272 Hooker.—Hydrotropism in Roots of Lupinus albtis. 
Specially constructed hygrometers were used to measure the percentages 
of relative moisture within the zinc compartment. A grain of Stipa 
pennata was inserted in a small cork and the awn was cut off at an 
appropriate distance, leaving from 1 cm. to 2 cm. of it projecting 
vertically from the cork. A glass capillary about 2 cm. long was fixed 
with sealing-wax to the cut end of the awn so that it extended horizontally. 
This served as an indicator, turning as the hygroscopic awn twisted tighter. 
Markings were made on a circular piece of cardboard fastened to the top 
of the piece of cork. This hygrometer was calibrated by placing it in a small 
closed compartment with a relatively large amount of a definite concentration 
of sulphuric acid. The percentage of relative moisture was determined by 
the vapour pressure of the sulphuric acid. 
Table I. 1 
Relative 
Percentage 
Vapour 
percentage of 
of 
pressure 
moisture 
H^SOi. 
at 20 0 C . 
approxhnately. 
57*65 
3-728 
21 
52-13 
5 - 79 2 
33 
43-75 
8*494 
49 
37*69 
10*831 
62 
33 -io 
12*317 
7 1 
24*26 
14*482 
83 
0*00 
17-363 
100 
By using several different sulphuric acid solutions, the hygrometers 
could be rendered accurate within 5 per cent., but they had to be 
recalibrated every three weeks. The instruments were carefully compared 
with one another and were found to vary from one another within 2 per cent. 
The measurements of psychrometric differences would then be accurate 
within 2 or 3 per cent., which is sufficient for the present purposes. 
A hygrometer was placed at either end on the floor of the compartment, 
their centres 25 cm. apart. Under ordinary circumstances a difference of 
8 per cent, was registered. 
The seeds of Lupinus albus were soaked twenty-four hours in water 
and allowed to germinate in sawdust until the roots had reached a suitable 
length. To be sure there are other seedlings which react better to hydro¬ 
tropism than lupins, as for example corn seedlings. But great difficulty 
was experienced in obtaining straight roots of corn that would suit for 
experimentation. In this and other respects lupin seedlings were found to 
be most satisfactory. 
The experiments were conducted for the most part at a temperature 
of 20° C. 
1 Compiled from Landolt and Bornstein : Physikalisch-chemische Tabellen, pp. 360, 426. 
Vierte Auflage. Berlin, 1912. 
