132 ANATOMY OF STEM, ROOT AND LEAF 



tous tissue termed callus (c). In the outer parts of the latter 

 there soon forms a cork-cambium while within it is developed 

 a new cambium from which wood and bast are ultimately pro- 

 duced. Year by year the new tissues produced by the cambium 

 extend further and further inwards over the exposed wood (iJ) 

 until the edges meet all round, after which time the cambium 

 exists as a continuous layer over the wounded surface (C, Fig. 6i). 



The new wood formed as a cap-like covering over the exposed 

 old wood {b) does not actually coalesce with the latter and the 

 position of old wounds into the wood can always be easily 

 recognised in sections, although they may be so completely 

 overgrown and buried in the succeeding growth that no external 

 sign of their existence is visible. 



The length of time necessary to cover a wound depends upon 

 its size, and the vigour and nutrition of the cambium. Clean 

 cut wounds heal more rapidly than jagged ones, and when large 

 branches are amputated with a saw it is advisable to trim the 

 exposed edges of the cambium with a sharp chisel or knife. 

 In the case of wounds where a considerable portion of old 

 wood is laid bare and which cannot therefore be overgrown 

 in a short time it is also important to cover this portion of 

 the wounded surface with Stockholm tar or some similar 

 antiseptic dressing to prevent its decay. 



Ex. 59. — Cut across one, two and three year old branches of ash, and 

 make the surface of the section smooth with a sharp knife : notice the annual 

 rings in each. 



Make longitudinal sections of similar pieces of ash twigs, and notice the 

 arrangement of the yearly growths where one piece joins another a year 

 younger (compare with Fig. 57). 



Make similar observations on as many common trees as possible. 



Ex. 60. — Prepare sections of a piece of a larch pole 4 or 5 inches in 

 diameter : cut with a saw and then carefully smooth with a sharp chisel or 

 plane. 



Transverse, longitudinal, and oblique sections should be made. 



Study the arrangement of the yearly rings in sections cut as in Fig. 58 to 

 illustrate the nature of a knot. 



