. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. 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
. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. 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 Fig. 127. —Dia- gram illustrating the annual growth of Please no
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