. Lessons in botany. Botany. BOW SOME PLANT PA UTS 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 21. 86. To demonstrate the tissue ten- sion.—Take a portion of the peti
. Lessons in botany. Botany. BOW SOME PLANT PA UTS 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 21. 86. To demonstrate the tissue ten- sion.—Take a portion of the petiole of a caladium, or of celery, or other plant, about 15OTZ long. Cut the ends off squarely. With a knife strip off a layer from the outside about 2-ynm in thick- „. „. „ _. 0 Fig. 37. Fig. 38. Fig. 39. ness, and the full length of the piece. Centre of Outside Outside strip Now attempt to replace it, comparing the petiole. strip. attached to 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 8cm 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 Please note that these images are extract
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