Pethybridge — An Improved Simple Form of Potometer. 151 



bored rubber cork. Into one of the holes of this (that in which 

 a glass rod bearing the letter A is shown in the figure), the leafy- 

 shoot is inserted ; the other carries the tapped funnel. The lower 

 outlet is also furnished with a rubber cork, through which passes 

 an L-shaped glass tube of narrow bore, with the longer limb of 

 the L vertical, and suitably supported about half-way up by 

 means of a wire fastened to the neck of the tower. Fastened to 

 this tube is a paper scale, which is a convenience in reading ; 

 but which may be dispensed with, and file or other marks on the 

 tube substituted in its place. It will be noted that the narrow- 

 bore tube is of the same height as the funnel, so that when filled 

 with water (as the apparatus is at the start of an 

 experiment), and the tap turned on, it forms one 

 limb of a U tube. As transpiration proceeds, 

 water is absorbed by the cut end of the shoot, 

 and its place is supplied by water from the fun- 

 nel and from the narrow-bore tube, so that the 

 level of the water in these slowly sinks. 



On turning off the tap, however, water can 

 only be withdrawn from the narrow-bore tube, 

 the level in which consequently sinks more or less 

 rapidly according as to whether transpiration is 

 rapid or otherwise. By means of this apparatus 

 then, the chief features of which are its compact- 

 ness, the ease with which it can be moved about, 

 and the fact that no specially constructed tube is 

 required, the broad effects of various external 

 conditions, such as heat and cold, light and 

 shade, draughts and still air, dry and moist air, 

 &c, can be very conveniently studied and re- 

 corded by noting the time taken by the column 

 of water in falling through a given distance in the tube in each 

 case. It should be noted that, before each fresh experiment, the 

 level of the water should be adjusted to the original starting-point 

 in the narrow-bore tube by means of the tapped funnel, since, 

 owing to differences in water-pressure, a fall of, say, 1 cm. in the 

 upper part of the tube is not strictly comparable with a fall of 

 1 cm. near the bottom of the tube. 



Expense may be saved by substituting for the tapped thistle- 



N2 



