18 AIR AND SUNLIGHT 



vessel of no material now known can confine it. In the limitless 

 ocean of ether surrounding the sun innumerable waves are set up 

 and transmitted in all directions. More than four hundred mil- 

 lions of them arrive at the earth every second, having come across 

 ninety-three millions of miles in about eight minutes. It is under 

 such hurried strokes as these that many of the processes of plant 

 growth are produced. 



To appreciate the true value of sunlight we have only to observe 

 the sickly appearance of plants in the shade or in dark places. No 

 doubt you have observed that the tops of many house plants 

 incline towards the source of light. In New Mexico, near Ros- 

 well, one may frequently see the same thing illustrated in the long 

 rows of cottonwood trees lining the banks of irrigating ditches 

 which border many of the roadways and drives. Florists some- 

 times arrange window plants on a revolving platform in order 

 to secure an equal distribution of the light and an even growth of 

 the plant. From this it will be seen that plants must have plenty 

 of sunlight in order to develop. 



EXERCISES 



1. Place some marble or limestone in a glass or a bottle, and add a little 

 hydrochloric acid. Test the gas that is formed and ascertain what it is. 



2. Blow your breath through limewater by means of a straw or a glass 

 tube. Note the results and explain. 



3. Plant a few grains of wheat or corn in two tomato cans filled with good 

 soil. Puncture the bottom of one of the cans and give the seed and soil 

 in this can only a moderate quantity of water from time to time as needed. 

 Keep the soil in the other can water-soaked so as to exclude the air as much 

 as possible. Note the results and explain. 



4. Place two cans or pots containing plants in the window where the 

 light can shine on both. Label one of the cans A and the other one B. 

 Adjust the position of the one marked A several times each day so that all 

 parts of the plant will have an equal chance at the light. Do not change 

 the position of B at all; and compare results in the development of fie two 

 plants. Explain. 



5. Grow plants both in the shade and the sun. Explain results. 



QUESTIONS 



1. Discuss the composition of the air. 



2. What part of plant food is derived from (a) the soil, (6) the air ? 



3. In what forms does carbon appear? 



