FOSSIL WOOD AND HOW IT IS STUDIED. 97 



of cedar, oak, walnut and pine in almost every collection. In some 

 cases of course these determinations may be correct, but usually they 

 are mere guesses. It may not be generally known to our readers, but 

 a piece of fossil wood, and for that matter any kind of fossil plant that 

 has the internal structure preserved, may be prepared and studied 

 under the higher powers of the microscope almost as satisfactorily as 

 if cut from a living tree. 



In order to demonstrate the internal structure of a piece of wood, 

 thin sections must be cut in three directions: Transverse, or at right 

 angles to the axis, thus cutting off the ends of the wood-cells; longi- 

 tiidmal radial, or lengthwise of the stem and parallel with the medul- 

 lary rays as they run from pith toward the bark, and longitudinal 

 tangential, or lengthwise of the stem at right angles to the medullary 

 rays, thus showing them cut across the ends. In the first section we 

 may note the size of the wood-cells and their arrangement to form the 

 rings of .growth, if these be present, and also the long, narrow view 

 of the rays. The second section shows the long, broad side of the 

 wood-cells with the markings, and the fiat face of the rays. The last 

 section shows the other side of the wood-cells and the square-cut ends 

 of the rays. 



The sectioning process may now be described. Selecting a piece 

 of wood of suitable size, preferably containing about one cubic inch, 

 the side from which the section is to be made is ground down smooth 

 on a rapidly revolving iron disk, by aid of water and corrundum. 

 When the inequalities have been all removed it is polished with very 

 fine powder until perfectly smooth and even. It is now cemented, by 

 means of hard Canada balsam, to a slip of glass a little larger than the 

 size of the piece of wood. When dry the wood is cut away by means 

 of a diamond saw or other similar device, leaving a slice of the wood, 

 perhaps one-eighth of an inch thick, adhering to the glass. This is 

 now ground down and polished with the corrundum until it is thin 

 enough to permit the light to pass through readily. This last process 

 requires skillful manipulation, for if it is too thick it will remain 

 opaque, and on the other hand when already very thin a few extra 

 turns of the disk may remove it entirely from the glass. When prop- 

 erly prepared it should be no more than one layer of cells in thickness. 

 By warming the glass the balsam is melted and the thin slice of wood 

 may be removed from it, and mounted again in Canada balsam on a 

 glass slide and covered with a cover-glass, as in ordinary microscopical 

 preparations. It may now be viewed under the higher powers of the 

 microscope precisely as though it had been cut from a living tree. 



If the natural fracture of the piece of wood to be sectioned is not 

 along the lines desired, it may be sliced in any direction with the dia- 



