THE STEM— ITS GENERAL STRUCTURE 



6i 



Fig. 71. — Diagram of 

 Wood Strands or 



FiBRO-VASCULAR 



Bundles in a 

 Root, showing the 

 wood (;f) and bast 

 (/) separated. 



and shriigth to the plant, and of long tubular interrupted 



canals that serve to convey sap upivard from the root and to 



convey food downward from the leaves to the stem and tlie 



roots. 



Monocotyledons, as shown by fossils, existed before 



dicotyledons appeared, and it is thought that the latter 



were developed from ancestors of the 



former. It will be interesting to trace 



the relationship in stem structure. It 



will first be necessary to learn something 



of the structure of the wood strand. 

 Wood Strand in Monocotyledons and 



Dicotyledons. — Each wood strand (or 



fibro-vascular bundle) consists of two 



parts — the bast and the wood proper. 



The wood is on the side of the strand 



toward the centre of the stem and con- 

 tains large tubular canals that take the watery sap upward 



from the roots. The bast is on the side toward the bark, 



and contains fine tubes 

 through which diffuses 

 the dense sap contain- 

 ing digested food from 

 the leaves. In the root 

 (Fig. 71) the bast and 

 the wood are separate, 

 so that there are tzvo 

 kinds of strandL 



In monocotyledons, 

 as already said, the 

 strands (or bundles) are 

 usiLally scattered iji the 



Stem tvitJi no definite arrangemc7it (Figs. 72, 73). In 



dicotyledons the strands, or bundles, are arranged in a 



Fig. 72. — Part of Cross-section of Root- 

 stock OF Asparagus, showing a few fibro' 

 vascular bundles. An endogenous stem. 



