CHAPTER IX 



THE CIRCULATION AND FINAL USES OF FOOD 

 BY PLANTS 



Problems : To find out where plants store food and what use is 

 made of it. 



To study the structure of stems and the passage of liquids up and 

 down them. 



To find out how plants digest and assimilate 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 Food in Green Plants. — We 



have seen that cells of green plants make food — especially the cells 

 that are 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 Dicotyledonous or 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 outermost 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 inner 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 sievelike ends. 



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