18 HISTOLOGY [VEGETABLE. supply of nourishment, soon wither ; imd thus occasionally the dead parts scale off, as in the Platanus, cherry, &c. The baik or rhytidome is thus a very complex structure, consisting of the secondary epidermal tissues either formed in the primary cortex alone or deep in the other tissues, and popularly it includes all the tissues outside the cambium layer, that is, the bast part of the fibro-vascular enticels. bundles and secondary epidermal tissues. Lenticels are special structures connected with the epidermal tissues, and are common on dicotyledons (Sambucus, Populus, Juglans, &c. ), and on some mono cotyledons, being formed on steins, branches, petioles, and roots. Below a stoma or group of stomata a few cells enlarge and divide, and form numerous colourless thin walled cells, which arise from the bent layer of lenticel cambium below. The epidermis becomes ruptured and the cells appear on the surface, forming a brownish wart-like marking. These lenticels are probably to be considered functionally as secondary stomata, as the cells have large inter cellular spaces and readily permit the passage of air into the interior. Lenticels have the marked peculiarity of being sometimes closed in autumn by the formation of cork cells, but open again in spring. 2. Tim Fibro-vascular System. ibro- String-like bundles, the fibre-vascular bundles, are common in oscular vascular cryptogams, gymnosperms, and angiosperms, and are undies, familiar in the leaves of plants as the veins. They run in the ground tissue either separately or united, as in many dicotyledons, and in most roots, &c., to form a central or hollow cylindrical vascular mass. "When the bundles are separate they often branch and anastomose as in leaves, or they may only anastomose at the nodes of steins. The bundles are easily separable by maceration, except in water plants, and a few others, in which the bundles are very soft ; or they may be examined in transverse and longitudinal sections of the part, more particularly in the latter case when the tissues have been rendered transparent by boiling in dilute caustic potash, or by being previously boiled in strong nitric acid. (See Niigcli and Schwendener, Das Mikroskop, p. 632. ) r ood Each bundle in its perfect state consists of two groups of cells,
- ylem) the wood or xylem portion, and the bast or phloem. Bundles are
id bast either closed or open. In the former the procambium cells, the ihloem). meristem, from which the permanent tissue of the bundle originates, entirely passes over into permanent tissue ; while in the latter the cambium remains between the xylem and phloem, and is capable of forming new cells for an indefinite period. Closed bundles thus rapidly assume a permanent form, while open bundles go on growing. Fibro- vascular bundles are divided by De Bary into four groups by the mode of arrangement of the xylem and phloem. The first and commonest form is the "collateral" bundle, where the xylem and phloem are placed side by side with or without cambium between them, the xylem being always towards the pith or the central part of the stem, the phloem external. In Ciicurbita, Solanum, and others the bundles are " bicollateral, " there being an additional phloem portion inside the xylem. "Concentric " bundles occur in many vascular cryptogams, the central xylem being completely sur rounded by the phloem. The last form is the " radial," where the bundles of phloem and xylem are arranged alternately in the central fibro-vascular axis, as in most roots. Irregular bundles also occur, and numerous intermediate forms connect the different types. In each of the portions of the bundle different kinds of tissue occur ; tructure but there is a marked similarity in the construction of the phloem f xylem. and xylem, at least in separate bundles and before circumferential growth takes place. In the wood, distinguished by the lignified hard brittle walls of the cells, there are four elements usually present : (1) the wood vessels or cell fusions filled with air, having the trans- verso walls more or less completely absorbed, and having thickened walls marked with rings, spirals, reticulations, or pits of different kinds ; the ends of the cells sometimes are more or less pointed and overlapping, with pitted markings, having, however, a free commu nication from cell to cell through the absorbed thin part of the pits ; (2) tracheides, or vessel-like wood prosenchymatous cells, having walls marked like the vessels, and with the cavity containing air, but never showing any absorption of the end walls and fusion into vessels ; (3) wood prosenchyma or libriform fibres, elongated, pointed, and overlapping cells, exactly resembling bast fibres, often with greatly thickened walls, these walls never having spiral or annular markings, but only small simple or occasionally exceed ingly minute bordered pits ; they are very common in the wood of dicotyledons, and may either be simple or have fine transverse parti tions forming chambers in the long cell ; (4) wood parenchyma, wood cells with thin walls, and simple pits ; these in winter con tain starch, and other reserve materials, along with the cells of the medullary rays, and at other times may contain tannin, chlorophyll, structure or crystals of calcium oxalate. In the bast or phloem portion of if the bundle there are three elements only, as there are no cells ihloem. equivalent to the tracheides. These are (1) the sieve tubes or bast- vessels, cell-fusions like the wood vessels, but having the trans verse portion forming the remarkable sieve plate perforated by the sieve pores, while occasionally such plates or similar markings occur on the side walls r the walls are soft and delicate, giving a cellulose reaction, and the cavity contains abundant protoplasmic contents with excessively minute starch granules ; (2) bast prosenchyma or bast vessels, elongated prosenchymatous cells, with pointed and overlapping ends, the walls so thick as almost to obliterate the cavity ; the walls are soft and flexible, often marked with tine pits ; like the libriform fibres of the wood, they have occasionally the cavity chambered with thin transverse walls, and not imfrequently they branch ; (3) bast parenchyma, repeating the wood parenchyma; but occasionally the cells are long and narrow, exactly like those of only slightly modified procambium, which they really are ; in this state they are often called cambiform cells. The sieve-tubes and bast parenchyma or cambiform cells form the soft bast. These different elements of the wood and bast are not always present, and the secondary wood and bast developed from cambium are often very different from the primary portions developed from procambium. Thus in Cucurbita there are no bast fibres, while in most coniferous woods the tracheides alone are present in the xylem. At the ends of the fibro-vascular bundles in the leaves the different laments gradually disappear until one or two spiral vessels and a K-v cam biform cells alone remain. In most roots the fibro-vascular bundles form a central mass with the phloem and xylem in separate groups and arranged alternately; the xylem masses generally project into the centre, and the oldest vessels are nearest the centre. The whole mass, which is either a single bundle or a group of bundles, is usually surrounded externally by a peculiar layer, thepericambium, in contact with the endodermis or sheath, the inner layer of the, ground tissue, which in roots forms the massive cortical portion. 3. Ground System of Tissues. The ground tissue comprises all that remains after the formation Groum of the epidermal and fibro-vascular systems; and is usually composed tissue, of parenchymatous cells, not in any way distinguishable except by their position from parenchymatous cells in the other systems. In other cases the ground tissue contains prosenchyma, or the cells in certain regions are more or less thickened. When the part contains closed fibro-vascular bundles, as in monocotyledonous stems and in leaves, the ground tissue forms the chief bulk of the part ; but in other cases, as, for instance, in the stems of conifers and dicotyledons, with circum ferential growth, the ground tissue is very feebly developed. In such stems the ground tissue forms the pith and cortex, with the primary medullary rays joining the two. In roots with a central fibro-vascular mass, the cortex is the only part of the ground tissue represented. The ground tissue immediately below the epidermis may be simply parenchymatous, or it may exhibit certain modifica tions. Either the cells form collenchyma, as in many stems and rollcn- petioles, a tissue consisting of mere elongated cells without inter- chyn;a. cellular spaces, and having special masses of thickening matter de veloped on the walls where neighbouring cells meet. These masses readily swell up in water, and probably act as a sort of erectile tissue. In other cases a greater or less development of hypodermais observed Hypo- in leaves and stems, the cells being elongated and greatly thickened derma. and sclerenchymatous, resembling in most points the bast-fibres of the fibro-vascular bundles. In some plants, as in ferns, separate, often dark-coloured, bundles of sclerenchyma occur in the ground tissue. These different elements form part of what has been distinguished as the "mechanical" system of tissues, hardened cells giving rigidity to the different parts of the plant, and although such cells occur in very different parts of plants, as in fibro-vascular bundles and in the ground tissue, still they have a marked external resemblance, and are closely related physiologically. Thick, short, sclerenchymatous cells occur in the ground tissue, as in the pulp of the pear ; in other cases the parenchyma is unthickened, and contains either colourless contents or develops chlorophyll. The part of the ground tissue next the fibro-vascular bundles forms the sheath or endodermis, Endo- a layer of cells often thickened or cuticularized, and surrounding dermis. either single bundles or the whole vascular mass or series of fibro- vascular bundles. In some cryptogams the endodermis is strength ened by numerous sclerenchymatous cells surrounding it either partially or completely. The ground tissue of tree LiliacecK, and even in some abnormal dicotyledons, forms a layer of secondary meristem cells capable of developing both new ground tissue and new fibro-vascular bundles ; and it is in this way that the secondary circumferential growth in the stems and roots of Dracccna, and pro bably of the fossil vascular cryptogams, took place. The secondary circumferential growth of gymnosperms and dicotyledons is the result of the activity of the cambium ring formed by the fascicular cambium and the interfascicular cambium in the ground tissue, as already described. The changes produced by secondary circum ferential growth are very numerous, and are fully described by De Bary (Vcrgleichcnde, Anatomic, chaps, xiv. and xv.). TiibJiorjraf>hii.1c. chief works to be consulted on the subject of vegetable histology are Heale, On the Microscope; Carpenter. On the Microscope; Do Bury, Vergle.ifhe.nde Anatomic (Hofmeister s Handbvch <ler I hysiologischen Jiotanik, vol. iii.) ; Dippel, Das Mikroskop; Lurssen, Grundziiye der Botanik (- tied.); KaRcli uml Schwendener, Das Mikroskop; Telletan, Le Alic.rofc.ope; Sachs, Text-Hook of Botany, or Lehrbiich der Tloiwiik (4th cd.); Sachs. Qtschichte der liotatiik; Strasburger, Ueber Zdlbihlnng und Zelltheitmig; Strasburger, /Me
Angiotpennen und die (Jiimnospermen; Van lleurek, Le Microscope.. (W. It. M N.)
