76 Roots: their Structure and Uses. 



Form, Functions) and Manner of Growth of the Roots. 



267. Eoots may be divided into two classes: Tap-roots, which 

 descend deeply into the soil, and derive their nourishment from the 

 sub-soil ; or Tracing-roots, that extend laterally and nearer the sur- 

 face. The latter will sometimes run to a great distance. 



268. It is sometimes found that a mixture of different species of 

 timber trees makes a better growth upon the whole, in a woodland, 

 than one kind alone. In these cases one has often a tap-root, and 

 the other a tracing-root, thus drawing their support from different 

 depths of the soil. The oak and the beech are examples of this 

 kind, and we shall have further occasion to speak of this subject. 



269. The roots of our common trees are made up of concentric 

 layers, and grow' by external addition under the bark, in the same 

 manner as the trunk and branches. Their bark is generally thin, 

 and they often grow one into another, and in a much more irregular 

 form than the branches. There is often no apparent difference in 

 texture, corresponding to sap and heart wood. The stumps and 

 roots of trees are in European countries often used for fuel, and in 

 making charcoal, and in the resinous kinds they, in some instances, 

 abound in turpentine, and are used in making tar and pitch. 



270. The fiber of root-wood is sometimes very strong and flexible, 

 as in the young spruce. The stump and roots in some trees have a 

 twisted and contorted grain that gives them great value for orna- 

 mental cabinet-work. The " Thuja," of the Atlas Mountains (Cal- 

 litris quadrivalvis) , one of the most highly prized of fine woods, is an 

 example. The black walnut often furnishes in its stump and large 

 roots, a beautiful grain in the wood. 



271. The roots of trees will insinuate themselves into the thinnest 

 crevices where they can find moisture and soil, and by their expan- 

 sion hasten disintegration, and sometimes raise and displace heavy 

 masses of rock. For this reason, trees should not be planted near 

 the lines of covered aqueducts, or other important works of masonry 

 that might be injured by displacement. 



272. The roots of poplars and willows will fill water-pipes, and 

 even wells, if they can reach them. This tendency to penetrate 

 damp soil, renders these kinds, including the cotton woods, very use- 

 ful in consolidating the banks of ditches, especially those used for 

 irrigation in arid regions. They doubtless evaporate a part of 



