196 Practical Plant Biology. 



remains as a tracheid. Resin-ducts are also seen in the trans- 

 verse section of the wood. They are found in thickened parts of 

 the rays, and may be cut transversely or longitudinally and are 

 lined with a layer of cells which secrete resin into the ducts. 



(2) A radial section of the wood shows the same structures as the 

 transverse, but naturally they appear very different, being viewed 

 from an aspect at right angles to that shown in the transverse 

 section. The tracheids are seen in longitudinal section, their 

 tangential walls being cut, while their radial walls are seen in 



FIG. 60. Finns silvestris, microphotograph of radial section of wood, x 32. 

 ew, early, or spring wood; Iw, late, or summer wood; r, medullary ray ; 

 rd, resin-duct. 



plan. In the summer wood the tangential walls are closer to- 

 gether than in the spring wood, consequently the wood tissue in 

 that region appears more dense and the successive layers of summer 

 wood appear as successive vertical bands alternating with the more 

 transparent bands of spring wood. It is on the radial walls of 

 the spring wood, between the more widely spaced tangential walls, 

 that the bordered pits are most plainly seen. Focussing down 

 upon them their structures come into view in the following sue- 



