STUDIES OF SEED-PLANTS 



171 



pith or medulla. In some woods, if examined with the unaided eye, 

 the rays appear to extend only to the bark ; but in other stems (as 

 beech) the rays extend out into the bark. The fact is that the same 

 kind of cells extend from the pith out into the bark in all dicotyledon 

 stems, but only in cases like beech are they compressed in the bark 

 into plates visible to the unaided eye. v 



Notice the difference in color between the outer and inner annual 

 rings. The lighter colored outer part of the wood is the sap-wood, 

 so called because it is active in transporting water and in freshly 

 cut stems appears to be filled with sap. Some of its cells contain 

 living substance. The darker central wood is the heart-wood, 

 composed entirely of dead cells. How do you explain the fact that 

 many large trees are hollow (because the heart-wood has rotted or 

 been burned away), and yet >hey appear to be healthy trees? 



(D or L) Split blocks from stems of various kinds of trees (espe- 

 cially oak, chestnut, beech, ash, pine) through the center and plane 

 the cut surfaces. The section 

 thus made is radial. Parallel to 

 the cut surface of one of the 

 halves plane off the bark until 

 some heart-wood is exposed. 

 This surface will be a tangential 

 section. In the radial section the 

 pith-rays will appear as glossy 

 plates of wood, and in the tan- 

 gential section it will be evident 

 that each ray extends some dis- 

 tance up and down the stem. 



a b c 



FIG. 47. a, transverse section ; 6, 

 radial; c, tangential. (U.S. Bureau 

 of Forestry.) 



Evidently the delicate radiating 

 lines or rays seen in the trans- 

 verse section were simply the 

 edges of flattened plates which are placed vertically and radially in 

 the stem. 



Make diagrams showing structure seen in radial and transverse 

 sections. 



(D or L) Examine the inner and outer layers of bark in various 

 woody stems. Soften pieces of bark by long soaking and boiling in 

 water, tear it into pieces and observe the position and direction of 

 the fibers. Some of these are bast-fibers (elongated cells). The 

 bast-fibers of many plants are of great value in the textile industries, 

 e.g., flax (linen), jute, ramie, and hemp are all obtained by rotting 

 the bark of certain plants and then separating the bast-fibers. 



