62 



INTRODUCTION TO BOTANY 



60. Internal structure of the young dicotyledonous stem. 1 It 

 is a difficult process, involving much careful work with the 

 microscope, to trace the earliest steps in the development of 

 stem structure in the seedling plant. It is therefore better, 

 for our purpose, to begin with the study of the stem at the 

 end of the first season's growth. 



In early spring, before the buds 

 begin to open, a twig of willow, 

 alder, or hickory is readily stripped 

 of its bark. When split through the 

 middle it shows a hollow cylinder 



FIG. 42, A. Diagrammatic cross 

 section of one quarter of a one- 

 year-old stem of Dutchman's- 

 pipe (Aristolochia) 



e, region of epidermis ; b, hard bast ; 

 o, outer, or bark, part of a bundle 

 (the cellular portion under the let- 

 ter) ; w, inner, or woody, part of bun- 

 dle ; c, cambium layer ; p, region of 

 pith ; m, medullary ray. The space 

 between the hard bast and the 

 bundles is occupied by thin-walled, 

 somewhat cubical cells of the bark. 

 Magnified about 15 diameters 



b 



...fv 



FIG. 42, B. Diagrammatic cross sec- 

 tion of one quarter of a sunflower stem 



p, pith ; fv, woody, or fibrovascular, bun- 

 dles ; e, epidermis ; b, bundles of hard-bast 

 fibers of the hark. Somewhat magnified 



of wood inclosing the cylindrical pith. These structural con- 

 stituents bark, wood, and pith make up almost the entire 

 bulk of the stem. 



Examined in section by the aid of a good lens, young dicoty- 

 ledonous stems are readily seen to be classifiable into two 



1 See also sections 61-65. The stems of many gymnosperms -for example, 

 trees of the Pine family in their general structure much resemble the 

 dicotyledonous stems. For a general account of the stem structure of dicoty- 

 ledons and monocotyledons see Coulter, Barnes, and Cowles's "Textbook 

 of Botany," Chap, iv, A. ANGIOSPERMS. 



