262 Plant Biology 



jects; houses and roofs; furniture and musical instruments; ships and 

 canoes; barrels and casks; vehicles; road materials; railroad ties; poles, 

 piling, and posts; industrial implements and tools; recreational equip- 

 ment; toys, canes, pencils, matches, toothpicks, clothespins, etc. 



The woods of many trees are of two kinds: sap wood and heart wood. 



The sap wood is formed in certain trees next to the bark in succes- 

 sive layers as new wood. It conducts sap and consequently is called sap 

 wood. Some of the plant foods are stored in this kind of wood. It is 

 usually more massive and resistant in larger trees. 



Sap wood after a certain time becomes stronger, more compact, and 

 somewhat drier. It no longer carries sap and is known as heart wood. 

 It differs in color from the sap wood because of the stored, useless plant 

 by-products. This color of heart wood frequently makes it more desir- 

 able for manufacturing purposes. 



Gums and Resins. — The two most common elastic gums are India 

 rubber or caoutchouc (pronounced koo'chuk) and gutta-percha. Both 

 are tough, waterproof, somewhat elastic solids which separate as a curd 

 from the milky juices of a number of tropical plants and of several of 

 our native plants, particularly goldenrod. It becomes hard when dried 

 or heated. The principal source is the Brazilian rubber tree (spurge 

 family) . 



Rubber was early used to rub out pencil marks; hence the name rub- 

 ber; the India part of the name was derived from the fact that it was 

 imported from the West Indies. 



Rubber has a great variety of uses, among which are manufacture of 

 boats, overshoes, waterproof garments, tires, bands, toys, bottles, cushions, 

 insulators, fountain pens, etc. 



The quality of rubber was improved by Charles Goodyear in 1844 by 

 adding sulfur to the caoutchouc and subjecting the mixture to consider- 

 able heat. This process was known as vulcanization. When a large 

 amount of sulfur is added to caoutchouc, a hard rubber known as vul- 

 canite is produced. 



Gutta-percha difTers from India rubber in being more firm and rather 

 inelastic below 50° C. Like caoutchouc, it is flexible, tough, a poor con- 

 ductor of electricity and heat, and impervious to moisture. It is obtained 

 from juices by tapping several dififerent species of trees closely related to 

 the taban tree (sapodilla family). It is similar in chemical composition 

 to caoutchouc in that they both contain hydrogen and carbon. Gutta- 

 percha contains in addition certain resinous substances which are formed 



