470 BOTANICAL GAZETTE | DECEMBER 
The temperature of the respiration chamber can readily be 
changed by cooling or heating the water surrounding it. 
Hygrometer. — When 
the writer was in the lab- 
oratory of Francis Darwin 
at Cambridge in 1888, he 
was shown a roughly made 
hygrometer,> which has 
since been used in the 
Purdue laboratory and 
modified into the form 
Fic. 3. Awn hygrometer. shown in IE. J iy coe: 
sists of a small thin glass 
vessel with a metal bar across the mouth bearing a part of a Stipa 
(S. spartea Trin.) awn. To the free end of the awn is attached an 
index of fine brass wire. When the air 
becomes moist, the awn untwists and 
the index is carried around. To use it 
two are selected with awns that untwist 
at the same rate, and fastened to oppo- 
site sides of a leaf by means of a mix- 
ture composed of wax, oil, and tallow. 
The leaf may be attached to the plant, 
or separated from it and kept from 
wilting by placing the end in water 
(fig.4). The position of the index on 
each side of the leaf is marked at the 
Start, and again when one of them has 
made acomplete revolution. The ratio 
between the number of degrees of the 
circle traversed by the two indices dur- 
ing the interval is approximately the 
ratio of transpiration from the two sides Fic. 4. Hygrometer in ¥5 
of the leaf. 
: , ical 
*Since described and its use fully explained; see Darwin and Acton’s Practica 
Physiology of Plants 
