2IO TEXTBOOK OF BOTANY 



ences in the thickness of the rings might not afEect the quahty 

 of the wood if the proportion of spring and summer wood in 

 each remained the same. But if a thicker ring should mean a 

 greater proportion of spring wood with its large, thin-walled 

 cells, then of course the thicker the ring, the lighter and 

 softer the wood as a whole will be. This is the case in the 

 pine and in other gymnosperms. In good soil, well supplied 

 with water, a pine tree grows rapidly ; each annual ring is 

 thicker than would otherwise be the case, and contains a 

 greater proportion of large cells ; consequently the wood is 

 comparatively soft and light. But if the tree grows in poor 

 or in dry soil, the annual rings are thinner, and since there are 

 fewer large, thin- walled cells, the wood is harder and heavier. 

 This rule, however, is modified by conditions of climate; 

 thus pine trees grown in cold countries have thin rings, but 

 their wood is comparatively soft. In angiosperm trees the 

 effect of rapid growth is just the opposite of what it is in gym- 

 nosperms. The thicker the annual ring of an angiosperm, 

 the greater is its proportion of summer wood. Therefore, an 

 oak tree living under favorable conditions of soil, moisture, and 

 temperature grows more rapidly and produces heavier, more 

 solid wood than does a tree that grows under unfavorable 

 conditions. The age of a tree also affects the thickness of its 

 annual rings. The older the tree, the thinner are its rings. 

 For this reason, a gjrmnosperm tree forms harder wood in its 

 old age than in its youth ; in an angiosperm tree, the later- 

 formed wood is softer. In tropical countries, where the 

 changes of season are less marked than with us, many trees 

 (some Indian oaks, for example) continue to grow in thickness 

 throughout the whole year. In such trees there is no distinc- 

 tion between spring and summer wood, and so of course 

 there is nothing that corresponds to the rings that we find 

 in the wood of trees of temperate countries. 



229. Vessels. — We have seen that the sap passes upward 

 in the pine trunk through certain long cells of the wood. 



