66 INTRODUCTION TO BOTANY 



63. Limited thickening of annual stems. In stems of large 

 dicotyledons which die to the ground every year, such as the 

 sunflowers, iroiiweeds, hemp, and giant ragweed, growth in 

 thickness goes on throughout the summer. The outer cells of 

 the cambium continually split up by the formation of tangen- 

 tial partitions parallel to the bark, and so form new layers of 

 bark. In a similar way and to a still greater extent the inner 

 cells of the cambium form new wood, and thus the stem goes 

 on increasing in thickness. But in such plants as those just 

 mentioned the activity of the cambium is strictly limited ; after 

 it has given rise to a certain amount of new tissue, growth 

 stops and the stem dies down to the ground. The death of 

 annual stems in the autumn is often thoughtlessly supposed 

 to be due to the arrival of winter, but it occurs just as cer- 

 tainly, and often after a briefer period of growth, in regions 

 where there is no cold weather. 



64. Annual thickening. In stems such as those of dicotyle- 

 donous trees and the trees of the Pine family and other cone- 

 bearers, which live for many years, the cambium forms each 

 year a new layer of bark and one of wood. These annual layers 

 are usually more noticeable in the wood than in the bark, 

 because the wood cylinders thus formed remain closely joined 

 together (fig. 45). The newer, lighter-colored portions of the 

 wood are known as sapwood, and the older portions, often 

 darkened by the deposit of coloring matter, are known as 

 heartwood. Not infrequently the heartwood decays and leaves 

 the tree hollow. 



How old is the stick of wood shown in figure 45? Did it 

 grow at the same rate during all the years of its life ? Dis- 

 cuss this question. Why is the name annual rings not an 

 accurate one ? What are they really ? Is each year's growth 

 uniform all round the stem ? Had this stem any branches 

 in the portion shown by the section ? How does figure 46 

 suggest a method of determining the age of the stem at the 

 time when a branch began ? What seems a probable cause of 

 the death of the branch, figure 46, b ? 



