THE STEM 



111 



These pores are the sections of ducts. They are very large 

 in the grapevine, and a cutting two or three years old will 

 show them distinctly. Examine sections of a twig that has 

 stood in red ink from three to twelve hours, and observe the 

 course the fluid has taken. How does this accord with the 

 facts observed in your study of the conducting tissues in 

 monocotyl and herbaceous stems? (Ill, 115, 116.) 



123. The rings into which the woody cylinder is divided 

 mark the yearly additions to the growth of the stem, which 

 increases by the constant accession of new 

 material to the outside of the permanent 

 tissues (116). The cambium constantly 

 advances outward, beginning every spring 

 a new season's growth, and leaving behind 

 the ring of ducts and woody fibers made 

 the year before. As the work of the plant is 

 most active and its growth most vigorous 

 in spring, the largest ducts are formed then, 

 the tissue becoming closer and finer as the 

 season advances, thus causing the division 

 into annual rings that is so characteristic of 

 woody dicotyl stems. Each new stratum of 

 growth is made up of the fibrovascular 

 bundles that supply the leaves and buds and 

 branches of the season. In this way we see 

 that the increase of dicotyl trunks and 

 branches is approximately in an elongated 

 cone (Fig. 127), the number of rings gradually diminishing 

 toward the top till at the terminal bud of each bough it is 

 reduced to a single one, as in the stems of annuals. 



Sometimes a late autumn, succeeding a very dry summer, 

 will cause trees to take on a second growth, and thus form two 

 layers of wood in a single season. On this account we can- 

 not always rely absolutely upon the number of rings in esti- 

 mating the age of a tree, though the method is sufficiently 

 exact for all practical purposes. 



Fig. 127. — Dia- 

 gram illustrating the 

 annual growth of 

 dicotyledons. 



