DISEASES OF TREES LIKELY TO FOLLOW INJURIES. 143 



through the vascular buudles shows that the cells of which they 

 are composed are, iu great part, very much elongated, so that 

 they maj' be called fibres, ducts, or vessels. When seeu in cross- 

 section, each vascular bundle is wedge-shaped, and, if carefully 

 examined, is found to consist of an outer and an inner part ; that 

 is, in respect to the circumference of the tree. The inner portion 

 develops into the bard wood of the stem, while the outer part 

 becomes a portion of what, for want of a better expression, we 

 may call the inner bark, or bast. Between the outer and inner 

 parts of the vascular bundle is a thin layer of small, colorless, 

 brick-shaped cells, the cambium. The cambium is the most 

 important part of the stem, since its cells during the season of 

 growth are constantly forming new wood cells on the inner side, 

 while those on the outer side are forming new cells of the inner 

 bark. The cambium itself does not vary much in thickness at 

 different ages, and, extending continuously throughout the length 

 of the stem, forms the circumference of a cylinder whose diameter 

 increases from year to year. It is important to bear in mind that 

 it is 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 iu the economy of the 

 plant, by giving strength and rigidity and serving as means of 

 passage to liquids and gases. 



We must consider especiall}' the action of the epidermis and the 

 cambium. As has been said, the colorless epidermal cells differ 

 from the cells beneath them in being thinner and flatter. The 

 latter include the chemicalh' 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. The}' serve in the first place to check 

 evaporation and, furthermore, their outer wall is usually trans- 

 formed into a cuticle which is nearly impervious to water and 

 is unaffected b^^ a good many substances which would injure the 

 walls of ordinary cells. An important property for us to consider 

 iu this connection is their ability to resist the growth of the 

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



