144 THE STEM PROPER 



direction. When it has reached the end of its stake suffer 

 it to grow about five centimeters (two inches, approxi- 

 mately) beyond, and watch the revolution of the tip. Cut 

 a hole through the center of a piece gf cardboard about 

 fourteen centimeters (five to six inches) in diameter, slip it 

 over the loose end of the stem, and fasten it to the stake 

 in a horizontal position with a pin. Note the position of 

 the stem tip every two houi's and mark on the cardboard ; 

 how long does it take to complete a revolution ? 



204. The Cause of Twining is believed to be unequal 

 growth on the two sides of the stem (Sec. 162) which 

 causes the tip to revolve slowly in a spiral toward the 

 side where growth is slowest. Run a gathering thread in 

 one side of a narrow strip of muslin, about a meter (one 

 yard, approximately) long, and notice how the ruffle thus 

 drawn will curl into a spiral when allowed to dangle from 

 the needle. In the same way the tension resulting from 

 unequal growth causes the stems and tendrils of climbing 

 plants to form themselves into spirals. 



Hardly any kind of stem grows at a uniform rate in all 

 its parts. Ordinarily the inner part grows most rapidly, 

 Split the stem of a fresh dandelion, hyacinth, or other herba- 

 ceous scape longitudinally, and immerse it in fresh water 

 for 30 to 45 minutes. Notice how the two halves curve 

 outward, or even coil up like a watch spring. This is on 

 account of the tension caused by the more rapid absorption 

 of the internal tissues, which, when relieved of the resist- 

 ance of the outer wall, or epidermis, stretch themselves, 

 as it were, but are held back and. drawn into a curve by 

 the resistance of the slower growing outer parts, as the 

 muslin of our ruffle was curled by the gathering thread. 



205. The Object of the Various Forms of Stem Growth 



is in all cases the same ; to bring the leaves into the best 

 possible relations with the light and air. The stem, besides 

 other important uses, serves as a mechanical support, or 

 framework, to bind the other organs together, and they are 

 largely dependent upon it for proper exposure to Hght 



