INTRODUCTION. 



A tree may 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 

 necessarily be somewhat arbitrary as nature draws no definite 

 division line. There are also trees which have no crown of 

 branches, like the palms. 



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

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

 support in the soil and for taking up water and various 

 mineral substances.- This is accomplished very largely by 

 delicate organs called root hairs developed near the root tips. 

 The root hairs are temporary structures developed from the 

 epidermis of the root (piliferous layer) near the tip and 

 dying off at the back as the root grows. Although largely 

 dependent on their environment the roots still have consider- 

 able 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 sometimes 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 animals, but the sap passes through by osmosis 

 from one cell to another or from one set of cells to another 

 as it frequently happens when some of the cross walls are 

 broken down in a vertical series of colls. The large cells in 



D. M. H»lii' UBItARY 



North Carolina SUt» Coil«C* 



