xliv OUTLINES OF BOTANY. 



gamous 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 sur- 

 rounds 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), d^ fibro-vascular system, or continuous mass of woody and vascular 

 tissue, which is gradually introduced vertically into, and serves to bind to- 

 gether, 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 phaenogamous plant. 



(3), an epidermis, or outer skin, formed of one or more layers of flat- 

 tened (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 imme- 

 diate action, but is wanting in those parts of aquatic plants which are con- 

 stantly 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 re- 

 gulate evaporation and respiration. They are chiefly found upon leaves, 

 especially on the under surface. 



195. "When a phaenogamous 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 

 phaenogamous 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 (Mono- 

 cotyledons) 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 be- 

 tween Exogens and Endogens, is often disguised or obliterated by irregulari- 

 ties 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-vascular tissue in Exogens are> 

 often very indistinct or have no relations 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, however, the distinction is already 



