Xl INTRODUCTION. 



dextrine, or vegetable mucilage, a gummy substance, between mucilage 

 and starch. 



starch or fecula, one of the most universal and conspicuous of cell-con- 

 tents, 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 JRaphides. 



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



Plants. 



193. Leaves, young stems, and branches, and most parts of phsenogamous 

 plants, during the first year of their existence consist anatomically of 



1, a cellular system, or continuous mass of cellular tissue, which is de- 

 veloped both vertically as the stem or other parts increase in length, and hori- 

 zontally or laterally as they increase in thickness or breadth. It surrounds or 

 is intermixed with the fibro-vascular system, or it may exist alone in some parts 

 of phsenogamous plants, as well as in cryptogamous ones. 



2, a fibro-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 phsenogamous 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. 



191. 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 atmosohere. They communicate with in- 

 tercellular cavities, and are obviously designed to regulate evaporation and 

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

 face. 



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 tw r o great classes of phamogamous plants 

 called Exogens and Endogens, which correspond with very few exceptions to 

 the two classes Dicotyledons and Monocotyledons (167), founded on the struc- 

 ture 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 



