176 PLANT PHYSIOLOGY 



by the Krutitzky apparatus (figured by Etjrgerstein, 22); the plant 

 in a glass jar is watered from below by a siphon fed from a movable float, 

 which may be made self-registering. The arrangement necessarily keeps 

 the roots saturated and thus gives conditions inimical to the health of 

 most plants. Far better in every way would be Livingston's self-water- 

 ing arrangement, described in the Plant World, n, 1908, 39, which could 

 readily be made self-registering (by making the feed-vessel a slender cylinder 

 carrying a float moving a pointer against a revolving cylinder), and which 

 may yet play a large part in the study of this subject. Another arrangement 

 of Krutitzky's, taking a cut shoot and self-registering, is figured by Good- 

 ale, 273. An older arrangement, taking a cut shoot, is Vesqtje's (figured 

 by Burgerstein, 21); the plant and its vessel are balanced with another 

 communicating vessel, and the amount of water needed to keep them in 

 equilibrium is measured as added. The absorption method, however, is 

 most commonly applied to cut shoots or water-culture plants; the cut end 

 or roots of these are sealed air-tight into the neck of a tall vessel having 

 a communicating slender graduated cylinder, in which the loss of water 

 may be read off at intervals. It is illustrated by Pfeffer, i, 232 (Burger- 

 stein, 18). A method of making this self-registering, by use of a float in 

 the cylinder carrying a pointer against a revolving drum, is described by 

 Stone in Torreya, 4, 1904, 19. The objection to this method consists in 

 the unnatural conditions of the absorption, which makes this transpiration 

 an unsafe index of transpiration under natural conditions, while errors may 

 also arise from the volume changes in the water under varying temperature 

 and accumulation of gas, etc. When the apparatus is made small and the 

 cylinder becomes reduced in bore to a degree sufficient to render the loss 

 of water observable by the eye, these instruments merge over to potometers, 

 which will be described in a later section. 



A method of weighing by small hanging springs, read by a horizon- 

 tal microscope, is described by Darwin and Acton, 101, but has marked 

 limitations. This arrangement is made self-registering by Linsbauer, 39. 



In studying the results yielded by the foregoing experiment, 

 and especially in comparing the results obtained by different 

 students, either for different plants or even for different sizes 

 of the same plants, it becomes at once evident that the result 

 must vary with the size and leaf spread of the plant. These dif- 

 ferences must be compensated before any comparison of the 

 transpiring powers of different plants can be made, and before 

 any definite statement can be made as to the transpiring power 

 of any given plant. This compensation is best made by deter- 

 mining the leaf area of the plant and reducing the transpiration 

 to grams per square meter per hour, which yields a valuable 



