usually coloured, and consists of two lobes. The anthers when ripe burst, and discharge a coloured dust; this is the pollen. Lastly, within the stamens is the central organ of the flower, the pistil, and sometimes there are several of them. The pistil consists of three parts, viz. a swollen base, the ovary (fig. 41 b), surmounted by a column or style (fig. 41 a), and which is crowned by a viscid and often hairy summit, the stigma (fig. 40*). In chickweed there are 3 styles.
It must be remarked that, in the flowers of some plants, stamens alone are present, while others contain pistils only, although most flowers contain both organs. When the stamens and pistils occur in separate flowers on the same plant, the plant is said to be monœcious (μὁνος, single, οἶκος, family); when all the flowers of distinct plants contain either stamens only or pistils only, the plant is diœcious (δις, twice, οἶκος); and when the stamens and pistils occur together in all the flowers of the same plant, the plant is said to be hermaphrodite. These terms had their origin in the idea that the differences of plants in respect to these organs were analogous to those of the sexes in animals. All the parts of a flower have their special uses: thus the calyx and corolla protect the delicate organs enclosed by them, until they attain maturity. The petals also, by their brilliant colours, attract insects which feed upon or collect the honey of the flowers; these at the same time conveying the pollen which adheres to their bodies from one flower to the stigma of another. The stamens and pistils are organs of fructification, it being essential for the fertilization of the flowers that the pollen should come into contact with the stigma. We will now consider some interesting points of structure in these organs.
Petals.—The petals often form most beautiful microscopic objects, on account of the curious shape and structure of the cells of their epidermis, and the splendid tints of the colouring matters contained in them. As petals are mostly too thick to allow of the cells being distinctly seen in the entire state, a little cut should be made in them while gently stretched on the finger, and the epidermis carefully stripped off with forceps; the strip should then be laid on the slide in water as usual: in this way the curious patterns of the epidermic cells will become very distinct. The petals of a red geranium (Pelargónium) may be used to illustrate them ([Pl. I.] fig. 24). The structure may be best understood by reference to the epidermis of the leaf of a geranium ([Pl. I.] fig. 13), in which the cells present wavy or undulate walls. In the petal (fig. 24), the walls are inflexed at tolerably regular distances, so as to give rise to the appearance of a row of teeth lining the cell. If the strip of petal be folded, so as to exhibit the side view, it will also be seen that the cells project outwards from the surface to form a bluntish point or papilla, or the petals are papillose as it is called; and the surface of the membrane around the papillæ is finely wrinkled, so as to present the appearance of very delicate radiating lines or striæ. Intermediate degrees of this inflexion may be found in various flowers, between the slight condition seen in fig. 13 and the extreme state of fig. 24, as in the snapdragon (Antirrhínum május).
Anthers.—The cavities of the anthers are lined with fibro-cellular tissue, the fibres of which aid in discharging the pollen; this may be seen by dissecting an anther of London pride (Saxif´raga umbrósa), or of a wallflower (Cheiran´thus cheíri) in water. It also exists in chickweed.