128. BOTANICAL GAZETTE [aucust 
It is advisable to have a number of tubes already filled where work is 
being done in the field, in order that several parallel experiments 
may be run before returning to the laboratory for making weighings. 
The same tube may be used repeatedly so long as there is a part of it 
filled with dry P,O,. Care should be taken, however, to set up the 
tube in the same way each time, that is, to have the air enter and pass 
out of the tube in the same direction. The reason for this is that the 
phosphoric acid, after being formed from P,O, plus water, will itself 
take up water. If now the tube be turned so that perfectly dry air in 
leaving the tube passes over the phosphoric acid, which has previously 
taken up an excess of water, it will itself take up water from the acid 
and the tube will lose weight. 
It requires some experience to know how much leaf area to 
include in the cylinder for each experiment. After a few trials, 
however, one will learn to estimate sufficiently closely the capacity of 
the apparatus so as not to overcrowd it. Overfilling is indicated 
by the collection of moisture drops on the side of the cylinder, show- 
ing that the air inside has reached the saturation point. This 
may be remedied either by reducing the amount of leaf surface 
inclosed, or by increasing the rate of flow of the water through the 
aspirator. 
This method of measuring transpiration may be said to be only 2 
t d adaptation of the methods used by L RE’ 
and E. and J. VeRSCHAFFELT,? in that air is drawn over the plant 
the same manner, and the transpired moisture collected in U-tubes 
containing hygroscopic substances which are not contained in the 
vessel with the plant, but are connected in the same aspirating 
series, so that the air, after passing through the evaporation cylinder, 
next passes through the U-tubes. However, a different absorbent 
is used and the apparatus, moreover, is adapted for measuring 
transpiration of plants on their own roots. The essential point 
difference, nevertheless, lies in the condition of the air as supplied 1 
« GANEAU DE LAMARLIERE, L., Recherches physiologiques sur les feut les, deve 
loppées & l’ombre et au soleil. VI. Transpiration. Rev. Gén. Botanique 43579 
1892. ; 
the 
? VERSCHAFFELT, E. en J., De transpiratie der planten in koolzuurorije ange 
Botanisch Jaarboek (uitgegeven door het kruidkundig genootschap “Dodonaea 
Gent) 2:305. 1890. 
