PRACTICAL BOTANY 



further that the newly-formed cell-walls cut the pre-existing 

 cell-walls at right angles, and that the two parts of the cells 

 thus divided are apparently equal to one another. A com- 

 parison of the general arrangement of the cell-walls with the 

 diagram shown in Fig. 3 will help to make clear the arrange- 

 ment of these cell-walls : in drawing the comparison, however, 

 it must not be forgotten that Fig. 3 is a diagram, and cannot be 

 expected to apply in detail. 



The whole merismatic mass is differentiated into parts, which 



FIG. 3. Diagram illustrating the plan of arrangement of cell-walls in the apex of 

 the stem of an Angiosperm. x, x = longitudinal axis, which is the organic axis 

 of the stem. E, E=external surface. P,P, P,P, are the periclinal curves. A,A=the 

 anticlinal curves which cut these at right angles. /,/=incomplete periclinals. 

 a., rt=incomplete anticlinals. The dermatogen is represented by the space 

 between the outer surface, E, E, and the outermost periclinal, p, p. (After Sachs.) 



may be distinguished more or less clearly from one another, 

 and it will be easy to trace their continuity with the several 

 tissue-systems of the stem and leaves, of which in fact they 

 are the formative layers. We may thus distinguish the 

 following : 



I. The dermatogen, as a single continuous layer of cells, 

 which divide only in a direction perpendicular to the external 

 surface of the organ (stem or leaf), which it covers completely : 

 it is easily seen to be continuous with the epidermis, of which 



