38 VEGETABLE PHYSIOLOGY 
of. size, new conducting tissue must be formed. The 
skeleton of a young plant is consequently much smaller 
Fig. 40.—Srction or DicoryLEponovus 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- 
