THE PLANT AND ITS ENVIRONMENT 179 



505. Since most stems are positively phototropic, why is it that all 

 the trees in the northern hemisphere are not bent somewhat toward the 

 south? 



506. Flowers are generally positively phototropic and fruits negatively 

 so. Of what advantage are these reactions to the plant? 



507. When nurserymen grow seedlings of forest trees, they cover the 

 plants with a lattice-work screen for the first few years. Why? 



508. Most plants in darkness grow abnormallj' long. Of what 

 advantage may this reaction be to the plant? 



509. Of what advantage is it to the plant for its roots to be positively 

 geotropic and its stems negatively so? 



510. Give an example of a stem which is not negatively geotropic. 



511. Give an example of a root which is not positively geotropic. 



512. How different is the direction in which the trunk of a tree will 

 point if it is growing on a steep hillside from that in which it will i)oint if 

 it is growing on level ground? Explain. 



513. Just how differently does the force of gravity act when it causes 

 a root to grow downward and when it causes a horizontally placed piece 

 of wood, or other dead object, to bend downward? 



514. Of what advantage to the plant is it to have its roots positively 

 hydrotropic? 



515. Why do desert plants usually have a ratlier high osmotic con- 

 centration in their cell-sap? 



516. Crocus, narcissus, and tulip plants flower and flourish in early 

 spring and by late spring have withered down, not to appear above 

 ground till the following year. For what sort of a climate do you think 

 such plants are well adapted? 



517. Why are arctic and alpine plants xerophytic? 



518. The leaves of many evergreens, such as juniper, are pressed 

 against the stem in winter and loosely spread in summer. Explain. 



519. The timberline (or line above which trees do not grow on a 

 mountain side) is generally higher on a mountain range than on an 

 isolated peak, and in ravines than on ridges. Why? 



520. The "rosette" habit of growth, where all the leaves arc in a 

 circle next to the ground and a stem is absent, is common in cokl, 

 arid regions. ExiMain. 



