30 THE STRUCTURE OF ECONOMIC PLANTS 



spiral elements in Ricinus communis, in which multinucleate cells 

 are formed from uninucleate procambial cells. After attaining a 

 maximum elongation, a fine spiral of cellulose is laid down 

 throughout the entire length of the vessel; and the development 

 of the spiral and its initial lignification proceed rapidly, keeping 

 pace with the deposition of cellulose. During lignification, the 

 protoplasm of the developing vessel does not appear to disintegrate. 



In the later stages of axial development, the thin-walled pro- 

 toxylem elements may be ruptured owing to the stretching and 

 subsequent collapse of the walls. This frequently occurs with 

 annular elements as in the bundle of Zea, where the disintegration 

 of the protoxylem and adjacent parenchyma results in the formation 

 of a large lacuna. In reticulate and pitted vessels, there may be a 

 penetration of the lumen of the vessel segment by the cytoplasm of 

 adjacent parenchymatous cells. This frequently occurs in Cucur- 

 bita and Solanum and results in the formation of tyloses which may 

 plug the vessel passage and render it functionless except for the 

 support afforded by its walls. (Fig. 318.) 



Xylem Fibers. — The structural details of fibers have been 

 discussed, but there is a special relationship existing between the 

 xylem fiber and the tracheid, since both are constituent elements of 

 the complex xylem tissue. The xylem fiber may be regarded as a 

 tracheid which has undergone changes in the course of phylogeny 

 so that it is non-functional as a conductive element, but has in- 

 creased properties for mechanical ^support. Because of this, no 

 sharp line can be drawn between xylem fibers and tracheids; and 

 there are intermediate types known as fiber-tracheids. Some of 

 these may have a limited conductive function in addition to their 

 mechanical qualities, since their pits permit some movement of 

 water and substances in solution. 



Xylem Parenchyma. — The parenchymatous cells of the xylem 

 which form an important constituent in the xylem complex, 

 especially in many herbaceous angiosperms, do not differ in general 

 characteristics from those described in the section on parenchyma. 

 They are usually arranged in longitudinal rows and may be elon- 

 gated in the axial direction, except in the xylem rays, where their 

 radial dimension is the greatest. The walls are relatively thin at 

 first, but frequently become thickened and lignified later. The pits 

 are simple in the walls of adjacent parenchymatous cells, while 

 half-bordered pits occur where they abut vessels or tracheids. 



