240 BOTANICAL GAZETTE [MARCH 
Specific transpiration is really an expression of the rate of drying of plants. 
It is the percentage of the total water content of the plant lost during a definite 
time. The loss would naturally be very high in plants with low water content 
and very low in plants such as succulents, which have very high water content. 
transpiration is much more rapid than leaves exposed in the shade, will continue 
fresh while those in the shade wilt 
Water requirement is used in ae different ways. In the broadest applica- 
openings and measuring the water loss. The epidermi rmis has been fixed nee 
in alcohol or picric acid and the openings measured by microscopic examina 
The rate of the flow of air through the leaf, or the porometer method, which 
has several modifications, has enabled the experimenter to estimate the rela- 
tive difference in the openings of the stomata. To this method has also been 
applied automatic recording devices 
One of the simplest and most oxefal methods for determining whether or 
not the stomata are open is the infiltration method. Absolute alcohol, 
petroleum ether, or other fluids, when dropped on a leaf with open stomates 
penetrate into the mesophyll. This penetration is easily observed, and the 
technique is so simple that observations can be made rapidly. 
Several modifications of the gas diffusion or gas infiltration method have 
been employed by different workers. A large variety of potometers and 
atmometers have been devised. Nothing especially new has appeared among 
the porometers, but a number of new types of atmometers have been employed. 
These consist chiefly of porcelain filters or of filter paper saturated with water 
which is allowed to evaporate and the loss determined by weighing. 
In the measuring of transpiration loss no entirely new methods have been 
devised, although great improvements have been made on the older methods. 
Automatic weighing devices have been greatly improved, and there has been 
considerable improvement also in the methods of measuring transpiration by 
collecting the transpired water. There are now several types of automatic 
instruments, chiefly of the step-by-step type, which give satisfactory records 
of transpiration loss. 
Efforts to find algebraic equations or formulae by which transpiration can 
be estimated from the observed environmental conditions, has resulted in a 
clearer understanding of the factors affecting transpiration, but no entirely 
satisfactory equations have been deduc n fact, experimental data are 
ardly sufficient at the present time to doable one to evaluate properly such 
factors as wind, light, etc., and this, combined with the uncertainty of the 
