GENERAL DIRECTIONS 21 



passages occur at such wide intervals, or are so grouped within 

 narrow areas, that a given specimen may show none of them in 

 transverse section, although a very considerable area is examined. 

 In a minor degree the tangential section is also useful but not 

 necessary in extending our knowledge respecting the distribution 

 of bordered pits in the tangential walls of the tracheids, but 

 whenever such features are to be studied critically it will be 

 necessary to provide two tangential sections for each species, 

 one passing through the spring wood and the other through the 

 summer wood. 



For the preparation of sections of fossil woods which are 

 strongly silicified or calcified, or otherwise infiltrated with mineral 

 matter, special apparatus for cutting and grinding is required, 

 and this part of the work is best accomplished by intrusting it 

 to one who has gained the necessary dexterity through long 

 experience. For those fossil woods of comparatively recent 

 deposits, such as the Pleistocene or later formations, which have 

 undergone little or no modification by the infiltration of mineral 

 matter, the methods applicable to woods from existing species 

 will be found to meet all the requirements of the case. Where 

 there has been a slight infiltration of mineral matter, boiling 

 with sodium carbonate will in most cases serve to remove the 

 carbonate or silicate, as the case may be, and bring the material 

 into such condition that it may be cut with facility by the micro- 

 tome knife. In all such cases, however, before mounting for 

 examination, care must be taken to fully neutralize the action 

 of the alkali by the action of dilute acetic acid, which serves to 

 restore the structure to its normal volume. 



In the case of recent coniferous woods it will meet all the 

 requirements of the case and amply provide for species deter- 

 minations if blocks about one centimeter cube are boiled in water 

 from one half to two or three hours and then sectioned while hot. 

 The sections should be cut as thin as possible, carefully freed 

 from air, stained, and mounted in Canada balsam. In thickness 

 the sections should be as nearly as possible of the diameter of 

 a tracheid or less, and this may be accomplished by means of a 



