122 PHYSIOLOGICAL PROCESSES IN PLANTS 



6. What, is meant by the "timber line"? Under what 

 conditions does it exist? 



7. Where are the conditions the more variable, in the arc- 

 tics or tropics ? Then -where would there be more variation 

 in plant life ? 



8. As ponds dry up, do different plant societies follow one 

 another ? Explain. 



9. How does the arctic willow vary from its relative of the 

 temperate zone ? Why ? 



10. How do floating plants get their supply of soil substance ? 



11. Where are there xerophytic forests in the United States ? 



12. What are halophy tes ? What are their characteristics ? 



Special Reports 



1. The factors that determine plant association. 



2. How plants protect themselves from drought. 



3. The effect of cold regions on plants. 



References 



Hunter, Essentials of Biology. Chap. XII. 



Coulter, Text-book of Botany. Chaps. XXI, XXII, XXIII, XXIV. 



Bergen and Davis, Principles of Botany. Chaps. XXXVI, XXXVII. 



Weed, C. M., Wild Flower Families. 



Bailey, L. H., "How Plants Live Together." Cornell University 

 Nature Study Quarterly, No. 6, October, 1900. 



Merriam, C. H., "Life Zones and Crop Zones in the United States." 

 Bulletin 10, U. S. Department of Agriculture, 1898. 



