GENERAL DIRECTIONS 19 



encounter in the case of fossil material, great care should be 

 exercised in order to secure the few truly radial sections which 

 the scanty material affords. When properly prepared such radial 

 sections supply some of the most important data for diagnostic 

 purposes. They furnish but little information as to regional dis- 

 tribution, a feature which is of secondary importance in this case. 

 They do, however, furnish evidence of the highest value as to 

 the form and structure of the tracheids, the character and extent 

 of the transition zone, the regional distribution of the bordered 

 pits and the various important details of their structure, size, and 

 aggregation, the disposition and character of the spirals, and, 

 above all, it gives the only adequate knowledge of the medullary 

 ray with respect to those features which are of the greatest value 

 in the final differentiation of species. It is true that some of the 

 features of the ray may be inferred from a transverse and more 

 particularly from a tangential section, but they cannot be utilized 

 fully except in generic differentiations. The radial section gives 

 a complete side view of the ray, exposing the entire structure in 

 all its details throughout the whole radial extent. It is from such 

 data that we obtain our final decision respecting the separation 

 of the first and second sections of the genus Pinus, the differen- 

 tiation of Sequoia, Libocedrus, and Taxodium, the recognition of 

 species wherever found, a differentiation of the Taxodiineae from 

 the Cupressineae ; and since it exposes the entire structure in all 

 its details, which are presented chiefly in side view, it permits us 

 to determine their relations to one another and to the activities 

 of the plant as no other section can. As features of subordi- 

 nate value the radial section completes our knowledge of the 

 bordered pit, which it presents in section wherever these struc- 

 tures lie in the tangential walls, a distribution which may be 

 more accurately ascertained in this plane of section than in the 

 tangential, since they are more certain to occur within the limits 

 of a given field. For similar reasons the radial section affords 

 the most convenient means of studying the longitudinal aspects 

 of resin passages, resin cysts, resin cells, and crystallogenous 

 idioblasts. 



