TEMPEKATURE AND LIGHT 233 



plants with acaulescent stems have leaves with petioles of vary- 

 ing lengths which prevents overlapping and shading to any 

 great degree. 



Influence of Light on Plant Distribution. — Light influences 

 the distribution of plants in a manner somewhat similar to 

 water, temperature, soil and other environmental factors. Some 

 plants are knovsm as lovers of stmshine and are usually found 

 where the sunlight is intense. Others thrive best in subdued 

 light and are very generally found growing in the shade of trees 

 and other plants larger than themselves. Therefore, many for- 

 ests show strata of plant growths: (a) the trees, (b) the shrubs 

 or bushes beneath them, and (c) the herbaceous plants and ferns 

 which carpet the ground. These light relationships are recog- 

 nized by the farmer, who knows that he can grow beans and 

 pumpkins advantageously with the corn. Many crops, such as 

 tobacco and pineapples, are sometimes grown under the shade 

 of cheesecloth or slats. Of course, these shading devices not 

 only protect the plants from too intense sunlight, but they also 

 protect against storms and conserve the water supply. 



LABORATORY EXERCISES. 



Exeroise 1. Grow some seedlings in potsi under the same conditions. 

 When the plants are well above ground, put one-half in a place where the 

 light is the same from all sides and the other half where the light comes 

 from one side only. After two weeks compare the two lots. What dif- 

 ferences do you observe? What conclusions can you draw from 

 this experiment? 



Exercise 2. Diagram the leaves of a dandelion or other acaulescent 

 plant. What per cent, of the lower leaf surface is shaded by the 

 upper leaves? 



Exercise 3. Put thin slices of beet and some thin leaves (nasturtium 

 or lettuce, e.g. ) in a beaker of water containing a thermometer. Heat very 

 slowly. At what temperature does the color escape from the beet cells? 

 At what temperature do the leaves lose their turgor? Can it be restored 

 if they are promptly replaced in cold water? Explain. 



Exercise 4. Count out two lots of 100 seeds each of wheat or other 

 grain. Weigh each lot and record the result. Dry one lot for several 



