OUTLINES OF BOTANY. xxi 



and nitrogen), which fills an important part in cell-division and growth. Within the cell (either 

 in the centre or exoentrical) is usually a minute, soft, subgelatinouB body called the nucleus, 

 whose functions appear to be intimately connected with the first formation of the new cell. As 

 this cell increases in size, and its walls in thickness, the protoplasm and watery cell-sap become 

 absorbed or dried up, thefiim cellulose wall alone remaining as a permanent fabric, either empty 

 or filled with various organised substances produced or secreted within it. 



192. The principal organised contents of cells are 



sap, the first product of the digestion of the food of plants ; it contains the elements of 

 vegetable growth in a dissolved condition. 



sugar, of which there are two kinds, called cane-sugar and grape-sugar. It usually exists 

 dissolved in the sap. It is found abundantly in growing parts, in fruits, and in germinating seeds. 



dextrine, or vegetable mucilage, a gummy substance, between mucilage and starch. 



starch oifecula, one of the most universal and conspicuous of cell-contents, and often so 

 abundant in farinaceous roots and seeds as to fill the cell-cavity. It consists of minute grains 

 called starch- granules, which vary in size and are marked with more or less conspicuous 

 concentric lines of growth. The chemical constitution of starch is the same as that of cellulose ; 

 it is unaffected by cold water, but forms a jelly with boiling water, and turns blue when tested 

 by iodine. When fully dissolved it is no longer starch, but dextrine. 



chlorophyll, very minute granules, containing nitrogen, and coloured green under the action 

 of sunlight. These granules are most abundant in the layers of cells immediately below the 

 surface or epidermis of leaves and young bark. The green colouring matter is soluble in alcohol, 

 and may thus be removed from the granules. 



chromule, a name given to a similar colouring matter when not green. 



wax, oils, camphor, and resinous matter, are common in cells or in cavities in the tissues 

 between the cells, also various mineral substances, either in an amorphous state or as 

 microscopic crystals, when they are called Raphides. 



§ 2. Arrangement of the Elementary Tissues, or Structure of the Organs of Plants. 



193. Leaves, young stems, and branches, and most parts of phtenogamons plants, during the 

 first year of their existence consist anatomically of 



1, a cellular system, or continuous mass of cellular tissue, wliioh is developed both vertically 

 as the stem or other parts increase in length, and horizontally or laterally as they increase in 

 thickness or breadth. It surrounds oris intermixed with the fibrovascular system or it may 

 exist alone in some parts of phtenogamous plants, as well as in cryptogamous ones. 



2, Sj fiiro-vascular system, or continuous mass of woody and vascular tissue, which is 

 gradually introduced vertically into, and serves to bind together the cellular system. It is 

 continued from the stem into the petioles and veins of the leaves, and into the pedicels and parts 

 of the flowers, and is never wholly wanting in any phasnogamous plant. 



3, An epidermis, or outer skin, formed of one or more layers of flattened (horizontal), firmly 

 coherent, and usually empty cells, with either thin and transparent or thick and opaque walls. 

 It covers almost all parts of plants exposed to the outward air, protecting their tissues from its 

 immediate action, but is wanting in those parts of aquatic plants which are constantly submerged. 



194. The epidermis is frequently pierced by minute spaces between the cells, called Stomates. 

 They are oval or mouth shaped, bordered by lips, formed of two or more elastic cells so disposed 

 as to cause the stomate to open in a moist and to close up in a dry state of the atmosphere. They 

 communicate with intercellular cavities, and are obviously desighed to regulate evaporation and 

 respiration. They are chiefly found upon leaves, especially on the under surface. 



195. When a phsenogamous plant has outlived the first season of its growth, the anatomical 

 structure of its stem or other perennial parts becomes more complicated and very different in the 

 two great classes of phsenogamous plants called Exogens and Endogens, which correspond with very 

 few exceptions to the two classes Dicotyledons and Monocotyledons (167), founded on the structure 

 of the embryo. In Exogens (Dicotyledons) the woody system is placed in concentric layers 

 between a central pith (198, 1) and an external separable bark (198, 5). In Endogens 

 (Monocotyledons) the woody system is in separate small bundles or fibres running through the 

 cellular system without apparent order, and there is usually no distinct central pith, nor outer 

 separable bark. 



196. The anatomical structure is also somewhat different in the different organs of plants. 

 In the Root, although it is constructed generally on the same plan as the stem, yet the regular 

 organisation, and the difference between Exogens and Endogens is often disguised or obliterated 

 by irregularities of growth, or by the production of large quantities of cellular tissue filled with 

 starch or other substances (192). There is seldom, if ever, any distinct pith, the concentric 

 circles of fibro-vaseular tissue in Exogens are often very indistinct or have no relation to seasons 

 of growth, and the epidermis has no stomates. 



197. In the Stem or branches, during the first year or season of their growth, the difference 

 between Exogens and Endogens is not always very conspicuous. In both there is a tendency to 

 a circular arrangement of the fibro-vascular system, leaving the centre either vacant or filled 

 with cellular tissue (pith) only, and a more or less distinct outer riud is observable even in 



