264 TEXTBOOK OF BOTANY 



water-conducting tubes (vessels), which are visible through the hand 

 lens, will reveal one of the two reasons why annual rings are evident to 

 the eye. Wood cells must be seen through a microscope to discover 

 the second reason. Annual rings are clearly evident in stems of pine and 

 other conifers that have no vessels. 



Usually radially arranged rays, the vascular rays, can be seen in the 

 split surfaces and ends of blocks of wood. The vascular rays in the 

 xylem may be called xylem rays. On the split surfaces they appear as 

 smooth, narrow, thin ribbons of different lengths extending crosswise 

 to the "grain" of the wood. The xylem rays may also be seen on pieces 

 of polished lumber one side of which was cut in a plane parallel to them. 

 We shall see presently, however, that much of the conspicuous grain of 

 polished wood is dependent upon differences in the wood cells in each 

 annual cylinder of xylem. 



Bark tissues. From the bark of a living woody twig or small branch 

 one may scrape or pick off an outer layer of brownish cork tissue, be- 

 neath which there is a layer of parenchyma, which is usually green 

 because many of the cells contain chlorophyll. These two layers of the 

 bark are generally referred to as the coHex. The parenchyma is the 

 cortical parenchyma, and any part of it that contains chlorophyll may 

 also be called cortical chlorenchyma. The cortex is just within the epi- 

 dermis, which, being only one cell thick, may be difficult to detect with- 

 out a microscope. If one carefully scrapes the cortical chlorenchyma 

 away with the edge of a knife, a number of hard fibers extending 

 lengthwise in the bark become visible. These are pericycle fibers. If the 

 point of the knife is slipped under these fibers, they may be lifted a 

 slight distance from the stem before they break. Beneath the pericycle 

 fibers is a layer of soft, usually colorless, tissue. When this layer is 

 scraped off, the outer surface of the wood cylinder is visible. This inner- 

 most soft tissue of the bark is the phloem, containing the food-conducting 

 cells. In the stems of some species, such as basswood and pawpaw, the 

 phloem also contains fibers, phloem fibers, comparable to the pericycle 

 fibers already noted. The tenu bast fibers is sometimes used commer- 

 cially to refer to any of the fibers in the bark, whether in the cortex, 

 pericycle, or phloem. In some twigs the radially arranged vascular rays 

 in the phloem, phloem rays, are visible, but they can usually be seen 

 only through a microscope. 



'The phloem and wood cylinder together constitute the vascular 

 cylinder, or stele, of the stem. Its outer boundary is the pericycle. The 



