The Structures and Processes of Stems 129 



stem into equal spaces, we may observe on the following day 

 that the uppermost space has elongated the most. The ad- 

 joining spaces below are less and less elongated. This indi- 

 cates that the greater part of cell division and enlargement 

 takes place very near the tip (the growing point), but that 

 some growth takes place in the cell layers for a certain dis- 

 tance below the end of the stem (the growing region). 



Annual stems increase in thickness until the plant matures. 

 This increase in size is brought about by the enlargement of 

 cells and by the formation of additional cells. Shrubs and 

 trees increase in thickness each growing season. This is often 

 called secondary growth ; as we have seen, it is brought about 

 by the continued growth of the cambium. This layer of cells 

 produces new water-conducting tissue and wood fibers on its 

 inner side, and it produces food-conducting tissue and bast 

 fibers on its outer side. As growth proceeds from year to 

 year, annual rings mark the successive additions to the wood. 

 The bark also develops annual layers, but in most woody 

 plants these are much thinner and less conspicuous than the 

 annual layers of the wood. Further, since growth takes place 

 inside the cortex, the cortex is continually being split and 

 broken. The outer layers may die and after a few years will 

 be gradually weathered off. The ridges and grooves of the 

 bark show how much too small the outer bark is to cover the 

 more recently formed wood. Smooth, thin-barked trees lose 

 their bark very rapidly. Trees with bark that is thick and 

 has large ridges are the ones that hold their bark more tena- 

 ciously. But in all large trees the bark contains only a part 

 of the cortical layers that have actually been formed ; much 

 material has scaled off and fallen away. It should be noted 

 that as a tree increases in diameter, the annual rings of wood 



