208 TEXTBOOK OF BOTANY 



of the stems of many herbaceous plants which, like hemp and 

 flax, contain but little wood. The bark of the basswood 

 (linden) likewise contains a large proportion of bast fibers. 

 In addition to the strengthening cells, in the wood and bast, 

 many plants have thick-walled cells in their cortex. The 

 runners of the strawberry, for example, have several layers 

 of thick- walled cells just within the epidermis. Similar 

 strengthening tissue is sometimes formed also in the pith. 



226. Differences in Woods. Everyone knows that the 

 woods of different kinds of trees differ from one another in 

 various ways. There are differences, for example, in hard- 

 ness. Generally speaking, the wood of the pine and of other 

 gymnosperms is comparatively soft ; that of many common 

 angiosperm trees, such as the oak, maple, walnut, and elm, 

 is hard. But there are great differences in hardness between 

 different angiosperm woods ; and some, like the basswood, 

 are as soft as the wood of most gymnosperms. Woods differ 

 also in weight and durability. Differences in hardness, 

 weight, and durability are to be found not only between woods 

 of different species, but also between the woods of two trees 

 of the same species that have grown under different condi- 

 tions. By examining the structure of various woods, we can 

 discover some of the causes of these differences in quality 

 and can see why each kind of wood is useful for certain 

 purposes and not for others. 



227. Sapwood and Heartwood. In most angiosperm 

 trees, as in the pine, the older wood changes to heartwood. 

 This is usually different from the sapwood in color, often 

 harder and heavier because of changes that have occurred 

 in its walls, and more durable because it is not so easily af- 

 fected by decay-producing fungi. It is these qualities that 

 make the heartwood valuable for building purposes. The 

 time at which the change of sapwood into heartwood begins 

 to occur varies greatly. The oak begins to form heartwood 

 early in life ; the ash begins much later. When the change 



