150 



X. THE CONIFEROUS STEM. 



appear exactly as in the cross-section (Fig. 27 B,t), and good sec- 

 tions of them are proportionally more easy to obtain than in the 

 latter case. In old trachei'des we can determine that the closing 

 membrane bulges to one side, and that the torus (t) is pressed up 

 against one of the openings of the bordered pits. Staining the 

 section with hsematoxylin will facilitate the proof of this. The 



sections of the medullary rays (Fig. 

 56) appear to have a spindle-like 

 outline (tin, sm) since the cells at 

 both edges (or ends) gradually be- 

 come narrower. The shallowest 

 medullary rays are about three cells 

 high ; most are about eight cells, 

 while with some the height can 

 increase up to as many as twenty 

 cells. The shallowest are always 

 but one cell broad ; the highest can 

 in their central part be several layers 

 thick, and these latter contain the 

 resin-canal, which is shown in sec- 

 tion. The section may have also 

 cut through a vertical resin-canal, 

 which will present the same appear- 

 ance as in the radial longitudinal 

 section. 



In this tangential section the 

 tracheidal elements of the medullary 



of the Scotch tir. t, bordered pit ; culty, be distinguished from those 



tin, tracheidal and sm, starch-con- 



taming medullary-ray cell et, uni- which contain protoplasm ; on the 



other hand careful 



a very thin section are required in 

 order to determine that the medullary ray is accompanied on both 

 sides by small intercellular spaces (i), which follow it along its 

 course. These accompany almost exclusively the medullary-ray 

 cells which are provided with plasmic contents, not the trachei- 

 dal cells. Through these interstices, which can be traced from 

 the periphery of the stem through the cambium right into the 

 woody mass, the gaseous interchange with the surrounding 

 atmosphere which is necessary for the living cells is carried on. 

 In sections through fresh material these intercellular spaces 



