116 THE PLANT 
of maintaining the normal relation of soil and roots,-a condition more or less 
impossible in a pot. In both instances the weighing should be done in the 
habitat, which was not the case in Hesselmann’s researches. 
The slight value of the potometer, which has had a vogue far beyond its 
merits, is indicated by the following table. These results were obtained 
from three plants of Helianthus annuus; II] was left undisturbed in the 
pot where it had been growing, IV was placed in a potometer, after the 
root had been cut off, and V was an entire plant placed in a potometer. 
The amount of transpiration is indicated in grams per square decimeter of 
leaf surface. The plants were kept in diffuse light, except for a period of 
two hours (8:00 to 10:00 A.M.) on the last day, when they were in full 
sunshine at a temperature of 75° F. Plant IV wilted so promptly in the 
sunshine that it was found necessary to conclude the experiment in 
diffuse light. 
8a.M.| 5P.M. | 8 A.M. | 5P.M. | 8A.M.| 5P.M. | 84.M./104.M,| 5 P.M. | 8 A.M.| Total 
III 2.9 7.3 2.4 6.0 1.7 1.6 2.0) 3.4] 2.0] 1.8 | 31.1 
IV 4.7 7.2 2.9 2.3 1.0 0.6 0.9) 0.5) 0.5 | 0.4 | 21.0 
Vv 3.7 5.3 3.2 4.8 2.5 1.6 3.0] 2.6] 1.6] 2.6 | 30.9 
The cut plant, IV, lost more water the first day than either of the others, 
but the water loss soon decreased, and at the end of the period was almost 
nil. The total transpiration for III and V is much the same, but the range 
of variation for pericds of 12 hours is from +2 to —1 gram. This ex- 
periment is taken as a fair warrant that the use of cut stems in potometers 
can not give accurate results. It is inconclusive, however, as to the merits of 
potometric values obtained by means of the entire plant, and further studies 
are now being made with reference to this point. 
159. Expression of results. From the previous discussion of the relation 
between them, it follows that an expression of the amount of transpiration 
likewise constitutes an expression of absorption. It is very desirable also 
that the latter be based upon root surface and chresard, but the difficulty of 
determining the former accurately and readily is at present too great to 
make such a basis practicable. In expressing transpiration in exact terms, 
the fact that plants of the same species or form are somewhat individual in 
their behavior must be constantly reckoned with. In consequence, experi- 
ments should be made upon two or three individuals whenever possible, in 
order to avoid the error arising from this source. 
