54 



THE AMERICAN MONTHLY 



[March, 



or parenchyma, as it is called, being seen. The parallel lines which 

 run vertically up and down the picture at irregular intervals from an 

 eighth to a half an inch apart, are the medullary rays which traverse the 

 wood from core to bark, carrying sap horizontally in the growing tree. 

 When they are numerous they are very prominent, forming what is 

 known as the silver grain. When a log is quarter-sawed the medullary 

 rays are specially marked. These wood cells, it will be noticed, are 

 much thicker, and also closer together at the edge of each circle, and 

 this denotes the wood formed in the fall when the sap is descending and 

 cool weather has partly arrested active growth. Probably the lignifica- 

 tion even goes on all winter. The arrangement and appearance of these 

 layers of growth vary greatly with different species of trees. Many, 



Poison oak {Rhus toxicodendron). 



such as the magnolia, cypress, and poplar, form more than one layer a 

 year. All form them of different thickness, from year to year, depend- 

 ing on the seasons and amount of annual rainfall, and microscopic ex- 

 amination shows that the seasons of slowest growth are those in which 

 the lignification is most complete. 



Fig. 2 shows a transverse section of red cypress from the rich, moist, 

 alluvial soil of southern Louisiana, far south of the frost line. Although 

 the annual cycles are uneven, yet the regular, evenly formed cell struc- 

 ture shows a soft, straight-grained wood, which keeps up its growth the 

 whole year through. The numerous small dark spots indicate cells filled 

 with the essential oil which gives the cypress its odor and is a cause of 

 its durability. 



Fig. 3 is a section of Oregon yellow pine {Plnus ponderosa) ^ one of 

 the most valuable of the pines of the Pacific coast, resembling closely 

 in nearly all respects the sugar pine. The winter growth, however, is 

 much more pronounced in the yellow pine, giving a prettier pattern, 



