"Sprin$wood" and "summerwood"

It is well known that the growth of a tree in diameter is shown in the so-called "rings" of annual growth. These rings are simply the material that is added aiound the tree underneath the bark each year. The outer cells, just inside the bark make up the living active portion of tht tree and each year's product is plainly marked off from that of the preceding year. It is in this manner that the rings are formed.

It is perhaps not so well understood popularll' that each ring. is made up of two parts : first the inner light-colored portion known as "springwood"; and, second, an outer darker portion knorvn as "sltmmerwood". During the spring months the tree grows faster than it does in the summer and the cells produced at that time are larger and hav'e thinner walls. The summerwood cells have thicker walls and are smaller. Summerwoocl, therefore, gets more solid wood substances than springrvood and is ?arker in color. The two parts of the annual ring are much more noticeable in the softwoods-that is the conifer<lus treesthan in the hardwoods.

The proportion of springrvood to summer.rvoocl lurtrber h-as a,n important effect upon the strength, properties and physical characteristics of the lumber.

In some species the springwood blends into the summer: wood and in this condition makes for uniform texture. easier working and a small amount of shrinkage and swelling of the lumber. In other softwoods the trJnsition from springwood to summerwood is more abrupt, the proporiion of summerwood is larger and the rvood is strongir, and the two parts of the annual ring can be seen clearly.

Owing to the importance of ihe composition of iumber rvith respect to the varying proportions bf springvvood and summerwood, the National Lumber Manufacturers Association has issued a leaflet-No. 9-in its Lumber Facts series thatfully treats of this subject.An earlier leaflet in the series, Lumber Facts No. 4, dealt with the subiect of softwoods and hardwoods in general, the relation of softwood to hardwood and the subject of annual growth rings. Thesg two leaflets presented in compact,- basily understandable, form the most essential facts rvhich puichasers and users of lumber should know regarding-the natural composition of the material.