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 sohd 

 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. 



