150 General Botany 



internode is mature. In these plants the growing point (primary 

 meristem) develops nodes, and short internodes that continue 

 growing independently. Instead of a continuous growing region 

 extending back from the growing point, there is a series of shorter 

 and shorter growing regions at the base of each internode. It 

 is this fact which explains why growing corn breaks easily 

 just above the nodes, and why it grows into an upright position 

 again when blown over during its period of development. 



Diameter growth of annuals. Annual stems increase in thick- 

 ness until the plant matures. This increase in size is brought 

 about by the enlargement of cells and by the formation of addi- 

 tional cells by the cambium. In many annuals, like mustard, 

 zinnia, onion, squash, and corn, the stem thickens by increase 

 of the size of cells. In the sunflower the cambium continues to 

 form woody tissue and bast for a considerable part of the growing 

 season, so that very large plants have stems i to 2 inches in 

 diameter near the base. 



Growth in diameter of trees and shrubs. 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 con- 

 tinually being split and broken. The outer layers may die and 

 after a few years will be gradually weathered oflf. Secondary 

 cambiums in the cortex may develop additional layers of cork. 

 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 



