STUDIES OF SEED-PLANTS 169 



the center are larger and more scattered. Make a sketch showing 

 the above points. Carefully examine a bundle near the center of 

 the section of the stem, and note that it consists of two parts, (1) that 

 directed toward the center of the stem is made up of wood-cells sur- 

 rounding the air- and water-tubes, while (2) the outer part of the 

 bundle consists of wood-cells surrounding tubes known as sieve-tubes. 

 Carefully observe the structure of the bundle, and compare with 

 Fig. 45. 



162. Stem of Dicotyledons. While a growing stem is 

 elongating in a given internode, the arrangement of the tissues 

 remains, practically the same as described for the young bean 

 stem ( 70). This is the primary growth, and is the condition 

 in many annual dicotyledonous plants. But in many plants 

 which live for a number of years the stem increases in diameter 

 long after the elongation has ceased in a given internode, 

 and this increase in diameter after the first woody tissue has 

 been formed is due chiefly to the formation of new wood- 

 cells between the cambium and the first-formed wood. At 

 the same time that the cambium is adding new wood-cells, it 

 adds new cells to the inner layer of the bark. These addi- 

 tional layers of wood and bark constitute the secondary 

 growth of the stem. It is the common method of increasing 

 diameter of the hard stems of perennial dicotyledonous and 

 cone-bearing plants. This growth of new layers commonly 

 occurs annually and results in the rings of wood seen in 

 transverse sections of many trees. These annual rings are 

 clearly marked in many trees by the extra-large wood-tubes 

 formed during the rapid growth in the beginning of the 

 growing season. 



The age of a tree cannot always be accurately determined 

 by counting the annual rings, for it has been found that some- 

 times two rings are formed in a single year. This may happen 

 if growth is checked for a time in early summer, as by drought 

 or destruction of leaves by caterpillars, and begins again 

 later in the summer. In tropical regions, where conditions 

 for growth are uniform throughout the year, it may happen 



