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? (111, 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 AES 
branches of the season. In this way we see hee 
that the increase of dicotyl trunks and annual growth of 
. . dicotyledons. 
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. 
