Plants in Relation to Man 477 



soil temperatures be taken ? 54. If the experiment with the tempera- 

 ture of soil in prepared pots has been performed, summarize your 

 conclusions. 55. What does a gardener mean by a warm soil? What 

 is the usual character of such a soil ? 56. Show the relation of water 

 to the temperature of the soil. 



SUGGESTIVE QUESTIONS 



1. Compare the soils of the South and those of the North as to 

 origin. 2. What is the prevailing type of soil in the vicinity of the 

 school building? 3. (a) Are there any peat swamps in your vicinity? 

 (6) Of what use are they ? 4. Sometimes rocks used about a camp fire 

 explode. Why? 5. What does this suggest as to the effect of tem- 

 perature changes in making soil from rocks? 6. If clay retains soil 

 water to the amount of 40 per cent of its dry weight, what per cent 

 of the wet clay is water? 7. (a) Mention some plants that require 

 a great deal of water ; some that are drought resisting. (6) What 

 are some of the differences in structure? 8. (a) Which is the better 

 for plants, chemically pure water or soil water? Why? (6) Should 

 carbon dioxid be considered the food of plants or the material out of 

 which they make food? 9. About what part of the average rain 

 falling on cultivated fields in your locality is returned to the air through 

 the leaves of plants? 10. In what way do farmers of your vicinity 

 recognize differences in soil on their farms? 11. What kind of land 

 and what exposure is selected for the following : wheat or oats, corn, 

 potatoes, hay, garden vegetables, orchard trees? 12. (a) Why do 

 orchards thrive better when the soil is cultivated? (6) What condi- 

 tions of growth are lacking when the soil is not cultivated? 



II. NATURAL AND ARTIFICIAL SELECTION 



227. Natural selection. In only a few regions of the earth 

 can even a small population find sufficient food materials in the 

 free course of nature to sustain life in ease and comfort. Human 

 population rapidly outruns natural productiveness and must 

 therefore in some way supplement or improve nature's pro- 

 duction, or face want and famine. In all the wealth of plant 

 nature, man finds a comparatively small number of plants 

 adapted to his needs, but these he selects and encourages by 

 cultivation. Plants which interfere, he destroys or restricts 

 in their range of growth. In the large areas set aside for grain, 



