THE PRIMARY PERMANENT TISSUES 41 



and the wood fibers. The parenchyma cells are, as a rule, the 

 longest lived of all xylem elements. They have been found still 

 carrying on the vital function of starch storage in rings of growth 

 nearly a century old. 



After the elements of the primary xylem and phloem have 

 been completed it is found that a layer of undifferentiated pro- 

 cambium cells remains between them in Dicotyledons, while in 

 Monocotyledons and Pteridophytes the w r hole of the procambium 

 enters into the composition of phloem and xylem. The layer of 

 undifferentiated cells in Dicotyledons is known as the cambium, 

 and its most important characteristic is that it retains the power 

 of cell division for a longer or shorter time, even indefinitely in 

 the case of woody perennials. 



The divisions of the cambium cells may take place trans- 

 versely, or vertically in radial and tangential planes; but the 

 tangential vertical division is by far the most frequent and gives 

 rise to cells known as the tissue mother cells, which by one or 

 more cell divisions produce other cells that differentiate into the 

 elements of the xylem on the one hand and those of the phloem 

 on the other. These new additions to the primary vascular 

 bundle constitute the secondary xylem and phloem. 



The medulla or pith is formed by transverse and vertical 

 divisions of the ground meristem, and the subsequent enlarge- 

 ment in all dimensions of the daughter cells. The pith cells 

 are not apt to be much elongated in any one dimension, and 

 their walls, as a rule, remain relatively thin and unchanged 



the pits have an overhanging border, the walls have become lignified as indicated by the 

 stippling, and finally the protoplasts have disappeared, and the tube is mature and dead ; 

 B, stages in the formation of tracheids from procambial or cambial cells. The steps are 

 the same as in A, excepting that the cross-walls remain and become pitted. C, steps in 

 the development of wood fibers from cambial cells; i, cambial cells; 2, the same grown 

 larger in all dimensions with cells shoving past each other as they elongate; 3, a later 

 stage with cells longer and more pointed and walls becoming thickened and pitted ; 4, 

 complete wood fibers with walls more thickened than in the previous stage and lignified, 

 as shown by the stippling. The protoplasts in this last stage have disappeared and the 

 fibers are dead. D, steps in the formation of wood parenchyma from cambial or pro- 

 cambial cells, i, group of cambial or procambial cells; 2, the same enlarged in all dimen- 

 sions; 3, the same with walls thickened and pitted; 4 and 5 show the same stages as 2 and 

 3, but here the cells have enlarged radially or tangentially more than they have vertically. 

 The walls of these cells are apt to become lignified, but the cells are longer lived than the 

 wood fibers. 



