INTRODUCTION. XXIX 



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



wax, oils, camjjhor, 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 RajMdes. 



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



193. Leaves, young stems, and branches, and most parts of ph»nogamous plants, 

 during the first year of their existence consist anatomically of 



1, a cellular system, or continuous mass of cellular tissue, which 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 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, ^ fihro-vasnilar 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 j)hrcnogamou9 

 plant. 



3, an epidermis, or outer skin, formed of one or more layers of flattened (horizon- 

 tal), firmly coherent, and usually empty cells, with either tliin 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 designed 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 baric (198, 5). In Endogens (Monocotyle- 

 dons) the woody system is in separate small bimdlcs 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 organization, 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 sel- 

 dom, if ever, any distinct pith, the concentric circles of fibro-vascular 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 rind is observable even in several Endogens. More frequently, how- 

 ever, the distinction is already veiy apparent the first season, especially towards its 

 close. The fibro-vascular bundles in Endogens usually anastomose but httle, passing 

 continuously into the branches and leaves. In Exogens the circle of fibro-vascular^ 

 bundles fomns a more continuous cylinder of network emitting lateral offsets into the' 

 branches and leaves. 



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