1'OREST UTILIZATION 



55 



large pores, or else "diffuse porous," if the ducts are 

 evenly distributed over the entire ring. Sapwood 

 and heartwood are merely distinguished by a differ- 

 ence of color, caused by incrustations of pigments,, 

 lignin. tannin etc., in the walls of rings formed a. 

 number of years before. The number of years elaps- 

 ing before incrustation takes place is small in catalpa, 

 chestnut, locust; and larger in yellow poplar, white 

 oak, walnut where it is about thirty or forty years 

 old. Beech, maple, basswood etc. do not form any 

 heartwood. 



GENERIC STRUCTURE OF HARDWOODS. 



Populus 



Ulmas 



Morns 

 Allanthus 



Broad. 



Tilia 



Acer 



Corylus 



Carplnua 



Ilex 



Platanus 

 Fagus 



II. Botanical structure of softwoods. 



(a) The tissue of softwoods is more homogeneous than that 



of hardwoods. It is mainly formed by tracheae. 

 The cell walls formed in early spring are thinner and" 

 the lumina formed in early spring are larger than those 

 formed in summer. 



(b) Parenchyma is found in the medullary rays and around 



the rosin ducts. 



(c) Ducts of the form found in hardwoods exist only close 



to the central pith column. 



(d) The medullary rays are very fine (microscopic), usually 



only one cell wide and about a dozen cells high. The 

 lowest string of cells in the ray is usually formed by 

 tracheae (exception red cedar). 



(e) Rosin ducts are not cells merely, but, unlike the ducts o 



hardwoods, hollow tubes, the walls of which are 

 formed by parenchymatic cells. These ducts are run- 

 ning horizontally as well as vertically in picea, pinus> 

 larix, pseudotsuga. 



The tissue of the genera abies, taxus, juniperus, thuja, 

 t?uga, chamaecyparis etc. lacks the ducts. 



