34 



ANALYTICAL CLASS-BOOK OF BOTAJSTY. 



194. Component Parts of Wood. — ^By examining 

 fig. 7, Plate VIII., it will be seen tliat wood is not 

 usually composed of woody fibre alone, but that ducts of 

 various kinds enter largely into its composition. In this 

 drawing of the Italian Reed, the diiFerent forms of the 

 cell are represented very clearly. The single woody 

 bundle here exhibited, though from the stem of an 

 Llndogen (224), shows their distinguishing points — be- 

 ginning at the left hand, cellular tissue ; annular and 

 spiral ducts ; then a series of bundles composed of woody 

 fibre, dotted and spiral ducts, and wood ; and, finally, 

 parenchyma again in the cells of the rind, the two outer- 

 most layers of which have some dots of chlorophylle. 



ANATOMY OP THE STEM. 



195. It will be seen by the above analysis that a woody 

 stem usually consists of three principal portions — the 

 Pith, Wood, and Bakk. The organic structure of these 

 must be considered separately. 



196. The Pith. — This, as you have doubtless often 

 seen, occupies the centre of the stem ; and by aid of the 

 microscope wo discover that it is composed of parenchyma, 

 the structure of which has already been examined, and 

 may be seen in fig. 8, at a a. The cut represents a 

 woody stem of three years' growth. The figures 1, 2, 

 3, mark the growth of each year which is deposited in 

 layers, and the letters refer to the same points in both 

 parts of the figure. In the young state, the pith is filled 

 with nutrimental juices ; but often, from a rapid growth, 

 or other causes, it is absorbed, and the centre of the 

 stem becomes hollow, as in the Grasses ; or the pith is 

 pressed inward by the vigor of growth in the outside, 

 and is finally obliterated, as in our forest trees. It ap- 

 pears to be directly serviceable to the plant only in the 

 early stages of life ; for as it advances in strength, it al- 

 ways gradually diminishes in volume, and at length be- 

 comes a dry spongy substance, filled only with air. To 

 the pith belongs 



197. The Medullary Sheath. — This consists of a 

 circle of spiral, or other ducts, immediately surrounding 

 the pith, and connecting it with the wood, as in fig. 8 at 

 d. But the circle of ducts is not complete, for inter- 

 cepting wedges of the woody tissue protrude themselves 

 here and there, sometimes crowding the ducts out of 

 place. Yet still the medullary sheath maintains itself 

 in a general way, sending off its spiral tissue to the 



Of -what not wholly composed ? OompositioQ of Pith. The Medullary 

 Sheath. Three principal parte of a woody stem. Describe Pith. At what 

 3tago most abundant? Probable use. In what plants obliterated? Its 

 changes, describe. Describe Medullary Sheath. What tissue chieHy pre- 

 vails in it? In what parts deposited? Medullary Rays How arranged? 



branches, and from these to the stalk and veins of the 

 leaves. 



198. The Medullary Rays consist of flattened 

 quadrangular cells built up in a single series, like bricks 

 in a wall. They belong especially to the wood, which 

 they traverse at regular intervals, radiating from the 

 Medullary Sheath to the Bark. The two lines of oblong 

 cells passing through fig. 5 are the Medullary Rays ; but 

 in this highly magnified representation, their convergence 

 is not to be seen. This is better shown at i i, in the trans- 

 verse section of figure 8, marked A, where the Medul- 

 lary Rays are represented passing from the Bark to the 

 Pith. But in the vertical section at B they are inter- 

 rupted, the six layers or lines that are represented, not 

 passing from the bark to the pith. The reason of this is 

 that their layers are not arranged in a perfectly vertical 

 direction, and owing to this slight flexure, their lines are 

 divided or cut by a vertical section of the stem. The 

 Medullary Rays are the remains of the Horizontal Sys- 

 tem (37). They have been compared to the woof of the 

 vegetable fabric, as they are delicately shot through the 

 woody fibres of the vertical system, which compose the 

 warp. They serve to keep up a communication between 

 all the growing parts, or the Wood, Pith, and Bark 

 generally. They are called by joiners the " silver 

 grain ; " and although the cells themselves are invisible 

 to the naked eye, yet the lines which they form are con- 

 spicuous in the cros.s section of many kinds of wood, and 

 especially the Oak and Maple, to which they give a 

 beautiful silvery lustre. The Medullary Rays do not 

 always reach entirely from the pith to the bark. 



199. You have already obtained some idea of the 

 general structure of the Wood, as well as its component 

 parts. These are very well portrayed in the Italian 

 Reed, at fig 7, and also in fig 8, where the wooden 

 tubes and ducts — maintain their relative positions in the 

 stem. In a section of the wood of the Plane-tree, fig. 

 10, the structure is seen as it occurs in a more condensed 

 form, but still more highly magnified. The mode in 

 which the tubes, both of the proper wood and ducts, over- 

 lap each other by their pointed ends, is well shown, 

 clearly representing the complete isolation of each cell, or 

 tube, and hence the identity of the general structure 

 with that of parenchyma, or the primitive cellular tissue. 

 The divided ends also show the respective calibre of the 

 ducts and wood-cells — that of the former being repre- 



To what parts belong? IIow deposited? What parts connect? In what 

 section of the iigurc are the lines complete? Why not in tho vertical? Of 

 what are they the remains? To what compared? Wliat called by Joiners? 

 In what woods conspicuous ? Of what is wood formed ? Wliich have greatest 

 calibre ? How are the tube<! arranged, and what does it prove ? 



