HOW SOME PLANT PARTS REMAIN RIGID. 49 



Longitudinal tissue 

 tension. 



Transverse tissue 

 tension. 



Succulent stems and petioles are often kept rigid be- 

 cause of a pull, or tension, of different layers of 

 cells in opposite directions. The outer layers of 

 cells tend to shorten, while the inner cells tend to 

 lengthen. 



These opposite tensions, or pulls, make the shoot 

 rigid. 



The cells of the shoots must be turgid with water or 

 the tension is not present. 



This occurs where the outer layers of tissue are 

 stretched transversely instead of longitudinally. 



Material. If fresh plants cannot be obtained out-doors, use leafy shoots 

 of rather succulent plants from the green-house, like the coleus plant, 

 garden balsam, or leaves with long petioles like the caladium of the green- 

 house, or stored celery. The shoots should not be cut from the plant until 

 the pupil is ready to begin the exercise. Wide-mouthed bottles, filled with 

 water, and if necessary some bell jars (one large bell jar will answer for 

 several students). 



