THE WALLFLOWER 83 



Now, the growing point and the very youngest 

 leaves consist entirely of thin-walled cells rich in 

 protoplasm, with relatively large nuclei. Such cells 

 have already been described (p. 28). All these cells 

 are in course of active growth and division, and 

 therefore the tissue of the growing point is called 

 meristem or dividing tissue. From this meristem all 

 other tissues are derived. In favourable sections we 

 can distinguish three distinct meristematic strata at 

 the apex ; the outermost of these is a single layer 

 of cells, dividing exclusively by walls at right angles 

 to the surface, and giving rise to the epidermis only. 

 This is called the dermatogen (Fig. 32, ep). It ex- 

 tends not only over the growing point of the stem, but 

 also over the young leaves, for the dermatogen forms 

 the epidermis of the leaves as well as that of the 

 stem. Next below the dermatogen we find a stratum 

 which may be only one cell thick at the extreme 

 apex, but which lower down divides by cell-walls 

 parallel to the surface as well as by walls at right 

 angles to it, and thus becomes several layers in thick- 

 ness. This is the periblem, or young cortex (Fig. 32, 

 i). Lastly, underneath this again, we find a group of 

 meristematic cells which divide in all directions, and 

 form the central cylinder of the stem, including the 

 pericycle, vascular bundles, pith, etc. This last group 

 is called the plerome (Fig. 32, pi). 



The dermatogen is always a distinct layer. The 

 periblem and plerome, however, cannot in all cases be 

 distinguished from each other at the extreme apex. 



