THE STEM AND ITS FUNCTIONS 101 



the thick, woody cell-wall is left. Definite thin areas or pits 

 occur at frequent intervals along this wall and facilitate the rapid 

 movement of water. In simpler types of wood, such a cell is 

 able to provide both the necessary rigidity and conductive 

 capacity and is known as a tracheid. In the higher types, how- 

 ever, this simple element has become specialized in two directions 

 and has given rise to long and very heavy-walled cells, the 

 wood-fibers, in which almost no cavity remains and which con- 

 tribute a high degree of mechanical strength to the wood; and 

 the vessel-cells or tracheal cells, much shorter, with wide cavities, 

 and walls which are comparatively thin and are provided with 

 large perforations in their ends. These cells, laid end to end in 

 vertical rows, constitute the ducts or vessels, so characteristic of 

 the wood of many plants, which carry the ascending stream of 

 water through the stem. Parenchyma cells sometimes occur 

 among the lignified elements and like them may be elongated 

 vertically (Fig. 52, G). Other parenchyma cells are elongated 

 at right angles to the stem (Fig. 52, F) and dispersed among the 

 woody cells in horizontal bands or ribbons running out through 

 the xylem along the radii of the stem. These structures are 

 known as the wood-rays, and in somewhat modified form 

 extend also into the bast. They facilitate the horizontal 

 transfer of materials in the stem and are of particular importance 

 as centers of food-storage. 



Cambium. — A narrow layer of thin-walled cells, the cambium, 

 separates the wood from the bast. Through its activity new cells 

 are added to the outside; of the wood and the inside of the bast, 

 and the thickness of the stem is thereby increased. Among 

 woody plants, such growth continues from year to year and each 

 season's increment, or annual ring, is easily recognizable. 



At each node a small but complete segment of the fibro- 

 vascular ring separates from the rest and passes out through the 

 cortex into the base of the petiole, causing a break, or leaf-gap, 

 in the ring. Into each leaf may enter one, three, five or more of 

 these leaf-traces which are destined to pass upward through the 

 petiole and to form the system of veins in the blade. 



Woody and Herbaceous Stems. — The perennial woody stem in 

 which the fibro-vascular cylinder, as seen in cross section, forms a 

 continuous and rather wide ring (except for the leaf-gaps), and 

 which receives adcUtions in thickness year by year through 

 cambial activity, is probably the most ancient stem-type among 



