40 BOTANY OF CROP PLANTS 



from (2) the formation de novo of vascular bundles from 

 cells that have retained their meristematic power. 



Annual Rings. — An annual ring, as generally understood, 

 is one year's growth of wood (xylem). The ring varies in 

 width, depending upon the time in the life of the plant it was 

 formed, and upon seasonal climatic conditions. Further- 

 more, it is known that some trees grow rapidly, producing 

 wide annual rings, while it is a specific character of others 

 to grow slowly, i.e., produce narrow annual rings. 



There is usually a marked difference in the wood formed in 

 the spring and early summer, and that produced in late 

 summer and fall. In early or so-called "summer wood," 

 tracheal tubes are large and quite numerous; in late or 

 "autumn wood," tracheal tubes are smaller and fewer, and 

 tracheids and wood fibers are relatively more abundant. 

 Hence, "autumn wood" has more strength than summer 

 wood. It is readily seen that the autumn wood of one year 

 (say 1916) is adjacent to the spring wood of the following 

 year (19 17). "Soft wood" is usually one which grows 

 rapidly, and is difuse porous, that is, tracheal tubes are 

 rather small and uniform in size and evenly distributed 

 throughout the year's growth. "Hard wood" is usually a 

 comparatively slow-growing wood, and is ring porous, that 

 is, the tracheal tubes of the spring and early summer are 

 large and numerous, while the autumn wood is solid as a 

 consequence of the relatively greater abundance of tracheids 

 and wood fibers. 



Bark. — The term "bark" with us includes all that portion 

 of the stem down to the cambium layer. When the bark 

 of a tree is peeled off, there are removed the following 

 layers in order from outside to inside: cork, cork cambium, 

 cortex, phloem, and portions of cambium. The cleavage Hne 

 is the cambium zone. 



