102 THE NATURE AND WORK OF PLANTS 



of form have they undergone ? Cut the stalks of 

 the calla lily or rhubarb into strips, and note be- 

 havior. 



134. Stems as conducting organs. The position 

 of the stem between the leaves where sugars and 

 other substances are manufactured and the roots 

 where water and mineral salts are taken up makes 

 it necessary for it to conduct the products of the 

 leaf downward, and the materials taken up by the 

 leaf upward. There is a stream of water upward 

 and a stream of food material downward, but the 

 two are not connected, and there is nothing like a 

 circulation of the sap. 



135. Upward path of sap. Cut off a stem of 

 the tomato or touch-me-not with a number of sound 

 leaves attached, and thrust the lower end of the 

 stem into a bottle of red ink or a tumbler of water 

 colored with aniline dye. After four or five hours 

 remove and dissect the stem. What portions are 

 colored? This will show through what tissues the 

 liquid has passed. It is to be seen that the dead 

 wood cells also serve as conduits for water in addi- 

 tion to their function of making the stem rigid. 



