SHOOTS. 



83 



STRUCTURE AND FUNCTION" OF THE STEM. 



57. Stem structure. — The aerial foliage stem is the most 

 favorable for studying stem structure, as it is not distorted 

 by its position or by being a depository for food. If an 

 active twig of an ordinary woody plant be cut across, it will 



Fig. 



Flower clusters of an umbellifer (JSvum).— After Strasburger. 



be seen that it is made up of four general regions (see Fig. 

 78): (1) an outer protecting layer, which may be stripped 

 off as a thin skin, the epidermis ; (2) within the epider- 

 mis a zone, generally green, the cortex ; (3) an inner zone 

 of wood or vessels, known as the vascular region ; (4) a 

 central pith. 



58. Dicotyledons and Conifers. — Sometimes the vessels 



