Practical physiology of plants / by Francis Darwin and E. Hamilton Acton.
- Darwin, Francis, Sir, 1848-1925.
- Date:
- 1894
Licence: Public Domain Mark
Credit: Practical physiology of plants / by Francis Darwin and E. Hamilton Acton. Source: Wellcome Collection.
99/354 (page 75)
![CH. IV] POTOMETER. ' 0 tube. As soon as this appears the small block is replaced, the tube once more dips into the water, and a bubble of air included in it travels up, and serves as an index of the rate of absorption. The bubble must be of uniform size in successive readings, because other things being equal a long and short bubble travel at different rates. To insure this, mark the tube with a file at 5 or 6 mm. from its end, and replace the vessel W when the air column has reached the file-mark. The movement of the air-bubble is timed from a mark on the tube: the upper limit of its course is the upper end of the capillary tube, the moment of its impinging against the column of water in the potometer being easily visible. It is for this reason that the tube projects above the cork, for otherwise a convenient place of ending for the course of the bubble would not be visible. The starting point of the course should be at least 2 inches above the file-mark, so that the bubble may settle down to a uniform pace before the course begins; and because time is needed for the observer to put down the block and the small vessel of water, and take up the stop-watch. In this way numerous readings can be taken in a short time; the air admitted collects at A, and can be removed after a time; it is obvious if the branch were placed in A and the cork in G, that the admitted air might diminish the surface of contact between the water and the absorbing surface of the branch. It is well to make graphic representations of one or more of the following experiments, using the reciprocals of the stop-watch readings, which will be proportional to](https://iiif.wellcomecollection.org/image/b21500162_0099.jp2/full/800%2C/0/default.jpg)
