38 DIFFERENTIATION OF THE TISSUES 



their origin, and in some instances certainly for several years, but 

 it may be stated as a rule that they soon become crushed by the 

 pressure of surrounding tissues, and their effectiveness is thereby 

 reduced, and after a time altogether destroyed (Fig. 24). 



The three kinds of cells taking part in the formation of the 

 phloem as described above do not always occur together; in 

 Monocotyledons, for example, the parenchymatous elements are 

 absent, and in Gymnosperms and Pteridophytes the companion 

 cells are lacking. 



The primary xylem (namely, that portion of the xylem that 

 has differentiated from the procambium and exclusive of that 

 which is added later by the cambium) may consist of three classes 

 of elements, the tracheal or water tubes, tracheids, xylem paren- 

 chyma, and wood fibers. These elements do not, however, com- 

 monly all occur together. The wood fibers are usually absent, 

 and tracheids are not common in the primary xylem of Angio- 

 sperms; while in Gymnosperms true tracheal tubes do not occur, 

 with few exceptions. 



A tracheal tube is produced by the absorption of the end or 

 transverse walls in a vertical row of cells, and at the same time 

 the enlargement in all dimensions of the cells composing the tube, 

 and the subsequent unequal thickening, and lignification of the 

 vertical walls. The thick places in the walls are for strengthening 

 the tube, while the thin places are for the easy passage in and out 

 of water and materials in solution. In the tubes first formed the 

 thick places are in the form of rings or a spiral coil. To realize 

 the use of these, imagine barrel hoops or a flexible spiral coil of 

 wood sewed inside a bag to make it stand open. This type of 

 tracheal tube is differentiated from the procambium not far from 

 the growing apex in internodes that have not yet ceased elongating, 

 and it will be seen that the corresponding growth in length which 

 these tubes must undergo will be but little resisted by the kind 

 of thickenings which their walls possess. In older internodes 

 where elongation has nearly or quite ceased stronger tracheal 

 tubes are laid down having thin places of more restricted area in 

 the form of pits or elongated meshes. 



