1910] WIEGAND—TRANSPIRATION 433 
the most important factor in transpiration is evaporation; indeed 
we may say that transpiration is really evaporation modified and’ 
' regulated by the plant. Therefore, it seemed that a detailed study 
of the relation of cutin and hairy coverings to evaporation would throw 
much light on the relation of these same coverings to transpiration. 
The first desideratum for such experimental work was a suitable 
evaporating surface. Several substances were tried, but the choice 
finally fell on good quality blotting paper. This possessed the desired 
property of wetting quickly and evenly, and of having a very homogene- 
ous evaporating surface. The pieces used were all cut, for convenience, 
exactly 7°" square. The plan followed was to expose saturated 
- Squares of this paper to the air, each having been previously weighed 
and then covered with the material to be experimented with. After 
a given time each was again weighed, and the difference in weight of 
course represented the quantity of water lost by evaporation. One set 
of readings in each case was taken in very quiet stagnant air, and 
another set in air actively in motion. The former condition was 
obtained by completely inclosing a portion of table top with botanical 
drying felts placed on edge and covered with the same. For the 
wind, a small electric fan was stationed at one end of a table on 
which the evaporating blotters were placed. After a large number 
of erratic readings were obtained, it was recognized that the following 
precautions must be taken to eliminate errors. The blotters should 
be rolled with a round pencil to remove surface water. They should 
be placed on slightly larger squares of glass and gently rolled again, 
While with the finger the edges are carefully pressed in contact with 
the glass. The glass plate and coverings should be weighed with 
the blotter both before and after. The substance in contact with 
the blotter should have been previously rendered waterproof in a 
dilute solution of paraffin in gasoline, otherwise water will be absorbed. 
The cotton coverings used were always separated from the blotter 
by a very thin linen cloth so treated. Various materials, in most 
cases composed of cotton, were used to simulate a hairy covering, 
but in place of cutin beeswax alone was finally employed. ‘This, 
while melted, was evenly and thinly spread over the damp blotter 
with a brush. In order to avoid reducing the supply of water in one 
case far below that in the other, and thus possibly introducing error, 
