HOW .SOME PLANT PARTS REMAIN RIGID. 47 



excellent for this purpose, and these may be had at almost 

 any season of the year from the greenhouses, and are thus 

 especially advantageous for work 

 during late autumn or winter. 

 The tension is so strong that a 

 portion of such a petiole 10- 

 i^cm long is ample to demon- 

 strate it. As we grasp the lower 

 end of the petiole of a caladium, 

 or rhubarb leaf, we observe how 

 rigid it is, and how well it sup- 

 ports the heavy expanded lam- 

 ina of the leaf. 



Exercise 2 1 . 



86. To demonstrate the tissue ten- 

 sion. Take a portion of the petiole of a 

 caladium, or of celery, or other plant, 

 about i$t'w long. Cut the ends off 

 squarely. With a knife strip off a layer 

 from the outside about 2-ymn in thick- p . g ^ Fig ^ pig ^ 

 ness, and the full length of the piece. Centre of Outside Outside strip 

 Now attempt to replace it, comparing the P etiole ' strip " attached to 



centre. 



length of each part. Remove another Figures 37-39. Showing longitudinal 

 strip lying next this one, and so on tissue tension, 



until all the outer portion has been removed. Describe what takes place as 

 the successive strips are removed. When all are removed, compare an outside 

 strip with the central portion. What has happened ? Is there now a greater 

 difference in length between the outside strip and the central portion ? What 

 is the cause of this ? Describe the tensions in the outside and inner portion 

 of the petiole. 



Cut a section of the petiole about $>cm long, remove strips on two opposite 

 sides and split the remainder down the middle, securing two pieces with the 

 center and outside portion attached. Place one of these in fresh water and 

 the other in a 5 per cent salt solution and note the result. If convenient 

 treat celery petioles in the same way. The flower stems of dandelions split 

 into quarters are .excellent objects to compare when placed in water, and in 

 a 5 per cent salt solution. 



