THE PLANT BODY 



81 



2. Stem 



Just as cell division in the meristem tissue of the growing 

 tip of the root proceeds in such a way that the direction 

 of growth is typically downward, so in the similar region at 

 the growing point (BUD) of the shoot (stem and leaves) cell 

 multiplication results in progress upward. In other words, 

 from the region where root and shoot merge, the growth of 

 the plant is in opposite 

 directions. 



The general character 

 of the embryonic tissue 

 is essentially the same 

 in both the root and 

 shoot regions, being 

 composed of densely 

 packed, more or less 

 cubical cells which are 

 completely filled with 

 protoplasm. At this 

 stage the cells have not 

 become modified for 

 special functions by the 

 formation of vacuoles of cell sap, or in other ways. But all 

 the tissues of the stem are derived from the cells of the 

 growing point, so that slightly below this region three areas 

 are to be noted in which differentiations are in progress. 

 These result in the formation of the outer cylinder (epider- 

 mis), the intermediate cylinder (cortex), and the central 

 cylinder (stele) in which the ground tissue (pith) is gradually 

 encroached upon by the developing vascular bundles. The 

 general arrangement of these fundamental tissue systems 

 may be seen in a transverse section of the young stem of a 

 Bean which affords an excellent working plan of stem anat- 



FIQ. 45. Portion of a cross section of the stem 

 of a young plant (Ricinus). ca, cambium; co, cor- 

 tex; e, epidermis; p, pith; ph, phloem; x, xylem. 

 (From Gager.) 



