INTRODUCTION. 



A tree many be defined as — a woody plant of any size 

 which produces naturally one main, erect stem with a definite 

 crown of branches. A shrub is a woody plant which produces 

 small irregular or slanting stems usually in tufts. In attempt- 

 ing to separate "trees" from the larger "shrubs," one must neces- 

 sarily be somewhat arbitrary as nature draws no definite division 

 line. 



A tree has three main parts, the root system, the stem or 

 trunk, and the crown of branches. The root system is for sup- 

 port in the soil and for taking up water and various mineral sub- 

 stances. This is accomplished by delicate organs called root 

 hairs developed near the root tips. Although largely dependent 

 on their environment the roots still have considerable selective 

 power in taking up the salts dissolved in the soil water. The 

 root system may have a main or tap-root extending deep into the 

 ground with smaller lateral roots ; or the tap-root may be only 

 slightly developed or entirely absent, in which case a number of 

 larger branch roots may extend downward from the base of the 

 trunk. The roots which extend laterally near the surface some- 

 times run to a great distance. Such roots are called tracing 

 roots. 



The trunk or bole is a supporting and conducting organ. 

 The water with dissolved mineral substances taken up by the 

 roots passes up through the young wood or xylem while the food 

 material from the young twigs and leaves passes up or down 

 through the phloem cells of the inner bark. There are no real 

 vessels for carrying food and water like the blood vessels of ani- 

 mals, but the sap passes through by osmosis from one cell to an- 

 other or from one set of cells to another as it frequently hap- 

 pens when some of the cross walls are broken down in a vertical 

 series of cells. The large cells in the wood, however, are called 

 wood vessels. Just how the water is able to pass up to the tops 



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