288 



INTRODUCTION 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 0.9 inch in diameter, at one 

 hundred years 17.2 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 -Jj 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 

 inch during the last forty years, or 

 -fa inch per year. 

 The tallest and least shaded white-pine trees at fifty years 



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 1J cubic 



feet per year. 1 



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 



1 For further discussion of the rate of growth of pine trees see "The 

 White Pine," Bulletin %2, U.S. Dept. Agr., Division of Forestry. 



FIG. 223. Seed cone of Scotch 



pine (Pinus sylvestrls) which 



has opened and dropped its 



seeds 



