14 WOOD AND FOREST. 



Bark may be classified according to formation and method of 

 separation, as scale bark, which detaches from the tree in plates, as 

 in the willows ; membraneous bark, which comes off in ribbons and 

 films, as in the birches; fibrous bark, which is in the form of stiff 

 threads, as in the grape vine; and fissured bark, which breaks up 

 in longitudinal fissures, showing ridges, grooves and broad, angular 

 patches, as in oak, chestnut and locust. The last is the commonest 

 form of bark. 



The bark of certain kinds of trees, as cherry and birch, has pe- 

 culiar markings which consist of oblong raised spots or marks, es- 

 pecially on the young branches. These are called lenticels (Latin len- 

 ticula, freckle), and have two purposes: they admit air to the internal 

 tissues, as it were for breathing, and they also emit water vapor. 

 These lenticels are to be found on all trees, even where the bark is 

 very thick, as old oaks and chestnuts, but in these the lenticels are 

 in the bottoms of the deep cracks. There is a great difference in the 

 inflammability of bark, some, like that of the big trees of California, 

 Fig. 54, p. 209, which is often two feet thick, being practically in- 

 combustible, and hence serving to protect the tree; while some bark, 

 as canoe birch, is laden with an oil which burns furiously. It there- 

 fore make? admirable kindling for camp fires, even in wet weather. 



Inside the cork is the "phloem" or "bast," which, by the way, 

 gives its name to the bass tree, the inner bark of which is very tough 

 and fibrous and therefore used for mat and rope making. In a liv- 

 ing tree, the bast fibers serve to conduct the nourishment which has 

 been made in the leaves down thru the stem to the growing parts. 



(2) The cambium. Inside of the rind and between it and the 

 wood, there is, on living trees, a slimy coat called cambium (Med. 

 Latin, exchange). This is the living, growing part of the stem, 

 familiar to all who have peeled it as the sticky, slimy coat between 

 the bark and the wood of a twig. This is what constitutes the fra- 

 grant, mucilaginous inner part of the bark of slippery elm. Cambium 

 is a tissue of young and growing cells, in which the new cells are 

 formed, the inner ones forming the wood and the outer ones the bark. 



In order to understand the cambium and its function, consider its 

 appearance in a bud, Fig. 5. A cross-section of the bud of a growing 

 stem examined under the microscope, looks like a delicate mesh of thin 

 membrane, filled in with a viscid semi-fluid substance which is called 

 "protoplasm" (Greek, protos, first; plasma, form). These meshes 



