62 INTRODUCTION TO BOTANY 
60. Internal structure of the young dicotyledonous stem.! 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 
e be p 
Fic. 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 Fie. 42, B. Diagrammatic cross sec- 
pith; m, medullary ray. The space tion of one quarter of a sunflower stem 
between the hard bast and the 
bundles is occupied by thin-walled, p, pith; fv, woody, or fibrovascular, bun- 
somewhat cubical cells of the bark. dles; e, epidermis; 0, bundles of hard-bast 
Magnified about 15 diameters fibers of the bark. 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 't Textbook 
of Botany,’’? Chap. iv, A. ANGIOsPERMS. 
