. Introduction to botany. Botany. 288 INTEODUCTION TO BOTANY constantly thicker, until finally, in older branches and stems, the light penetrates through it poorly, if at all, and chloro- phyll ceases to be developed. The ridging of bark is due to the fact that bark on young branches and stems is so spread by growth within that longitudinal cracks are formed. As more wood is developed within, the spreading and thickening are increased, and ridges and crevices become more pronounced (fig. 222), as is true in most perennial stems. 272. Rate of thickening of the stem. Two of the most im- portant
. Introduction to botany. Botany. 288 INTEODUCTION TO BOTANY constantly thicker, until finally, in older branches and stems, the light penetrates through it poorly, if at all, and chloro- phyll ceases to be developed. The ridging of bark is due to the fact that bark on young branches and stems is so spread by growth within that longitudinal cracks are formed. As more wood is developed within, the spreading and thickening are increased, and ridges and crevices become more pronounced (fig. 222), as is true in most perennial stems. 272. Rate of thickening of the stem. Two of the most im- portant of our gymnosperm trees are the white pine and the long-leaf pine. A white-pine tree overtopping most of its fellows in the forest is, on the average, at ten years inch in diameter, at one hundred years inches, and at two hundred years 31 inches. The average thickness of the annual rings during the life of the tree throughout its second century is therefore about J^ inch. In the Southern pine the growth is slower. The increase in thickness of a tree two hundred twenty years old and 17-| inches in diameter was only 1 mch during the last forty years, or. Fig. 223. Seed cone of Scotch pine (Firms sylvestris) which has opened and dropped its seeds ^Iq- inch per year. The tallest and least shaded white-pine trees at fifty j^ears develop new wood at the rate of about -^ cubic foot per year; at seventy-five years at the rate of about 1 cubic foot per year, and at one hundred years at the rate of about 11 cubic feet per year.^ 273. Significance of the stem. The gymnosperm stem is more complex than that of any other plant in the series of groups that we have been studying. There was vascular ' For further discussion of the rate of growth of pine trees see White Pine," Bulletin Sg, Dept. Agr., Division of Forestry. "Tlie. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appe
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