THE STEM— ITS GENERAL STRUCTURE 



6l 



Fig. 71. — Diagram of 

 Wood Strands or 

 fibro-vascular 

 Bundles in a 

 Root, showing the 

 wood (pc) and bast 

 (/) separated. 



and strength to the plant, and of long tubtdar interrupted 

 canals that serve to convey sap upward from tlie root and to 

 convey food downward from the leaves to the stem and 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 center 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 two 

 kinds of strands. 



In monocotyledons, 

 as already said, the 

 strands (or bundles) are 

 usually scattered in the 

 stem with no definite arrangement (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. 



