THE "POTETOMETER," AN INSTRUMENT FOR MEASURING THE TRANSPIRATION OF WATER BY PLANTS.
In view of the interest now attaching to recent advances in vegetable physiology, it seems not unlikely that a description of the instrument bearing the above name, lately published by Moll (Archives Neerlandaises, t. xviii.), will serve as useful purpose. The apparatus was designed to do away with certain sources of error in Sachs' older form of the instrument, described in his "Experimental Physiologie"--errors chiefly due to the continual alteration of pressure during the progress of the experiment.
As shown in the diagram, the "potetometer" consists essentially of a glass tube, a d, open at both ends, and blown out into a bulb near the lower end; an aperture also exists on either side of the bulb at or about its equator. The two ends of the main tube are governed by the stopcocks, a and d, and the greater length of the tube is graduated. A perforated caoutchouc stopper is fitted into one aparture of the bulb, e, and the tube, g k, fits hermetically to the other. This latter tube is dilated into a cup at h to receive the caoutchouc stopper, into which the end of the shoot to be experimented upon is properly fixed.
The fixing of the shoot is effected by caoutchouc and wire or silk, as shown at i, and must be performed so that the clean-cut end of the shoot is exactly at the level of a tube passing through the perforated stopper, e, of the bulb; this is easily managed, and is provided for by the bending of the tube, g h. The tube, f, passing horizontally through the caoutchouc stopper, e, is intended to admit bubbles of air, and so equalize the pressure and at the same time afford a means of measuring the rapidity of the absorption of water by the transpiring shoot. This tube (see Fig. 2, f) is a short piece of capillary glass tubing, to which is fixed a thin sheath of copper, b', which slides on it, and supports a small plate of polished copper, a', in such a manner that the latter can be held vertically at a small distance from the inner opening of the tube, and so regulate the size of the bubble of air to be directed upward into the graduated tube, a b.
The apparatus is filled by placing the lower end of the main tube under water, closing the tubes, f and i (with caoutchouc tubing and clips), and opening the stopcocks, a and d. Water is then sucked in from a, and the whole apparatus carefully filled. The cocks are then turned, and the cut end of the shoot fixed into i, as stated; care must be taken that no air remains under the cut end at i, and the end of the shoot must be at the level, k l. This done, the tube, f, may then be opened.
The leaves of the shoot transpire water, which is replaced through the stem at the cut end in i from the water in the apparatus. A bubble of air passes through the tube, f, and at once ascends into the graduated tube, a c. The descent of the water-level in this tube--which may conveniently be graduated to measure cubic millimeters--enables the experimenter at once to read off the amount of water employed in a given time.
It is not necessary to dwell on obvious modifications of these essentials, nor to speak of the slight difficulties of manipulation (especially with the tube, f). Of course the apparatus might be mounted in several ways; and excellent results for demonstration in class could be obtained by arranging the whole on one of the pans of a sensitive balance. H. MARSHALL WARD.
Botanical Laboratory, Owens College.