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 

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

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





Fig. 77. Flower clusters of an umbellifer {Sium).—MiQT SxRASBrROER. 



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 reqio7i ; (4) a 

 central pith. 



58. Dicotyledons and Conifers. — Sometimes the vessels 



