Ecological Groups of Stems 



i6i 



Fig. 91. Amaryllis bulb. 



possible in most parts of the United States. For this reason 

 nearly all our tulip bulbs are brought from Holland. The 

 importation of bulbs from countries 

 where they grow particularly well is 

 an important industry and enables 

 us to have many flowers which can- 

 not be so successfully propagated in 

 our climate. 



Tubers are the enormously thick- 

 ened ends of short underground 

 stems. The potato, the Jerusalem 

 artichoke, the dahlia, and the com- 

 mon white water lily develop tubers. 

 The scale leaves of the ordinary 

 rootstock are in tubers reduced to ridges, and the buds them- 

 selves to mere points. The scales and buds together form 

 the eyes of tubers. Tubers serve the same purposes for the 

 plant as do other fleshy underground stems : surplus food 

 accumulates in them, and by them the plant is multiplied. 

 The potato tuber has become one of the most important 

 sources of food for man. 



Commercial products derived from stems. We are all 

 familiar with the important products derived from the trunks 

 of trees. Lumbering is one of the most important industries 

 of the United States. Closely associated with it are the 

 furniture industry, which uses the hardwoods, — walnut, 

 oak, maple, sycamore, and birch, — and the wood-pulp in- 

 dustry, which utilizes soft woods — such as spruce and 

 poplar — in the making of paper. The Southern pines fur- 

 nish rosin and turpentine ; the bark of oaks and hemlocks sup- 

 plies tannic acid for the manufacture of leather ; and Span- 



