OF THE STRUCTURE OF WOOD AND HERBS. 81 



the wood. In the pine, tor instance, the tubes containing the turpentine exceed 

 the common sap-vessels in magnitude by three or four hundred times, and are 

 surrounded by a ring of smaller tubes. In drawing 143, the proper vessels of the 

 bark containing the milky juice of the sumach are arranged in arched clusters, 

 each cluster consisting of several hundred distinct tubes, and so small are these 

 tubes that a single turpentine vessel of the pine sometimes vies in magnitude 

 with an entire arched cluster of the sumach. 



In figure 126 the vessels of a certain species of pine are Fjg 126 



displayed, where the large turpentine tubes of the bark are 

 seen encircled by a ring of smaller tubes; the woody part of 

 the tree having been cut away, so that the longitudinal as 

 well as the transverse structure can be clearly seen. Dr. 

 Hill discovered by close investigation, that each of these 

 large tubes was nothing more than an opening in a bun- 

 dle of small tubes, and he remarks "that if we conceive 

 any small bundle of tubes to be opened in its centre, and 

 the vessels driven every way outward until they are stop- 

 ped by the substance of the bark, we shall have an idea of the structure of this 

 larger vessel; which is nothing more than a great cylindrical tube, passing 

 through the centre of a bundle of smaller ones." This structure is plainly per- 

 ceived in the tigure. 



The vessels containing the fluids peculiar to the bark are often found dispersed 

 through the wood from bark to the pith. Thus, in the fir and pine, the turpen- 

 tine and gum-tubes are seen in the wood, arranged in a circle around the centre, 

 tn nearly the same way as the sap-vessels. These vessels are regarded by na- 

 turalists as having once belonged to the bark, which, changing into wood by the 

 natural growth of the tree, in the manner soon to be explained, and becoming 

 encased annually in successive layers of wood, was gradually removed farther 

 and farther from the exterior surface of the tree. The skin or rind of the bark 

 when taken from young shoots, appears in most cases to consist of a single layer, 

 but in many kinds of wood it is found to be complex ; as in the case of the white 

 birch, in which it consists of distinct layers that are readily separated from each 

 other, amounting not unfrequently to sixteen or eighteen in number. 



In some trees the layers are still more numerous, for Ulloa speaks of a tree 

 in Peru, from the rind of which he peeled off no less than one hundred and 

 fifty envelopes ; when, tired of his task, he refrained from counting the remain- 

 der, as the layers he had detached did not constitute more than half the thick- 

 ness of the rind. 



THE MODE OF GROWTH IN THE TRUNK AND BRANCHES OF TREES. Commencing 

 on the outside of the tree, the exterior covering is the skin or rind, consisting, as 

 has already been stated, of several distinct layers. Beneath this is the bark, compo- 

 sed of cellular tissue and bundles of tubes or vessels running longitudinally ; and at 

 lirst parallel to each other. When a cross section is made of a shoot of a year old, 

 6 



