1896. ] Anatomical and Physiological Researches. 27 
method have failed, probably because it was so briefly de- 
scribed by him. Stahl found a much more satisfactory ma- 
terial in the cobalt salts, especially cobalt chloride.* Strips 
of Swedish filter paper were soaked in from I-—5 per cent. 
water solution of this salt and then dried. When dry the 
color varies from a light blue in the I per cent. solution toa 
deeper blue in the stronger solutions. The color fades from 
very light blue to pink, then white, as the paper becomes 
moist. The paper impregnated with the stronger solution is 
best for public demonstration but that soaked in the one per 
cent. solution is best for comparing small differences in evap- 
oration from various parts of the same leaf. A small piece 
of cobalt paper having been thoroughly dried, is placed on 
the part of the plant desired, then to hold it firmly against 
the surface and protect it from the air a thin piece of glass 
or mica is placed over it and held on with small clamps. 
After the observation is made the piece may be dried ready 
for use again. ® 
The author describes a number of experiments with this 
Paper as an indicator which I will briefly summarize. 
1, Stomatic and cuticular transpiration.—In cases where 
leaves have stomata only on the under side this side reddens 
the cobalt paper very quickly, often in a few seconds, while 
Paper on the upper side of the same leaf holds its color often 
for several hours. The contrast between the upper and under 
Surface is present even in the smallest leaves accessible to 
treatment, even those still in vernation. Among the plants 
used are Syringa vulgaris, Cornus stolonifera, Ribes alpinum, 
Populus fastigiata, and Quercus pedunculata. Where stom- 
ata occur on both sides the reddening of the paper is propor- 
tional to the number. In Trifolium repens, which has more 
stomata on the upper than on the under side, the paper faded 
Most rapidly on that side. In the case of slightly wilted 
_ *Cobaltous chloride was used by Marget but abandoned because the change 
in color passes quickly and can not be fixed. See Annales de la Soc. de Agric. 
ete. de Lyon 1. c. 
* In using the paper I have found it necessary to exercise caution in regard to 
several points. Any soft filter paper that takes up moisture readily may be 
used. Care should be taken to wet the paper evenly with the cobalt. Strips of 
Paper may be dipped in the solutions then laid on blotting paper to absorb the 
1 ‘i 
Must be taken that the cobalt paper is pressed with equal force at all points 
against the surface to be tested, otherwise the parts most closely pressed against 
= plant will fade first. This is particularly liable to occur where mica is used 
. 
© Protect the paper from the air. 
