STRUCTURE OF THE STEM 67 
the same weight per foot. The oat straw, the stems of 
bulrushes, the cane (of our southern canebrakes), and the 
bamboo are hollow cylinders; the cornstalk is a solid 
cylinder, but filled with a very light pith. The flinty 
outer layer of the stalk, together with the closely packed 
sclerenchyma fibers of the outer rind and the frequent 
fibro-vascular bundles just within this, are arranged in the 
best way to secure stiffness. In a general way, then, we 
may say that the pith, the bundles, and the sclerenchyma- 
tous rind are what they are and where they are to serve 
important mechanical purposes. But they have other uses 
fully as important. 
84. Growth of Monocotyledonous Stems in Thickness. 
In most woody monocotyledonous stems, for a reason 
which will be explained later in this chapter, the increase 
in thickness is strictly limited. Such stems, therefore, as 
in many palms and in rattans, are less conical and more 
cylindrical than the trunks of ordinary trees and are also 
more slender in proportion to their height (Plate II). 
STEM OF DICOTYLEDONOUS PLANTS 
85. Gross Structure of an Annual Dicotyledonous Stem. — Study 
the external appearance of a piece of sunflower-stem several inches 
long. If it shows distinct nodes, sketch it. Examine the cross- 
section and sketch it as seen with the magnifying glass or the dissect- 
ing microscope. After your sketch is finished, compare it with Fig. 39, 
which probably shows more details than your drawing, and label 
the parts shown as they are labeled in that figure. Split a short 
piece of the stem lengthwise through the center and study the split 
surface with the magnifying glass. Take a sharp knife or a scalpel 
and carefully slice and then scrape away the bark until you come to 
the outer surface of a bundle. 
