STRUCTURE OP EXOGENOUS STEMS. 



97 



and partly of woody fibre 

 centre, and are the parts 

 earliest formed ; the 

 latter protects them on 

 the outside, and is pro- 

 duced towards the end 

 of the season. The 

 mode in which these 

 are arranged, however, 

 varies in different trees ; 

 and it is principally 

 this, which gives that 

 beautiful variety, ob- 

 served in sections of 

 many woods, when ex- 

 amined with the micro- 



The former always lie next the 



e d c d cd. c 6 



de 



a o c a C. a C 



FIG. 49. HORIZONTAL OR TRANSVERSE, AND PERPEN- 

 DICULAR SECTION, OF THE STEM OF AN EXOGEN OF THRKK 



SCOpe. The ducts are YEARS' GROWTH. In the centre of each is seen the pith, 



f A' +* " Vi A a com P se< i f cellular tissue; surrounding it is the 



at Once QlStingUlsnea, me< iullary sheath, 6 ; and exterior to it are three rings 



by the larae size of their * wood eactl consisting of c, e, dotted ducts, and d, d, 



p i it j woody fibre. The last-formed is in contact with the 



onnces; and the WOOCly bark, e, e, in which the layers are indistinct. 



fibres by their com- 

 parative minuteness. Amongst the ducts, however, we usually 

 find lying some elongated cells, which fill up the spaces between 

 them, and which sometimes resemble woody fibres in form, but 

 differ in the firmness of their walls. "Whatever be the number 

 of annual layers, they are always traversed by the medullary 

 rays, which continue to extend outwards, with every addition 

 to the diameter of the stem, even when the pith and the inner 

 layers of wood have decayed away. 



131. In most trees, of which the wood is used as timber, 

 the inner and older portion is much harder and drier than the 

 exterior. Sometimes there is an evident line of demarcation, 

 between the heart-wood, or duramen, as it is called, and the sap- 

 wood, or alburnum; this is seen, for example, in the lignum- vitae, 

 and coco-wood, which are much employed by turners. But in 

 most cases, the change of character is more gradual. This change 

 is due to the consolidation of the interior wood ; by the depo- 





