STEMS, ROOTS, AND LEAVES 



85 



as radial lines in the wood. The annual rings arise from the 

 cambium layer and increase the diameter of the stem annually, 

 thus stretching and cracking the bark jacket of older trees. 

 The wood of each annual ring is divided into two zones, called 

 spring wood and summer wood. The spring wood is distin- 

 guishable by being porous ; that is, it is made up of cells with 

 larger openings, which give greater porosity to the spring wood 

 than to the summer wood. 

 These larger cells, or pores, 

 are the cavities of the large 

 water ducts, which are 

 usually more numerous in 

 spring wood, since it is 

 formed when a large sup- 

 ply of water is needed for 

 the growth of buds, leaves, 

 and flowers. The summer 

 wood, on the contrary, is 

 made up of thick-walled 

 cells (which are smaller 

 than those of the spring 

 wood) and has fewer water FlG 

 ducts. Its chief functions 

 for the stem seem to be 

 mechanical and storing. 

 Since the last-formed and 

 denser summer wood of 



one season abuts upon the first-formed porous spring wood of 

 the next season, the junction line of the annual rings is usually 

 clearly marked. The width of the annual ring for any given 

 season is determined in part by the age of the tree and in part 

 by external conditions. Periods of drought or the loss of leaves 

 by frost or by insects check the annual growth and may even 

 result in a double ring of wood in one season if growth is 

 resumed in the latter part of the summer after such a checking 

 process. Most trees also grow slowly in extreme youth or old 

 age, while growth is most rapid in the middle life. 



Gross structure of a lilac shoot two 

 years of age 



The limits of the pith, wood, bark, and epider- 

 mis are shown in the sector of the stem at the 

 left. The principal tissue layers in each of the 

 above subdivisions are shown at the right in 

 the upper and lower sectors 



