Hooker.—Hydrotropism in Roots of Lupinus albus . 271 
a psychrometrical difference about the roots. For relatively rough work 
this method was found very effective. 
In order to make careful measurements a more complicated apparatus 
was constructed. A zinc vessel 10 cm. broad, 20 cm. high, and 30 cm. long 
was used. The sides were of glass, and were covered with flaps of black 
paper to shut out the light in case the experiment was not made in a dark 
room. The box was supplied with a tightly fitting cover. One end of the 
vessel was covered on the inside with filter-paper. On the floor 2 cm. from 
this end, a piece of glass 2 cm. high and 10 cm. long was fixed upright 
and the connexions with the sides and bottom were made water-tight. 
This formed a reservoir, which collected water at the base of the filter- 
paper. Two short tubes penetrated the zinc wall behind the filter-paper, 
one near the top, which served as an inlet, and one near the bottom, which 
acted as an overflow from the reservoir. The former could be connected 
with the faucet and running water supplied to the filter-paper from above, 
which flowed down it to the reservoir at its base and made its exit through 
the lower opening. In the earlier experiments the water could not safely 
be left running, so a constant supply of water was obtained by other means. 
A bent glass tube, the lower end of which was drawn out to a capillary, 
supplied water from a beaker. By breaking off the capillary at the proper 
point, the amount of water supplied could be perfectly regulated. In this 
way a constant source of moisture was produced at one end of the closed 
compartment. The air within never became completely saturated, although 
the cover was kept on. This ensured a definite decrease in the percentage 
of relative moisture from one end to the other. This decrease could be 
augmented or diminished by placing at the drier end of the compartment 
a tray filled with sulphuric acid or water respectively. Mouldings 
extended the length of the two sides about 2 cm. from the top. These 
supported strips of cork to which seedlings had been pinned, which 
could thus be placed at any desired distance from the filter-paper. 
Evaporation from the cotyledons was found to have considerable influence 
on the prevailing moisture conditions, so a method was devised to 
obviate it. Zinc trays were constructed 9-5 cm. long, 2 cm. wide, and 
3 cm. deep. These had covers, and were arranged to hang from the 
mouldings on the opposite sides of the compartment. Five holes were 
made in the bottom of the trays, through which the roots of the seedlings 
were inserted or allowed to grow. The cotyledons were packed with 
moist sawdust, with which the trays were completely filled. The bottom 
of the tray was coated on the outside with paraffin that any hygroscopic 
action of the zinc might be avoided. In this way only the roots which were 
to be acted on hydrotropically came under the artificial conditions of the 
zinc compartment. The flank sides of the roots were turned towards the 
filter-paper. 
