DISEASES OF TREES FROM MECHANICAL INJURIES. 157 



essentially the cambium which is the growing formative, part of the stem, 

 whereas the wood cells formed constantly on its inner side soon cease to 

 grow and, although their walls become thick and hard, the cell contents 

 disappear, so that the cells of the hard wood are practically dead and unable 

 to produce new cells. They form a series of hard tubes very important in 4 

 the economy of the plant, by giving strength and rigidity and serving as 

 means of passage to liquids and gases. 



We must consider especially the action of the epidermis and the cam- 

 bium. As has been said, the colorless epidermal cells differ from the cells 

 beneath them in being thinner and natter. The latter include the chem- 

 ically active cells which in the younger parts of plants transform the food 

 elements into special substances of use to the plant. 



The epidermal cells, on the other hand, form merely a thin, protective 

 membrane. They serve in the first place to check evaporation and, 

 furthermore, their outer wall is usually transformed into a cuticle which is 

 nearly impervious to water and is unaffected by a good many substances 

 which would injure the walls of ordinary cells. An important property for 

 us to consider in this connection is their ability to resist the growth of the 

 mycelium of many fungi, which, when the epidermis is removed, are able 

 to make their way to the more delicate and succulent cells beneath, a point 

 to which I shall refer later. 



As the cambium constantly increases in circumference and the new wood 

 and inner bark increase correspondingly in bulk, it is plain that the 

 epidermis, unless endowed with the power of increasing in circumference, 

 must soon be ruptured, thus exposing the more delicate cells beneath. 

 The epidermis does not possess this power except to a very limited extent, 

 but to avoid the danger which must follow an exposure of the sub-epider- 

 mal cells to the air after the rupture of the epidermis, which must inevita- 

 bly take place early in the life of a plant, nature makes provision for the 

 transformation of the sub-epidermal cells into a zone of cork cells, which act 

 as a protective sheath after the epidermis proper is ruptured. The way in 

 which cork cells are formed is seen on a small scale when a potato tuber 

 is cut in halves. The wounded cells shrivel and die, but the more or less 

 spherical cells beneath become divided into a series of thinner, flatter cells 

 by the formation of new cell walls parallel to the cut surface, and the 

 walls themselves become tough and resistant. The epidermis of the stems 

 and branches in reality remains intact but a short time, usually only one 

 year, and then is ruptured and soon disappears; but, meanwhile, the sab- 

 epidermal cells, having been changed into a series of cork cells like those 

 mentioned in the cut potato but on a larger scale, form a new proctective 

 covering which replaces the epidermis. Furthermore, the new cork layer 

 itself is only to a moderate extent capable of extension, and as the inner 

 parts of the stem continue to increase it is in turn ruptured, and the breaks 

 are closed by the formation of a second layer of cork cells beneath. This 

 process is repeated indefinitely, so that in stems several years old, we have 

 what is in popular language called the bark, composed of several different 

 layers of cork cells more and more split up and cracked externally. 



If we now recognize the structure of the normal stem or trunk in its 

 essential points, we can next consider the primary effects of wounds. In 

 the first place, whenever a trunk or branch is wounded, no matter whether 

 by the action of wind or snow, by the bites of animals, by pruning, or by 

 wilful violence of man, nature itself attempts to heal the wound if pos- 

 sible. If the wound is not too great, it heals by natural processes, but 



