Rind 



Pith 



Vascular 

 bundles 



Conductive 



Rind Pith bundles 



Kislit. i? General Binlogical Supply House. Inc. 



CONDUCTING TISSUES IN CORN STEM 



The tough fibrovascular bundles of conducting cells ore surrounded by tender pith 

 cells; these con be readily shredded away and the bundles exposed. The arrange- 

 ment of the bundles clustered toward the outer rind is analogous to the hollow-tube 

 construction of a bicycle frame as a supporting structure 



In dicotyledonous stems the fibrovascular bundles are arranged sym- 

 metrically around the center. As in the monocots, the xylem tubes are 

 toward the center, and the phloem tubes are toward the outside. In the 

 dicots, however, these two sets of vessels are separated by a layer of un- 

 differentiated, growing cells. This layer is called the cambium layer. The 

 new cells which the cambium produces toward the center become woody 

 fibers and xylem tubes. Cells formed on the outer side of the cambium 

 become bast fibers and phloem tubes. As the stem grows in thickness, the 

 cambium layer is pushed away from the center. As the bark is pushed out- 

 ward, the outermost layers split or peel in various ways. This results in the 

 characteristic markings of various species, such as a birch tree or an oak, 

 for example. 



Circulation of Sap in Plants The rise of water to the tops of tall trees 

 has always puzzled people. There was no systematic study of the problem 

 before about 200 years ago, when Stephen Hales (1677-1761), an English 

 preacher, first used mercury gauges to measure the pressure with which sap 

 rises in plants. Hales came upon the idea of measuring the sap pressure 

 when he tried to stop the "bleeding" of a vine. He tied a piece of bladder 

 over the cut end, and then noticed that the bladder swelled up. He continued 

 his experiments and showed that the root pressure, which we now recognize 



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