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 4; inch. In the 
Southern pine the growth is slower. 
S The increase in thickness of a tree 
Fie. 228. Seed cone of Scotch two hundred twenty years old and 
pine (Pinus sylvesiris) which 178 inches m diameter was only 1 
has opened and dropped its . : 
neetie inch during the last forty years, or 
gy 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 11 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 
‘For further discussion of the rate of growth of pine trees see ‘The 
White Pine,’ Bulletin 22, U.S. Dept. Agr., Division of Forestry. 
