DIFFERENTIATION OF TISSUES 



283 



At the apex of the growing stem there is a group of 

 active meristem cells, from which all the tissues are de- 

 rived (Fig. 266). This group is known as the apical group. 

 Below the apical group the tissues and regions of the stem 

 begin to appear, and still farther down they hecome dis- 

 tinctly differentiated, passing into permanent tissue, the 

 apical group by its 

 divisions continually 

 adding to them and 

 increasing the stem 

 in length. 



Just behind the 

 apical group, the 

 cells begin to give the 

 appearance of being 

 organized into three 

 great embryonic re- 

 gions, the cells still 

 remaining meristem- 

 atic (Fig. 266). At 

 the surface there is a 

 single layer of cells 

 distinct from those 



within, known as the dermatogen, or "skin-producer," as 

 farther down, where it becomes permanent tissue, it is the 

 epidermis. In the center of the embryonic region there 

 is organized a solid cylinder of cells, distinct from those 

 around it, and called the plerome, meaning u that which 

 fills up." Farther down, where the plerome passes into 

 permanent tissue, it is called the central cylinder or stele 

 ("column"). Between the plerome and dermatogen is 

 a tissue region called the periblem, meaning "that which 

 is put around," and when it becomes permanent tissue it 

 is called the cortex, meaning "bark " or "rind." 



Putting these facts together, the general statement is 

 that at the apex there is the apical group of meristem cells ; 



FIG. 266. Section through growing point of stem of 

 Hippuris : below the growing point, composed 

 of a uniform meristem tissue, the three embry- 

 onic regions are outlined, showing the dermato- 

 gen (d, d), the central plerome (p, p), and be- 

 tween them the periblem. After DK BARY. 



