Lecture XXIIL 



195 



dark bands are separated from one another by light ones formed 

 by the thin-walled spring tracheids. Comparison of the abruptness 

 of the transition on either side of the bands will enable you to 

 determine in which direction the pith and the cortex lie. In the 

 transverse section, the pits are seen almost exclusively on the radial 

 walls. At the level of a pit this wall in section looks like two Ys 

 placed so that the ends of the two branches of one are approxi- 

 mated to the ends of the two branches of the other and connected 

 across the angle of the Ys by a thin line. The branches of the 



FIG. sg.Pinus silvestris, microphotograph of transverse section of wood, 

 x 32. ew, early, or spring wood ; lw y late, or summer wood; r, medul- 

 lary ray ; rd, resin-duct. 



Ys represent the section of the domes on each side of the pit. The 

 vertical limb of the Ys corresponds to the section of the radial wall. 

 In the transverse section of the wood the medullary rays are 

 also seen. They appear as radial bands of cells, one cell thick, 

 interposed between the radial rows of tracheids. The medul- 

 lary-ray-cells appear oblong, their greatest extension being in a 

 radial direction. Usually they are filled with protoplasm in which 

 is seen a nucleus and often starch grains and oil globules. Some- 

 times the protoplasm has disappeared and the wall of the cell alone 



13* 



