AERIAL STEMS 



153 



tendrils. They are roots as to origin. They penetrate 

 the tissue of the plant on which the ivy climbs. 



Beans, peas, and the morning glory are examples of plants 

 which climb by means of twining stems. In these plants 

 the young elongating part of the stem swings free. 

 It does not stand erect. Its end seems to be describing 

 an arc of a horizontal circle. It moves slowly, follow- 

 ing a horizontal 

 curve, until it 

 comes in contact 

 with something. 

 Around this some- 

 thing it swings it- 

 self if it can, grow- 

 ing as it swings. 



You have 

 already read a 

 little about ten- 

 drils. You have 

 learned of their 

 delicate response 

 to contact. (See 

 Sections 41 and 

 42.) The chmb- 

 ing or star cucum- 

 ber is a plant whose tendrils have been carefully studied as to 

 the delicacy of their response. Its tendril tips have been 

 found to respond to the touch of a swinging object which 

 weighs only one eighth as much as the lightest swinging 

 object which our skin is able to perceive. Such tendrils 

 as those of the star cucumber respond to contact in the 

 following manner. First the touched side of the tendril 



Fig. 51. — A tropical forest showing many lianas. 

 hy Whitjord in the Philippine Islands. 



-Phot]. 



