PHYSIOLOGICAL PROCESSES IN PLANTS 



91 



whence they run up through the stem to the leaves, where they spread to 

 form the veins of the leaf. They conduct fluids containing dissolved food 

 matter from the roots to the leaves, and from the leaves to the stem and 

 root. These tubes are closed when the leaves fall in the autumn. Can 

 you tell why ? 



b. Lenticels 



Observation. Look for small openings or cracks in the bark (len- 

 ticels). These permit the passage of air, and are therefore breathing holes 

 through the bark. Are there few or many ? 



Conclusions. 1. If there are also breathing holes in the leaf, when are 

 the lenticels of most importance to a plant ? 



2. Would there probably be as many lenticels in evergreen or tropical 

 plants? Explain. 



c. Internal Structure of a Dicotyledon 



Observations. 1. Cut a cross section of box elder or horse- 

 chestnut twig, or other similar stem. How does the inner part 

 (pith) differ from the circle bounding it (wood) ? 



2. Which is the stronger, pith or wood ? Which is the most 

 porous ? 



3. Find lines radiating 

 from the pith through the 

 wood (medu llary rays) . 

 These serve as channels 

 of communication between 

 the pith or wood cells 

 and the cells of the outer 

 portion of the stem even 

 the lenticels. They thus 

 convey oxygen as well as 

 food (sugar and water) 

 to the interior of the 



stem. 



4. Just outside the FIG. 23. Cross section of twig of box elder 



wood, find the bark. three years old. Note three annual growth 



Strip off a bit of it. Can rings in , *?" vascular . cylinder " c ' cortex ; 



ra, medullary or pith rays; w, wood. 



you find fibrous structures (After Coulter.) 



