VIII. PLANT GROWTH AND NUTRITION — THE CIR- 

 CULATION AND FINAL USES OF FOOD BY PLANTS 



Problem. — How green plants store and use the food thei 

 niahe. 



id) What are the organs of circulation ? 



{h) How and where does food circulate ? 



(c) How does the plant assimilate its food? 



Laboratory Suggestions 



Laboratory exercise. — The structure (cross section) of a woody stem. 



Demonstration. — To show that food passes downward in the bark. 

 Demonstration. — To show the condition of food passing through the 

 stem. 



Demonstration. — Plants with special digestive organs. 



The Circulation and Final Uses of Foods in Green Plants. — We 



have seen that cells of green plants make food and that such cells 

 are mostly in the leaves. But all parts of the bodies of plants grow. 

 Roots, stems, leaves, flowers, and fruits grow. Seeds are store- 

 houses of food. We must now examine the stem of some plant in 

 order to see how food is distributed, stored, and finally used in the 

 various parts of the plant. 



The Structure of a Woody Stem. — If we cut a cross section 

 through a young willow or apple stem, we find it shows three 

 distinct regions. The center is occupied by the spongy, soft pith; 

 surrounding this is found the rather tough wood, while the outer- 

 most area is hark. More careful study of the bark reveals the 

 presence of three layers — an outer layer, a middle green layer, 

 and an inner fibrous layer, the latter usually brown in color. This 

 layer is made up largely of tough fiberlike cells known as hast 

 fibers. The most important parts of this inner bark, so far as the 

 plant is concerned, are many tubelike structures known as sieve 

 tuhes. These are long rows of living cells, having perforated 



HUNTER, CIV. BI. — 7 97 



