Lecture XX1IL 



cession : (a) The opening of the dome, 

 which is more .or less slit-like ; (b} the 

 circular outline of the pit membrane 

 within which is a smaller concentric 

 circle, the outline of the torus; (c] the 

 opening of the dome on the further side 

 of the membrane. In this radial section 

 the medullary rays appear as walls of 

 cells crossing the tracheids, varying from 

 one to fifteen tiers of cells in height. 

 The outlines of the cells are oblong and 

 their greater extension is horizontal. 

 Their cytoplasm with its nucleus and 

 granular contents appear to fill the cells ; 

 sometimes, however, empty tracheids are 

 found replacing these cells, especially 

 those of the uppermost and lowest tiers. 

 Occasional resin-ducts are seen rising 

 vertically from ray to ray, or running 

 horizontally in a ray. 



(3) In tangential sections of the stem 

 the tracheids are again seen in longi- 

 tudinal section. This time, however, it 

 is their radial walls which are cut and 

 their tangential walls are seen in plan. 

 The tangential walls are usually smooth 

 and show no special structures, but on 

 the cut radial walls are seen the bordered 

 pits. Being radially symmetrical the 

 pits present the same appearance on this 

 section as they do on the transverse sec- 

 tion. In tangential sections of the stem 

 the medullary rays are cut at right angles 

 to their course. Being walls of one cell 

 thick and of a variable height they ap- 

 pear as vertical series of cells. The 

 height is somewhat greater than their 

 width. Most of them are filled with 

 protoplasmic and granular contents, and 

 often seem to bulge into the empty 

 cavities of the adjacent tracheids. Often 

 the uppermost and lowest tiers of cells 

 have lost their contents and have 



b 



<2 



FIG. 61. Pinus silvestris, 

 diagrammatic tangential 

 section of wood transmit- 

 ting transpiration-current, 

 x 60. a, medullary ray; 

 b, bordered pit with mem- 

 brane in median position 

 and capable of transmitting 

 water ; c, bordered pit with 

 membrane drawn against 

 dome and capable of re- 

 sisting tension of sap ; d, 

 tracheid full of water and 

 transmitting current ; e, 

 tracheid emptied of water 

 showing the bordered pits 

 on its walls with membranes 

 drawn against the domes. 



