INTRODUCTION. XXvU 



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2. Arrangemenl of the Elementary Tissues, or Structure of the Organs of Hants. 



193. Leaves, young stems, and branches, and most parts of phicnogamous plants, 

 during the first year of then- 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 nicrease in thickness or breadth. It surrounds or is intermixed with the fibro- 

 Tascular system^ or it may exist alone in some parts of phaenogamous plants, as well 

 as in cryptogamous ones. 



2, ^fibro-vascular sj/stem, or continuous mas? of woody and vascular tissue, which 

 18' gradually introduced vertically into, and serves to bind together, tlie cellular system. 

 It is contnuied from thp stem into the petioles and veins of the leaves, and into the pe- 

 dicels and parts of the flowers, and is never wliolly wanting in any plurnogamous plant. 



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

 tal), firmly coherent, and usually empty cells, with cither tliin and transparent or thiek 

 and opaque walls. It covers ahnost 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 freqiiently pierced by minute spaces between the cells, called 

 Stomaies. TJicy are oval or month-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 phnenogamous plant has outlived the first season of its growth, the 

 anatomical structure of its stem or other perennial parts becomes more complicated 

 ^id very different in the two great classes of pha^nogamous plants called Exogens and 

 Andogens, 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 

 ptth (198, 1), and an external separable bark (198, 5). In Endogens (Monocotyle- 



.^? 1 ^^^ ^'oody 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. 



190. Tlic anatomical structure is also somewhat difl'erent in the dilTcrcnt organs of 

 plants. Ill the Root^ although it is constructed generally on the same plan as the 

 Btem, yet the regular organization, and the dtiicrence between Exogens and Endogens, is 

 otten disguised or obhternted by irregularities of growth, or by the production of large 

 quantities of cellular tissue fdled with starch or other substances {192)- There is sel- 

 dom, if ever, any distinct pith, the concentric circles of flbro-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 

 tliJerence between Exogens and Endogens is not always very conspicuous. In both 

 Jnere is a tendency to a circular arrangement of the fibro-vasciJar system, leaving 

 the centre either vacant or filled with cellular tissue (pith) only, and a more or less 

 ttititmet outer rind is observable even in several Endogens. More frequently, how- 

 ^'er, the distinction is already very apparent the first season, especially towards its 

 Ciose. Tlie fibro-vascular bundles in Endogens usually anastomose but little, passing 

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

 Dundles forms a more continuous cylinder of network emitting lateral offsets mto the 

 ^I'anches and leaves. 



198. The Exogenous stem, after the first year of its growth, consists of^ 



1, thc^/rt, a cylinder of cellidar tissue, occupying the centre or longitudmal axis 

 ot the sfciu. It is active only in youn^r stems or branches, becomes dried up and com- 

 presscd as the wood hardens, and often finally disappears, or is scarcely distmguishable 

 in old trees. ^ a ir 



. 2, the medullar^/ slieatli, which surrounds and encases the pith. It abounds in 

 spiral vessels (ISS, 3), and is in direct connection, when young, with the leaf-buds and 



