THE STEM ITS MODE OF GROWTH. 



37 



207. It is the corky envelope which exfoliates -in 

 large continuous sheets from the Birch tree. This is of a 

 peculiar structure, the firm layers of tabular cells alter- 

 nating with others less compact and durable, which, 

 when existing in contact with a vital layer below, crumble 

 to dust, and thus cause the bark to peel off. 



208. CORK. Sometimes an unusual development of 

 this tissue occurs, as in the Cork-Oak. The bark of this 

 tree, which is a native of Southern Europe, exfoliates 

 once in eight or nine years, affording that very useful 

 article known in commerce as CORK. Some species of 

 Elm, and the Sweet Gum tree, also yield cork. 



209. At fig. 4 is a section from the stem of the 

 Maple, at the commencement of the second year's growth. 

 A is the transverse section ; B is the vertical section. 

 The spiral vessels around the pith are seen at t ; the 

 ducts are marked v p, and the bundles of woody fibre, 

 f ; c the cells of the cambium layer; pc pc, the cellular 

 layers of the new wood and bark ; / the fibrous cells, or 

 liber, between; s the corky layer; and, outside of all, is 

 the common integument, with projecting hairs. At fig. 

 9 is a transverse section of the Oak, showing, in the six 

 white concentric circles of wood, the growth of six years, 

 while the same number of rings are seen in the bark. 

 The circleo of dark dots inside of the woody tissue show 

 the position of the ducts. 



210. COMPARATIVE CONTINUITY OF THE BARK AND 

 WOOD. The same circumstance which solidifies and 

 makes the wood durable, contributes to the destruction 

 of the Bark namely, the GROWTH. As this chiefly 

 occurs in the Cambium Layer, between the wood and 

 bark, it must be external to the first, and internal to the 

 last Hence, while the old wood on the one hand is 

 rendered compact and hard by the inward pressure, the 

 bark, on the other, is greatly distended and torn by the 

 outward pressure. Thus while the one is continually 

 confirmed in its solidity, the other is continually under- 

 going changes, both from this cause and the corroding 

 influence of the elements without. Hence the bark on 

 old trees is never entire. It is marked by cracks through 

 its whole extent, and often by deep fissures. The dead 

 parts fall off in layers ; and as the cells of the bark soon 

 cease to grow, the proportion of bark to wood is much 

 less in old than in younger trees. 



211. The three layers of bark which have been de- 

 scribed are not to be distinguished in old trees, or rather 

 in old parts of the tree, for there the middle or green 



What layer next? Describe. Colon, Spring-walk. What substance pro- 

 d*d! From what tree? What other trees ? Longest life has Bark or Wood? 

 Bcuot of the difference. Trace the component parts of the Stem. What layer 

 bliunud In old bark ? 



layer is generally obliterated ; but in young woody stems 

 and branches they are very distinct and clear. 



CHAPTER XVI. 



THE STEM ITS MODE OF GROWTH. 



212. BY the first and most obvious principle of divi- 

 sion, stems are ranked according to their SIZE, as consti- 

 tuting Trees, Shrubs and Herbs. Between the two first 

 there is no absolute line of division, as a small tree may 

 be a large shrub, and the reverse. There are, however, 

 some general distinctions worth observing. 



213. A TREE is a plant with a woody stem, that 

 rises a considerable height from the ground before it 

 puts forth any branches ; and some authors suppose 

 that it should be about five times the height of a man to 

 be assured of its title. But sometimes large trees branch 

 close to the ground ; and there are many which it would 

 scarcely do to call shrubs, that are less than twenty feet 

 high. 



214. A SHRUB is a plant with a woody stem, that 

 branches near the ground, and is less than twenty-five 

 feet high, as the Currant. 



215. UNDER-SHRUBS are more diminutive forms of 

 the shrub, as the Whortleberry and Sweet Fern. 



216. AN HERB is generally a plant of smaller growth, 

 composed chiefly of cellular tissue, and having compara- 

 tively a small portion of wood in its system, as the Daisy 

 and Sunflower. 



217. The Herb, Tree and Shrub, are well contrasted 

 in figs. 1, 2 and 3, Plate VIII. But there is a funda- 

 mental difference in stems, by which they are ranked 

 according to their 



218. MODE OF GROWTH. All Phaenogamous plants 

 are thus divided into two great ranks Exogens, or Out- 

 side Growers, and Endogens, or Inside Growers. The 

 first correspond with Dicotyledones, and the second with 

 Monocotyledones ; .for all plants with two cotyledons 

 have exogenous stems, and all with one cotyledon have 

 monocotyledonous stems. Exogens may be represented 

 by the Almond tree, fig. 2, and by most of our com- 

 mon herbs, shrubs and forest trees ; and Endogens by 

 fig. 6, and by the Lily, Grass, and Palm tribes gene- 

 rally. We will now proceed to notice the differences of 

 structure that distinguish these strongly defined classes. 

 All their varieties radically depend on the MODE OF 



General subject Most obvlons division of stems. Define Tree Shrub 

 Under-Shrub. Herb. What division? Correspondence. Synonyme*. Ex- 

 plain. 



