THE CELL WALL

The original cell wall may, during the differentiation of the cell, undergo various chemical modifications that profoundly change its physical properties. Principal among these are the deposition of further cellulose or hemicellulose and incrustation of the wall by lignin, cutin or suberin. Algal cell walls, which commonly contain pectin mixed with cellulose, xylose, mannose or silica, may contain also hemicellulose, alginic acid, fucoidin and fucin (Phaeophyta), geloses (Rhodophyta) and chitin.

PARENCHYMATOUS TISSUE

Meristematic tissue is usually composed of cells characterized by isodiametric form (except in the case of the provascular tissues), by possessing a protoplast capable of division and a primary cell wall composed of cellulose. The fundamental parenchyma occurring in various parts of the plant is potentially meristematic, and such cells achieve maturity without further differentiation except for an increase in cell size and wall thickness and a restricted change of form. The pith, cortex and rays of the plant axis and the mesophyll of the leaves are composed, at least in part, of such parenchyma. The mesophyll cells often contain abundant chloroplasts, and may be differentiated into palisade and spongy mesophyll. An early stage of differentiation may be seen in the lignified pitted parenchyma constituting the pith of the stems of Lobelia inflata and Cephaelis ipecacuanha, and the pitted cellulose parenchyma of the pulp of Citrullus colocynthis.

THE EPIDERMIS

The epidermis consists of a single layer of cells covering the whole plant. The epidermis of the root constitutes the piliferous layer and that of the shoot is a highly differentiated and compact layer of cells. The epidermal cells, in contrast to the stomatal guard cells, are often devoid of chloroplasts. Epidermal cells show great variety in form, giving characteristic patterns when seen in surface view. In transection they are often flattened parallel to the surface, and square or rectangular in shape. The outer walls are often convex and the most markedly thickened.

The epidermis of the stems of trees and shrubs is usually obliterated early by the development of a cork cambium, but on the stems of herbaceous plants and in leaves, fruits and seeds the epidermis persists and often yields highly diagnostic characters.

For leaves in particular, the shape of the epidermal cells in surface view and in section (Fig. 42.1A–D), the nature and distribution of the wall thickening, the presence or absence of cuticle and its form, the distribution and structure of the stomata, the presence or absence of well-differentiated subsidiary cells to the stomata, the presence of characteristic cell inclusions such as cystoliths, the presence or absence and form, size and distribution of epidermal trichomes and the presence and distribution of water-pores should all be carefully noted in describing the characters of an epidermis.

Fig. 42.1 A, Epidermal cells of Urtica dioica containing cystoliths of calcium carbonate. B, Epidermal cells of the leaf of Barosma betulina showing sphaero-crystalline masses of diosmin and a thick deposit of mucilage on the inner tangential walls. C, Cells of the lower epidermis of Arctostaphylos uva-ursi showing thick cuticle and sunken stomata. D, Upper epidermis of Cassia angustifolia showing mucilage cells and stomata; a cell of the underlying mesophyll contains a cluster crystal of calcium oxalate. E, Aleurone grains showing crystalloid and globoid, from the endosperm of the seed of Ricinus communis. F, Sphaerocrystalline masses of inulin in dahlia tuber. G, Cells of the endosperm of the seed of Strychnos nux-vomica showing walls of reserve cellulose, traversed by plasmodesmata. H, Parenchymatous cells from the rhizome of Zingiber officinale containing starch grains.

The structures of the epidermis and stomata are of first importance in the microscopical identification of leaves (see Fig. 42.2). Straight-walled epidermal cells are seen in, for example, jaborandi, coca and senna leaves; wavy-walled epidermal cells in stramonium, hyoscyamus and belladonna; beaded walls in Lobelia inflata and Digitalis lanata; a papillose epidermis in coca leaf. A thick cuticle is developed in Aloe leaf and bearberry leaf; a striated cuticle in belladonna, jaborandi, Digitalis lutea and D. thapsi. Mucilage is present in the epidermis of senna and buchu leaves. Cystoliths of calcium carbonate occur in the epidermal cells of Urticaceae and Cannabinaceae; sphaero-crystals of diosmin occur in buchu epidermis (Fig. 42.1B).

Fig. 42.2 Epidermis of leaves: A, lower epidermis of Digitalis purpurea; B, lower epidermis of Hyoscyamus niger, C, upper epidermis of Atropa belladonna; D, lower epidermis of Cassia angustifolia; E, lower epidermis of Rosmarinus officinalis; F, lower epidermis of Mentha piperita; G, lower epidermis of Pilocarpus jaborandi; H, upper epidermis of Lobelia inflata; I, lower epidermis of Digitalis lanata; J, lower epidermis of Erythroxylum coca. A, Anomocytic type of stomata; B and C, anisocytic type of stomata; D, paracytic type of stomata; E and F, diacytic type of stomata; G, actinocytic type of stomata.

The stomata may be surrounded by cells resembling the other epidermal cells (anomocytic, formerly ranunculaceous, type), but in other cases definite subsidiary cells may be distinguished. Three main types are distinguishable: the anisocytic (formerly cruciferous) type, with the stoma surrounded by three or four subsidiary cells, one of which is markedly smaller than the others; the paracytic (formerly rubiaceous) type, with two subsidiary cells with their long axes parallel to the pore; and the diacytic (formerly caryophyllaceous) type with two subsidiary cells, with their long axis at right angles to the pore of the stomata (Fig. 42.2). There are variations among these types (e.g. the actinocytic type, in which the subsidiary cells are arranged along the radii of a circle) and altogether some 31 types have been recognized. (For a survey of the classification of morphological types of stomata see M. Baranova, Bot. Rev., 1992, 58, 49).

Often, when viewed under the light microscope as cleared preparations, the outlines of the epidermal cells and stomata do not appear as definite as the line drawings (Fig. 42.2) might suggest. This is due tothe convoluted arrangements of cells on the leaf surface and is illustrated by the scanning electron micrographs included in the digitalis and Solanaceae descriptions in Part 5.

The distribution of stomata between the upper and lower epidermis shows great variation. The stomata may be entirely confined to the lower epidermis, as in Ficus species, bearberry, boldo, buchu, coca, jaborandi and maté leaves. The leaves of savin show stomata confined to two localized areas of the lower surface. The floating leaves of aquatics have stomata confined to the upper epidermis. Sometimes they are evenly distributed on both surfaces; most commonly they are more numerous on the lower surface. For ‘stomatal number’ and ‘stomatal index’ see Chapter 43.

The epidermis of fruits and seeds may yield characters of diagnostic value (see Fig. 41.7). The outer and inner epidermi of the pericarp of the umbelliferous fruits are highly characteristic structures. Characteristic cells with thickened pitted walls form the outer epidermis of the pericarp in vanilla, juniper and capsicum. The outer epidermi of the pericarp of coriander and vanilla contain prisms of calcium oxalate. A striated cuticle is seen in aniseed, caraway and star anise fruits. Thickened palisade-like cells form the epidermis of the testa of colocynth and fenugreek seeds. Characteristic elongated tapering cells form the epidermis of cardamoms. Thickened lignified cells form the epidermis of lobelia seed, and mucilage cells that of linseed and of white and black mustard.

EPIDERMAL TRICHOMES

Most leaves and many herbaceous stems, flowers, fruits and seeds possess hairs or trichomes of one kind or another. Many show hairs of more than one type. Hairs may be grouped into non-glandular or clothing hairs, and glandular hairs. Clothing hairs may be unicellular or multicellular. Unicellular hairs vary from small papillose outgrowths to large robust structures (Figs 42.3, 42.4). Multicellular hairs may be uniseriate, biseriate or multiseriate or complicated branched structures (Fig. 42.4). The chemical nature of the cell wall, and the presence of pits or protuberances or of cell inclusions, such as cystoliths, should be noted.

Fig. 42.3 Epidermal trichomes. A, Papillae of lower epidermis of Coca leaf. B–G, Unicellular hairs; B, papillose epidermal cell with cystolith from leaf of Cannabis; C, cystolith clothing hair from floral bract of Cannabis; D, Lobelia inflata leaf; E, senna leaf; F, lignified hair of Ailanthus; G, comfrey. H, Group of unicellular hairs from Hamamelis leaf. I, T-shaped hair of Artemisia absinthium.

Fig. 42.4 Epidermal trichomes. A–H, Uniseriate clothing hairs; I, multicellular branched hair; J, biseriate hair. A, Datura metel; B, Datura stramonium; C, Mentha piperita; D, Thymus vulgaris; E, Plantago lanceolata; F, Hyoscyamus niger; G, Digitalis purpurea; H, Xanthium strumarium; l, Verbascum thapsus; J, Calendula officinalis.

Glandular hairs may have a unicellular or a multiseriate stalk; the glandular head may be unicellular or multicellular (Fig. 42.5). The cuticle of the gland may be raised by the secretion (Fig. 42.5E and F). In peppermint the oil secretion beneath the cuticle contains crystals of menthol. A particular type of hair is often characteristic of a plant family or genus—for example, biseriate hairs of the form shown in Fig. 42.4J are common in the Compositae, while glandular hairs such as Fig. 42.5A, B and C are found in the Solanaceae, and such as Fig. 42.5E in the Labiatae. For types of hairs found on seeds, see sections on cotton, strophanthus seeds and nux vomica seeds.

Fig. 42.5 Glandular hairs: A and B, Atropa belladonna; C, Datura stramonium; D, Digitalis purpurea; E, multicellular labiate glandular hair; F, Hyoscyamus niger; G and H, Primula vulgaris; I, Digitalis lutea; J, Cannabis sativa; K, Artemisia maritima.