94 GENERAL BOTANY 



composed of chains of cells in which the transverse walls have 

 wholly or partly disappeared, thus forming a long tube for the 

 conducting of water. These water ducts are usually not more than 

 three or four inches in length, although they may be from three 

 to six feet in length in oaks and many feet in length in some 

 climbing plants. The first-formed ducts are also characterized 

 by thickenings on their lateral walls in the form of spirals, rings, 

 and netlike or reticulate thickenings. In the later-formed ducts 

 these walls are often marked with peculiar pits, which give the 

 name dotted ducts to such water vessels. These thickenings on 

 the wall of a duct serve to strengthen it, while the thin places 

 between the thickenings allow water and salts to move later- 

 ally through the duct wall. The ducts are therefore admirably 

 adapted for strength and for the rapid conduct of water. Between 

 the ducts are the smaller fibrous cells of the xylem, some of 

 which are living, while others are dead. The living cell elements 

 are termed wood parenchyma ; they either serve for storage of 

 reserve foods during periods of inactivity or they conduct foods 

 locally to and from the wood rays. These wood-parenchyma 

 cells are usually thick-walled, with fine perforations in the cell 

 walls, through which protoplasmic strands connect the proto- 

 plasts of adjacent cells. The lifeless elements are fibrous cells 

 resembling closely those of the phloem and serve mainly to 

 strengthen the stem. In some trees, however, they remain living 

 for a considerable time and serve for storage, while in others 

 they conduct water and soil salts. The walls of wood fibers are 

 usually thick, with minute pits or pores which mark thinner 

 places in the cell wall. 



The wood rays are composed of thin-walled living cells with 

 their long axes running radially in the stem. In a tangential sec- 

 tion of wood the rays are very numerous and are made up of 

 more or less elliptical masses of living cells. In radial view they 

 are plates of cells running across the section. Their primary 

 function is the conduction and storage of organic food, although 

 they serve as lateral water carriers in some plants. 



