A laboratory course in plant physiology, especially as a basis for ecology; . 6, and close their tops byrubber stoppers cut to such a lengththat when forced tightly in they justfill the space above the graduation, andfor additional security against leakagecover them and the burette end withsealing-wax. In each one place now along shoot with numerous small leaves(I find Ficus repens grown in mostgreenhouses particularly available forthis), to the lower end of which a veryfine wire is attached by means of whichit may be withdrawn. The wire is thentaken outside the burette and held inplace by a r
A laboratory course in plant physiology, especially as a basis for ecology; . 6, and close their tops byrubber stoppers cut to such a lengththat when forced tightly in they justfill the space above the graduation, andfor additional security against leakagecover them and the burette end withsealing-wax. In each one place now along shoot with numerous small leaves(I find Ficus repens grown in mostgreenhouses particularly available forthis), to the lower end of which a veryfine wire is attached by means of whichit may be withdrawn. The wire is thentaken outside the burette and held inplace by a rubber band. Each burette isthen placed over a small mercury res-ervoir, a very small stiff rubber tubebeing first run up inside the burette withthe other end outside. The burette is FIG. 20.—ARRANGEMENT FOR DE-now depressed in the mercury reservoir TKRMINING THE GAS-EXCHAN<;Kuntil the mercury stands at 4 cc. above IN~ PHOTOSYNTHESIS. One-fourththe zero-mark inside and out, with end ^ie true size-of the rubber tube just at the level of the liquid. The tube is now with-. 94 PLANT PHYSIOLOGY, drawn and the burette is raised until the zero-mark is at the outsidelevel (it will fall somewhat inside); enough CO2 must now be allowed toenter the burette through a slender bent glass tube held in the mercurybeneath the burette, to depress the mercury to the zero-mark. If prop-erly carried out, the level of the mercury will be exactly the same (andat the zero-mark) inside and out, while the burette will contain 8% ofCO2, an amount which most plants can readily use. One of the bu-rettes is now to be darkened by a covering of tin-foil (better than blackpaper because it reflects heat), the exact temperature is to be noted fromthe attached thermometer, and the whole is to be set in bright light fora day. Towards sunset (in winter, a shorter time is ample in summer)the plants are to be carefully withdrawn by the wires and the resultantchange in volume, marked by the rise of mercury, carefu
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