SUNFLOWER STEM 75 



c. Parenchyma, which is to be found more espe- 

 cially around the vessels near the central limit of the 

 bundle. The phenomenon of thyloses is the result 

 of the encroachment of these cells on the cavity of the 

 vessels. The normal individual cells are oblong with 

 square ends, they have cellulose walls (note their 

 reactions), and retain their protoplasmic contents. 



The cells termed thyloses (Tiillen) are properly included under 

 the term xylem-parenchyma, being derived directly from this 

 tissue in the following way. When fully developed the vessels 

 have lost their protoplasmic contents and their turgescence ; their 

 walls are pitted, at some points being thin at others strongly 

 thickened. If thin-walled tissue, the elements of which are 

 active and turgescent, abut on such a wall, it is obvious that 

 but slight resistance to the internal tension will be offered at 

 the pits, where the wall of the vessel is thin. As a result the 

 wall bulges at these points, and the cells encroach as papillae 

 upon the cavity of the vessel. Cell-divisions may occur in 

 these papillae, and the whole process be continued till the 

 cavity of the vessel is completely filled with a cellular tissue. 



Look in the longitudinal sections of the old stem of Helianthus 

 for instances of such encroachment of cells upon the cavity 

 of the vessel. Good results may be obtained from the old 

 stem, or root, of Cucurbita, and from the stems of Robinia, or 

 Vitis. 



5. The central pith is composed of parenchymatous 

 cells, with thin walls consisting of cellulose : the walls 

 are slightly pitted : these cells have lost their proto- 

 plasmic contents in many cases, and especially near the 

 centre of the stem. Occasional resin-passages may be 

 found in the pith. 



