CHAPTER X. 





 STRUCTURE OF THE CONIFEROUS STEM. 



PRINCIPAL MATERIALS USED. 



Twigs of Finns sylvestris, or other Pinus, not more than .J inch in thick- 

 ness ; fresh. 



Pieces from a thick stem of the same, cut in June or July ; preserved in 

 alcohol, and laid for two or three days before use in half-and-half 

 glycerine and alcohol. 



PRINCIPAL REAGENTS USED. 



Chlorzinc iodine Alkanet root Bohmer's logwood, or ham-alum Aniline 

 blue Iodine glycerine. 



Pinus sylvestris. We will now take the Scotch Fir (Pinus 

 sylvestris), and make a careful study of the structure of the stem. 

 It is characteristic of the Pinaceae that the secondary growth 

 of the wood consists almost wholly of but one kind of element, 

 typical tracheides, between which in a certain number of species 

 only, as, e.g., the Scotch Fir, strings of wood parenchyma are 

 intercalated. True tracheae are wanting in the Pinaceae, and 

 spirally- thickened vessels, or vascular tracheides, are found only 

 in the medullary sheath, in the protoxylem of the vascular 

 bundle. Even in stems of half an inch in thickness, we can find 

 these very easily. In cross-sections passing through the pith 

 (which is readily indicated to the naked eye by its darker colour) 

 it can be seen that the inner limits of the ligneous mass consist 

 of portions projecting into the pith and composed of elements 

 with narrow cavities, and with somewhat brownish walls. In 

 delicate radial longitudinal sections passing through the same 

 region we can determine that these elements are spiral vessels or 

 vascular tracheides. Some such vessels, which possess at the 



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