288 PLANT PHYSIOLOGY 



voir. In the cow tree of Venezuela, Brosimum galadodendron, 

 the latex is palatable and nutritious and is used as food much like 

 ordinary milk. Thus the latex, like the tannins, glucosides, and 

 pigments may have very different functions in different plants. 

 Plants are living organisms, and there is no more reason to be- 

 lieve that those with latex all use it for the same purpose than to 

 think that all men with automobiles have them for the same 

 purpose. 



Summary. — In plants where manufacture exceeds consump- 

 tion, food will be stored. In transporting this food from the re- 

 gions of manufacture as well as in carrying colloidal materials 

 (proteins) to the regions where foods are used but not made, the 

 phloem cells play the most important part; but they may be sup- 

 plemented in certain plants by the latex, and in some cases (up- 

 ward movement) other regions such as the xylem may be the more 

 important. 



QUESTIONS 



1. Why does a girdled tree ultimately die? Why not immediately? 



2. Certain bark diseases, for example, the chestnut bark disease, attack 

 only the bark and yet are fatal. Why? 



3. Why do the tissues above swell, when a trunk is bound tightly with 

 a wire? 



4. What is " layering "? Why is it successful? 



5. Exhibition growers sometimes girdle a branch some distance below 

 the blossom to produce an exceptionally large fruit. What must be present 

 above the girdle besides flowers? Why? 



6. Give an example of a product which in one plant is a temporary product 

 and in another is a storage product. 



7. At the time of leaf fall, why is a larger percentage of nitrogen saved than 

 of carbohydrates? 



8. What is meant by the "morphologically upward" movement of food in 

 fruit peduncles? 



9. It is commonly said that for each good apple fruit there should be at 

 least 30 leaves. Why this connection between number of leaves and fruit? 



10. Does it seem reasonable that much translocation should occur in the 

 walls of the phloem cells? With this in mind, read critically the articles by 

 Crafts. 



1 1 . Name six possible functions of latex. 



12. Why should there be any connection between the region of storage 

 and the duration of life of the plant? 



