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, ironweeds, 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. £5). 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 annul 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, 6? 
