. Biology; the story of living things. 248 THE MAINTENANCE OF THE INDIVIDUAL. Wright Pierce The characteristic lenticels of the white birch {Betiila populifolia). Note the placement of the lenticles. Scattered over the surface of twigs and young tree trunks are found many lenticels, openings in the corky layer which become filled with loose masses of cells. They are found both on roots and stems and act as pores which allow for the exchange of gases be- tween the living cells of the cortex and the me- dium outside. Lenticels are often spoken of as "breathing jDores" and experimental
. Biology; the story of living things. 248 THE MAINTENANCE OF THE INDIVIDUAL. Wright Pierce The characteristic lenticels of the white birch {Betiila populifolia). Note the placement of the lenticles. Scattered over the surface of twigs and young tree trunks are found many lenticels, openings in the corky layer which become filled with loose masses of cells. They are found both on roots and stems and act as pores which allow for the exchange of gases be- tween the living cells of the cortex and the me- dium outside. Lenticels are often spoken of as "breathing jDores" and experimental evidence seems to make this title valid. As the stem or trunk of a tree grows larger in diameter, there is an in- creasing area that uses water and foods. Cells cannot grow without food, and food in a growing plant cannot be made without water. The structures which put the water-conducting tissue of the inside of the stem in connection with the phloem of its outer part are known as vascular rays. They may be seen in almost any cross section of a tree which has produced secondary xylem and phloem. Here the cambium has rows of irregularly placed cells that instead of forming xylem and phloem produce ingrowing masses of more uniform parenchymatous cells making vertically placed strings of tissue. These bands act as conducting pathways for water from the xylem to the phloem and also as chan- nels for elaborated food from the phloem to the xylem, thus dis- tributing these materials to the growing trunk. Experiments by. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hunter, George W. (George William), 1873-1948; Walter, Herbert Eugene, b. 1867; Hunter, George W. (George William), 1902-. New York, Cincinnati [etc. ] American book company
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