118. Longitudinal tension in dandelion stems.—Take long and fresh dandelion stems. Split them. Note that they coil. The longitudinal tension is very great. Place some of these strips in fresh water. They coil up into close curls because by the absorption of water by the cells the turgescence of the individual cells is increased, and this increases the tension in the stem. Now place them in salt water (a 5 per cent solution). Why do they uncoil?
119. To imitate the coiling of a tendril.—Cut out a narrow strip from a long dandelion stem. Fasten to a piece of soft wood, with the ends close together, as shown in [fig. 62]. Now place it in fresh water and watch it coil. Part of it coils one way and part another way, just as a tendril does after the free end has caught hold of some place for support.
120. Transverse tissue tension.—To illustrate this one may take a willow shoot 3-5 cm diameter and saw off sections about 2 cm long. Cut through the bark on one side and peel it off in a single strip. Now attempt to replace it. The bark will not quite cover the wood again, since the ends will not meet. It must then have been held in transverse tension by the woody part of the shoot.
[CHAPTER VII.]
STARCH AND SUGAR FORMATION.
[1. The Gases Concerned.]
121. Gas given off by green plants in the sunlight.—Let us take some green alga, like spirogyra, which is in a fresh condition, and place one lot in a beaker or tall glass vessel of water and set this in the direct sunlight or in a well lighted place. At the same time cover a similar vessel of spirogyra with black cloth so that it will be in the dark, or at least in very weak light.
Fig. 63.
Oxygen gas given
off by spirogyra.