Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/99

consist of lime in combination with carbonic or oxalic acids, and are in many plants very abundant. In Mono cotyledons they usually assume tho form of needle-like crystals, and are termed raphidcs ; they consist of oxa- late of lime (fig. 29). They are remarkably well seen in the Banana. The Squill bulb and the bulb of the Onion exhibit raphidian cells, which are easily separated during the decay of the plants. The crystals are also arranged in a conglomerate form (fig. 30), as may be seen in the root of Turkey Rhubarb, to which they impart grittiness ; and in Old-Man-Cactus they constitute GO to 80 per cent, of the dried tissue. In a single cell of the Poke (Phytolacca decandra) twenty to thirty crystals may be seen. In the epidermal cells of species of Ficus, and other allied plants, , prolongations inward of the cell-wall occur, at the extremity of which small crystals of carbonate of lime are deposited (fig. 31); to these the name cystoliths has been applied. Siliceous matter occurs in the walls of cells, as in Grasses

and Horsetails, and especially in Diatomacese.

2. Integumentary System.

A more or less marked division of the tissues into an

outer layer bounding an inner mass is visible in all plants. Amongst the lower cellular plants this division is not very distinct ; the circumferential cells are perhaps only a little smaller and more compacted than those near the centre. The higher cellular plants, however, exhibit great diversity, idermis. In them the cells of the circumference may be arranged in layers so as to constitute a true epidermis, the component cells having a definite relation to one another and to the exterior in the respective families. In all vascular plants an epidermis is found. In many cases, however, it is diffi cult to recognize it, as in the stems of submerged plants and in most roots. It usually consists of a single thin layer of cells closely compacted, and leaving no interspaces except at definite points (stomata), where openings lead into inter cellular cavities. The cells composing this layer have their outer wall much thickened, the inner wall remaining thin, and they contain no starch or granular matter, and usually are colourless. In many aquatic plants, however, and in Ferns, chlorophyll is present. The apparent colour of the epidermis depends on that of the parenchyniatous cells below, from which it can be separated as a colourless layer. Sometimes the cell-sap of the epidermal cells has a red tinge. The outer lamella of the outermost cell-wall of the epi dermal cells usually becomes cuticular or corky, and thus is formed an external separable layer or cuticle upon the surface of the epidermis proper. This layer has different chemical properties from the epidermis, being insoluble in sulphuric acid. Upon this cuticle wax is frequently de posited in various forms, serving as a protective from mois ture ; of this nature is the bloom of the Plum. The cuticle in aquatic plants is very thin ; in aerial plants it is much thicker. The single layer of cells forming the epidermis is n:)t unfrequently strengthened by the addition to the inside of other layers of cells. In the leaves of Begonia, Ficus, and the outer covering (velamen) of the aerial roots of Orchids, this constitutes the hypodcrma, the cells being of various forms. It is well seen also in vascular Cryptogams, many Bromeliacea?, Ilex, &c. On those parts of the plant which live long and have vigorous growth in thickness the efficacy of the epidermis as a protective covering is in creased by a largo formation of cork. Each epidermal cell divides into an outer and an innei cell. The former at once becomes a cork cell, losing all its succulent matter ; the latter remains capable of division. When a layer of these merismatic cells occurs we have a cork cambiiim or plidlogen. If several layers of cork cells be formed a cork tissue or periderm is the result, which supersedes the epi dermis, and which from variations in the several layers may be stratified. Not unfrequently the phellogen cells, in addi tion to giving off cork cells outwardly, give rise on the inner face to cells containing chlorophyll ; and if a layer of such is formed it is termed a pkelloderm. In such cases the phellogen lies between the phelloderni and the periderm. If phellogen lamella) are formed deeper in the tissues of the plant, the internal layers of tissue become dry and con stitute the bark. Periderm is thus replaced by bark. One Stomata. important character of the epidermis is the presence of stomata or breathing-pores. These exist abundantly upon the stems and leaves of plants ; they also occur on the parts of the flowers ; but they arc absent from all root structures, though present on underground axial structures. Each consists of a central pore bounded by two or more cells (guard-cells), which contain chlorophyll, starch, and matters distinct from the surrounding epidermal cells. The pore has various forms, and opens into an intercellular cavity (fig. 32). It may be round (Primrose), oval (Liliaceae), quadrangular (Yucca). The arrangement of the stomata on the plant varies much. They may be in lines as in Equisetum, or they may be sr irrprprl irrpcmlivlv o in Balsam (fig. oo), Orill definite Clusters as 111 UiaSbUia ailCl Saxifraga (fig. 34). In Equisetum the stomata, which are about -^th of an inch in their greatest diameter, consist of four guard-cells; two of which are arched and thick at their outer convex margin,

becoming thin at their inner concave edge, where two other

}&*&x&rs*,~<*^ty^X3l >7>

/ . i *e v ^- * ,,"1*1 - *- c <- >. 7l * . .,"/ XA-"."*^ / Vertical section of epidermis, from the rreguiaiiy as in lowcr sui . face of tl { o lcaf ; f Mn(1(lci . showing the intimate union of the cpi- plnqtpr<; is in fVinli irirl dc al cells e < e tllc loose subjacent par enchymap, with intercellular canals 777,

and lacuntt l ; *< stoma -

Fig. 33. Fig. 34. FIG. 33. Epidermis of the garden Balsam (Bahamina hortensis), showing stomata it, of an elliptical form. FIG. 34. Epidermis of leaf of Saxifraga sarmentosa, showing clusters of stomata 5, , surrounded by large epidermal cells e, e. The cells among which the stomata occur arc very small.

Fig. 35.

Fig. 36. Fio. 33. Formation of stomata from leaf of Hyacinth, seen from the surface. e, epidermis cells; s , mother-cell of stomatic guard-cells; s, bipnrtition of mother-cell into two guard-cells, (fiaclis.) FIG. 30. Splitting of partition wall between guard-cells to form the stoma (Sachs.)