38 VEGETABLE PHYSIOLOGY 



of size, new conducting tissue must be formed. The 

 skeleton of a young plant is consequently much smaller 



FIG. 40. SECTION OF DICOTYLEDONOUS STEM (Helianthus}. 

 A, young condition, with the primary vascular bundles only developed ; 

 B, older, after secondary bundles have been developed between the 

 primary ones by the interfascicular cambium, forming a ring of wood. 



than that of an old one. The difference between the con- 

 dition of a stem at two periods may be seen by comparing 

 fig. 40, A and B, the former of which shows the arrange- 



