36 



ANALYTICAL CLASS-BOOK OF BOTANY. 



sented by open mouths a, and that of the latter by minute 

 specks, , showing that they have nearly passed into the 

 solid state, the bore being almost obliterated. In fig. 5 

 is a transverse section of wood from the same tree, show- 

 ing more clearly the wide bore of the ducts, and the dot- 

 like calibre of the wood-cells, which are almost entirely 

 filled up by an accumulation of the Secondary deposit 

 (38), that has greatly thickened their walls. 



200. LIGNINE. The wood-cells and ducts collec- 

 tively, or the substance of wood, is termed LIGNINE. 

 This substance is richer in carbon and hydrogen, and 

 hence in the materials of combustion, than any other 

 tissue in the vegetable frame. But it does not consist 

 entirely of organic material, for in the thickening of its 

 cells a large quantity of earthy matter is deposited in 

 their walls, as you have before learned. This consists 

 chiefly of salts of Potash, and is shown by the deposit 

 left after burning, for the chemical basis of common 

 wood ashes is potash ; and if there were no earthy element 

 there would be no ashes. The harder kinds of wood, as 

 Oak, Maple and Hickory, abound in the ligneous deposit, 

 while they have less of water than the softer kinds ; and 

 this gives them their great value, not only as fuel, but 

 in the various mechanic arts. 



201. HEART-WOOD AND SAP-WOOD. The old wood 

 that occupies the centre of the stem is called the HEART- 

 WOOD; while that which lies next the bark, from its 

 beautiful white color, is named the ALBURNUM, or Sap- 

 wood. In the latter alone the vital forces of the wood 

 exist ; for the Heart-wood is so far a dead substance, that 

 it appears useful to the tree chiefly, if not entirely, in 

 affording mechanical support, and preserving the unity 

 of the system, by keeping the form entire. We may see 

 the truth of this assumption in those trees that live, and 

 bear fruit, many years after their trunk has become a 

 hollow cylinder. Whenever the distinction between 

 the Heart-wood and Sap-wood is strongly marked, the 

 latter assumes a particular and definite color. The yel- 

 low hue of the Fustic, the black of the Ebony, the beau- 

 tiful dark red of the Mahogany, and the rich colors of 

 the Black Oak and Black Hickory, all exist in the 

 Heart-wood. 



202. THE CAMBIUM LAYER is a stratum of extremely 

 delicate and vital cells, which are interposed between 

 the wood and bark, and belong equally to both. They 

 are of an oblong form, and may be seen in the lines e e, 



Of what besides organic material does -wood consist ? How shown ? Of what 

 salt chiefly consist? Why are hard woods better than others? The sub- 

 stance of wood name in what is it rich ? two principal parts of wood. 

 In what part are vital actions maintained? Why called Alburnum? Po- 

 sition of Heart-wood. What proves it a dead substance? Colors. Cam- 



fig. 8. They are formed of a mucilaginous liquid called 

 the CAMBIUM, which has already been noticed (54). 

 From these cells is developed a layer of Wood on the 

 one hand, and a layer of Bark on the other; while 

 another supply is sent off to replenish and reinvigorate 

 the Medullary Kays. 



203. THE BARK is at first composed entirely of 

 parenchyma, and the external layer is green, from the 

 chlorophylle (65) which it contains. But when the stem 

 begins to form woody tissue, a portion of woody fibre is 

 distributed in the Bark, the inner layers of which become 

 fibrous, while the outer layer gradually loses its green 

 color, and assumes an ashen gray. The Bark is com- 

 posed of three distinct layers the Liber, the Cellular 

 Envelope, and the Corky Envelope. 



204. THE LIBER. This is the white inner layer of 

 the bark, and is so called, not only because it exfoliates 

 in thin plates resembling the leaves of a book, but be- 

 cause the liber of many trees has been used instead of 

 paper for writing. The fibres of this integument are 

 long and very tenacious, as you have seen in the de- 

 scription of the Bass-tissue (42); and for this reason 

 they are used for many purposes in the manufacture of 

 cordage, and various woven fabrics, a more particular 

 account of which will be found in that section of the 

 Economical Uses of Plants that treats of the Bark. The 

 position of the Liber is seen &tf, in fig. 8, being next 

 outside of the Cambium Layer. 



205. THE CELLULAR ENVELOPE, or Middle Layer, 

 of the Bark comes next in the outward direction. This, 

 in the young state, may be known by its green color, and 

 from this circumstance is sometimes called the Green 

 Layer. It is shown at g, in fig. 8, and does not usually 

 increase after the first year or two. By the thickening 

 of the Corky deposit next outside, it is excluded from the 

 light, without which chlorophylle cannot be formed ; and 

 it gradually perishes, never to be renewed. 



206. THE CORKY ENVELOPE.- This, in woody stems, 

 surrounds the Cellular Envelope, and consists entirely of 

 flattened tabular cells, as may be seen at h, fig. 8. The 

 peculiar colors that distinguish the new bark and young 

 twigs of different species, are due to this envelope. 

 Every one who has had the taste for a walk in the woods 

 early in spring, must have observed the pleasing variety 

 of tints which they assume, changing from sober ash to a 

 a lively brown, crimson, or bright yellow, thus giving to 

 the monotonous landscape the vivid effect of flowers. 



binm Layer, define. Of what formed ? What does it develope T Bark, 

 of what first composed ? How many layers ? What called ? Inner bark, de- 

 scribe. Why so called ? Its fibre. Its particular tissue. Position of Liber. 

 What layer next ? How known in its young state ? Is it permanent ? 



