SUNFLOWER APICAL BUD 81 



clear the arrangement of these cell- walls : in drawing 

 the comparison, however, it must not be forgotten that 

 Fig. 5 is a diagram, and cannot be expected to apply in 

 detail. 



The whole merismatic mass is differentiated into 

 parts, which may be distinguished more or less clearly 

 from one another, and it will be easy to trace their con- 

 tinuity 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 : 



1. 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 con- 

 tinuous with the epidermis, of which it is the for- 

 mative layer. Within this is a solid mass of tissue, 

 which looks for the most part dark, owing to its being 

 permeated by intercellular spaces filled with air. It is 

 traversed at a short distance from the external surface 

 by transparent, longitudinal bands of 



2. Procambium, which is the formative tissue of 

 the vascular bundles. Trace its continuity with these. 

 Between the procambial bands and the dermatogen 

 lies 



3. The formative tissue of the cortex (periblem) 

 which is (partially at least) characterized by dark- 

 looking intercellular spaces. 



4. Centrally lies a dark bulky cylinder, which is 

 continuous with, and formative of, the pith. 



Observe carefully the mode of origin of the leaves. 

 They appear at the periphery of the cone as protuber- 

 ances of the dermatogen and the subjacent cells: the 



G 



