86 



VEGETABLE HISTOLOGY. 



of the wood. The tracheids are so peculiar in structure that 

 one may distinguish a gymnosperm from other plants. A few 

 genera of gymnosperms can be recognized by the number and 

 regularity of the markings on the tracheids. 



Soften a piece of pine wood by soaking a long time in dilute 

 alkali. After washing thoroughly in water, make longitudinal 

 radial sections by cutting at right an- 

 gles to the rings of growth. Mount a 

 thin section in water or glycerin. The 

 tracheids are long, tapering fibres, sim- 

 ilar to wood fibres, but larger. The sur- 

 faces of the cells are marked by a row 

 of pits, each pit being surrounded by a 

 smaller circle inside of a larger one. At 

 times the pits are close together; at 

 other times they are wide apart. Lying 

 across the fibres at intervals are rows 

 or plates of cells of the medullary rays. 

 Try the action of phloroglucin and hy- 

 drochloric acid on a section. 



Now make longitudinal tangential 

 sections by cutting parallel with the 

 rings of growth. In such a section no 

 pits are found on the faces of the cells, 

 but they may now be seen in section on 

 the edge in the cell-wall. In other 

 words, the peculiar pits occur only on 

 the radial faces of the cell-walls. 



Find a pit cut exactly through the middle. It forms a lens- 

 shaped cavity in the wall between two cells, opening into the 

 two cell cavities by circular orifices which in flat view appear 

 as the inner small circle of the pit (see above). Running 

 lengthwise through the pit and closing off the cavities of the two 

 neighboring cells is the middle lamella. Here and there be- 

 tween the tracheids occur rows of two or three rounded cells, 

 which are not to be mistaken for the pits. They are much 

 larger, being the lignified parenchyma cells which form the 

 so-called medullary rays. 



Fig. 52. Tracheids of pine 

 showing bordered pits, with 

 medullary rays crossing the 

 tracheids (Sachs). 



