92 PHYSIOLOGICAL PROCESSES IN PLANTS 



(bast) (best seen in old bark) ? Just outside this may be found 

 the rind or cortex. In most stems this is made up in part of an 

 outer layer of cork. 



Conclusions. 1. Can you think of any use for the woody part 

 of the stem ? 



2. Review the function of the medullary rays. 



3. (Home work) A potato is an underground stem. How 

 can you tell ? Select two potatoes of unequal weight. Peel one 

 of them until it weighs as much as the other. Set both in a 

 warm place, and after a day or so reweigh. Results ? What 

 is one of the uses of the epidermis of a stem ? Would the corky 

 layer do likewise ? 



d. Circulation in Stems 

 1. THE UPWARD PATH OF RAW FOOD MATERIAL 



Observation. Get an entire plant with a taproot, as a 

 parsnip. Cut off the root tip, and place the cut end in a solu- 

 tion of red ink or eosine. Let it stand a number of hours and 

 observe the cut end. Cut various sections of the- root and stem 

 and see if you can trace the path of liquids to the leaves. Try 

 the same with sections of a corn stalk. 



Conclusion. Where are the tubes (fibrovascular bundles) 

 through which liquids rise situated ? 



Note. A stem with fibrovascular bundles arranged as in the 

 box elder or horse-chestnut is characteristic of plants with two 

 cotyledons in the seed (dicotyledons). Stems with these bun- 

 dles scattered through the pith as in the corn are characteristic 

 of plants with one cotyledon in the seed (monocotyledons). 



2. THE DOWNWARD PATH OF RAW FOOD MATERIAL 



Note. Start this experiment two or three weeks ahead of the 

 time needed. 



Observations. 1. Place some fresh willow twigs in water 

 until they form roots. Girdle them (remove rings of bark about 



