GENERAL DIRECTIONS 17 



of a new species. In making a diagnosis for comparison with 

 that of a previously described species similar care is a neces- 

 sity, particularly in the case of the different hard pines, where a 

 comparatively slight deviation may involve one in considerable 

 difficulty. Until one is thoroughly familiar with the course of 

 procedure to be followed, and has an extensive knowledge of the 

 anatomical details in all their varying aspects as characteristic 

 of different genera and species, the only safe course to follow, 

 when attempting to identify a species, is to make a carefully 

 written diagnosis in full. After this is done comparison with 

 the key or with the supposed species may be made in detail. A 

 comparatively brief acquaintance with the systematic portion of 

 this work will enable one to recognize most of the genera at 

 sight, since the characters are in most cases very clearly defined 

 and easy of recognition ; but the same does not hold true of 

 species, since these are defined by a larger number of characters 

 which vary somewhat widely, and exceptional cases are of much 

 greater frequency. Where there is a final doubt as to the iden- 

 tity of a given species, the specimen should be compared with a 

 type section or be submitted to an expert for decision. 



The transverse section exhibits in the main an end view of the 

 various component elements. It should always be the first of 

 the three to be examined, since it immediately permits a separa- 

 tion of the genera into two great groups and affords suggestions 

 of such a nature as to permit of economy of time at a later stage 

 of the examination. It conveys a correct conception of the pres- 

 ence or absence of certain structural features, such as resin 

 passages, resin cells, or resin cysts and the presence or absence 

 of thyloses ; it affords the only accurate measure of regional dis- 

 tribution and of the general character of the growth rings, the 

 relative volume and character of the spring and summer woods, 

 and of the variations which distinguish the tracheids of those 

 regions of growth. When elements have similar terminal aspects, 

 as the spiral and pitted tracheids, as well as wood parenchyma 

 and parenchyma tracheids, the transverse section has no special 

 value beyond that which is to be found in a recognition of 



