GROWING POINTS I 9 



cells are arranged in several layers (Fig. 8) which can be traced 

 back into the different regions of the mature plant. As the cells 

 of the growing point multiply, those that remain near the apex 

 retain their power of division, whilst those that come to lie 

 further back gradually pass over into permanent tissue. 



In the stem the outermost layer of the meristem usually 

 divides only by walls at right angles to the surface to produce 

 a single layer of cells which in the more mature region can be 

 recognised as the epidermis (Fig. 8, ep.). The innermost cells 



C. 



B.J.S. 



FIG. 8. Growing point of the stem of the Mare's Tail (Hippuris) in 

 longitudinal section, showing the regions of vascular cylinder (V.C.), 

 cortex (C.), and epidermis (ep.), and the single meristematic cell (i.) 

 from which the central cylinder arises. 



derived from the growing point divide in all directions, and can 

 be traced back into the central region of the stem, which latter 

 consists of the vascular strands and pith (Fig. 8, V.C.). The 

 middle layers of the meristem also segment in like manner, and 

 develop into the tissue situated between the epidermis and the 

 vascular strands, a part of the mature stem known as the cortex 

 (Fig. 8, C.). There are thus, at the growing point of the stem, 

 three meristematic regions, known as the dermatogen, periblem, 

 and plerome, which respectively give rise to the epidermis, 

 cortex, and vascular cylinder of the adult. 



