202 Bulletin 194 



tmployed, the sections of wood must be cut very thin and the customary 

 thin cover-glass must be used between the wood-section and the micro- 

 scope lens. 



In the key to the woods which follows, it will be noticed that two 

 large divisions have been made, one the porous woods, being the great 

 mass of broad leaved woods, the other the non-porous or coniferous 

 woods. Porous woods have again been divided into two groups, the 

 ring-porous woods and the dififuse-porous woods. The key is con- 

 structed on the same principles which have been employed throughout 

 in other keys in this bulletin. For an explanation and illustration of 

 the manner of using a key, the reader is referred to the chapter on 

 "Artificial Keys, How Made and Used" (pages 13 to 15). Attention 

 is called to the fact that both naked-eye and microscopic characters have 

 been introduced into the key to the woods. Naked-eye characters are 

 printed in Roman type, the microscopic in italics. While keys based 

 solely upon naked-eye or macroscopic characters have been pub- 

 lished, their use often leads the beginner to wrong conclusions 

 or utter confusion. The insertion of microscopic characters may 

 be considered, then, as a necessary affliction which has been im- 

 posed to make accurate determinations easy, rather than to complicate 

 the key. The more common or important woods have been illustrated 

 by cross-, radial- and tangential-sections of the woods. In the prep- 

 aration of these drawings, photomicrographs of very thin, stained 

 sections of typical wood were taken. Blueprints were subsequently 

 made from the negatives and the outlines of the cells were drawn in 

 with India ink. Finally the prints were bleached, the result being a 

 silhouette of the original wood-section drawn accurately to a scale on 

 a white background. 



