STEMS, KOOTS, AND LEAVES 97 



thickening of the cell walls and some increase in diameter. The 

 cells destined to form the strengthening fibers of the wood 

 undergo much greater changes in form and in the thickening of 

 their cell walls to produce the pointed, thick-walled permanent 

 fibers. These fibrous cells also lose their living contents and are 

 finally useful only as mechanical tissue elements of the wood. 



The vertical rows of cells % produced by the cambium which 

 are to form the ducts undergo very great changes in size, in 

 cell contents, and in the character of the cell walls. Unlike the 

 fibrous cells the ducts are compound structures formed of a 

 vertical row, or chain, of cambium cells joined end to end. Each 

 one of these cells enlarges greatly in diameter during the matur- 

 ing stage, and the protoplasm begins to deposit new cell-wall 

 substance in definite patterns on the thin cell wall of the original 

 cambium cell. These new cell-wall deposits may take the form 

 of spiral or reticulate (netlike) thickenings, or they may take 

 place over most of the cell wall, leaving thinner places, called 

 simple pits, on the cell wall. These simple pits then become 

 arched over, forming what are called bordered pits. When the 

 growth in diameter and the thickening of the walls of the duct 

 are completed, the protoplasm dies and the transverse walls 

 between the constituent cells partially or wholly disappear. 

 These changes finally result in a long, tubular duct, which 

 allows the easy vertical flow of water and soil salts. The thin 

 portions of the lateral walls also allow of lateral migration of 

 fluids, while the thickened parts secure strength to the tree and 

 prevent the collapse of the duct when the stem is bent by the 

 wind or by other agencies. 



Phloem elements. The fibers and living cells of the phloem 

 not directly connected with the sieve tubes have much the same 

 history as that just outlined for similar elements of the xylem. 

 The sieve tubes, however, have a somewhat different series of 

 developmental stages. The mother cells of sieve tubes, which are 

 formed by the cambium, divide once after their formation, giving 

 rise to a future sieve-tube cell and a living companion cell. The 

 companion cell undergoes very little farther differentiation, but 

 the sieve-tube cells, like the cells composing the ducts of the 



