SHOOTS 



83 



STRrCTl'RE 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 

 bv its position or liy being a depository for food. If an 

 active twig of au ordinary woody plant be ctit across, it will 



c^^.- 



^•i"^. 



Fig. 77. Flower clusters of an umbellifer iShtm).— After STR.\sBrnGER. 



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

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

 oft' as a thin skin, the epidermis: (3) within the epider- 

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

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

 central pith. 



58. Dicotyledons and Conifers. — Sometimes the vessels 



