[Chap. XXVIII TISSUES AND PROCESSES OF STEMS 279 



entrance of water. In the zone of cell enlargement, and especially toward 

 its lower end, some of the cells are visibly different from others; that is, 

 the results of cell differentiation are becoming evident. 



Various types of cell differentiation in stem tips may be noted; ( 1 ) 

 some of the cells soon become the primordia of leaves and axillary 

 buds; (2) these axillary buds, and sometimes the terminal bud, while 

 still meristematic, may become either vegetative buds or flower buds 

 (Fig. 83 in Chap. XXVI); (3) some of the cells may retain their meri- 

 stematic condition and become the vascular cambium; (4) manv other 

 cells merely enlarge with or without evident change in form and com- 

 position, and, because of their position, origin, shape, and differences in 

 cell walls, may be recognized as belonging to the epidennis, cortex, 

 pericycle, phloem, xylem, or pith; and finally (5) the cross walls of 

 some of the xylem cells partly disintegrate, leaving long tubes, or vessels, 

 the lateral walls of which become thickened and lignified, with spiral 

 thickenings that are particularly conspicuous. Numbers (1) and (2) 

 above may be considered organ differentiations in meristematic tissue, 

 while numbers (4) and (5) are tissue differentiations in enlarging and 

 maturing cells. All the tissues formed by the differentiation of cells that 

 originated in the apical meristem are primary tissues. 



The above description applies to both woody and herbaceous stems 

 of dicots. It applies also to growing stem tips of pines and other conifers, 

 except that xylem vessels do not develop in conifers. Secondary xylem 

 and phloem soon begin to develop from cambial cells. In stems of 

 monocots, such as grasses, in addition to the apical meristem there is a 

 temporary zone of meristematic cells at the base of each internode. 

 This is the softest and weakest part of a growing grass stem. In young 

 woody stems a cork cambium develops from cells in the cortex, and a 

 layer of cork may develop on the new stem segment before the close 

 of the first growing season. In some species of plants latent root pri- 

 mordia develop from parenchyma cells in the pericycle. 



Just below the apical meristem short internodes, as well as nodes bear- 

 ing primordia of leaves and axillary buds, are recognizable. Ultimately, 

 flower primordia develop either from the apical meristem or from the 

 meristems of axillary buds. The cells in the center of the apical meristem 

 are forerunners of pith, which is surrounded by longitudinal strands of 

 primary xylem. 



The pith. Since the pith develops directly from the central cells of 

 the apical meristem, it increases in length as the stem elongates; thus, it is 



