. Elementary botany . 50 PHYSIO log v. algae and treat them as described above gas will be given off in a similar manner. 114. We may now take one of the higher green plants, an aquatic plant like elodea, callitriche, etc. Place the plant in ? the water with the cut end of the stem uppermost, but still immersed, the plant being weighted down by a glass rod or other suitable object. If we place the vessel of water containing these leafy stems in the bright sunlight, in a short time bub- bles of gas will pass off quite rapidly from the cut end of the stem. If in the same vessel we place another
. Elementary botany . 50 PHYSIO log v. algae and treat them as described above gas will be given off in a similar manner. 114. We may now take one of the higher green plants, an aquatic plant like elodea, callitriche, etc. Place the plant in ? the water with the cut end of the stem uppermost, but still immersed, the plant being weighted down by a glass rod or other suitable object. If we place the vessel of water containing these leafy stems in the bright sunlight, in a short time bub- bles of gas will pass off quite rapidly from the cut end of the stem. If in the same vessel we place another stem, from which the leaves have been cut, the number of bubbles of gas td iu/lg'r45' given off will be very few. This indicates that Bubbles of oxygen gas ° J given off from elodea in a iarge part of the gas is furnished by the presence of sunlight. ° r ° J leaves. 115. Another vessel fitted up in the same way should be placed in the dark or shaded by covering with a box or black cloth. It will be seen here, as in the case of spirogyra, that very few or no bubbles of gas will be set free. Sunlight here also is necessary for the rapid escape of the gas. 116. We may easily compare the rapidity with which light of varying intensity effects the setting free of this gas. After cutting the end of the stem let us plunge the cut surface several times in melted paraffine, or spread over the cut surface a coat of varnish. Then prick with a needle a small hole through the paraffine or varnish. Immerse the plant in water and place in sunlight as before. The gas now comes from the puncture through the coating of the cut end, and the number of bubbles given off during a given period can be ascertained by counting. If we duplicate this experi- ment by placing one plant in weak light or diffused sunlight, and another in the shade, we can easily compare the rapidity of the escape of the gas under the different conditions, which represent varying intensities of light. We see then that not on
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