TRANSPIRATION STUDIES. 9 
In general it may be said that the evaporating surfaces, both external and 
internal, are largest in the plant which has carried on its entire development 
in the green-house, next in the plant which has been in the green-house for 
one year after germination, then in the open for six weeks, and least in the 
adult tree. Of the different parts of the tree examined, the leaflets lose 
water by evaporation most easily, the rachises next, and the branches least 
easily. 
TRANSPIRATION STUDIES. 
METHODS. 
The best method yet devised for measuring transpiration is doubtless 
the determination, by weighing, of losses sustained by sealed potted plants. 
In the case of Parkinsonia this method was not practical, since the plants 
raised in the green-house differ from those growing in the open, both in leaf- 
structure and in the size of stems and leaves. The influence of the two 
environments on the size of seedlings is shown in plate I B and on the size 
and structure of the leaves in figure 1. It proved almost impossible to 
transfer small seedlings from their natural position in the ground to pots, 
there being only onesurvival out of some thirty which were transplanted. It 
thus seemed necessary to use a method for the measurement of water-loss 
in situ. Cannon’s method" is adequate for a rough estimate of comparative 
water loss from different plants under similar conditions, and seems to have 
several advantages over Stahl’st method, yet for the purposes of the present 
study, which require accurate quantitative measurements of water loss 
under natural conditions, a more accurate method had to be sought. The 
older methods, based on the absorption of moisture by means of sulphuric 
acid or calcium chloride, have many of the same objections as the polymeter 
method, ?. e., the plant is not under normal conditions of humidity, wind, 
and sunshine; moreover, the gain in weight of the absorbing material may 
not represent the actual amount of water given off by the plant, but is 
equal to this amount plus or minus the loss or gain of moisture in the air of 
the bell-jar during the experiment. If the bell-jar is a large one and the 
part of the plant under the jar is losing a small amount of water, this error 
may amount to as much as 100 per cent. 
A method was finally worked out which contains some of the features of 
both Cannon’s method and the absorption methods. By a reference to fig. 
2 it will be seen that the plant or part of the plant to be measured passes 
through a platform and is covered by a bell-jar, within which are an open 
dish of calcium chloride, a maximum thermometer, and a dew-point appa- 
ratus, the last being so arranged that the dew-point within the jar may be 
taken without any disturbance of the conditions within the jar. 
*Cannon, W. A., A new method of studying the transpiration of plants in place, Bull. 
Torr. Club, vol. xxx, p. 515. ee . 
{Stahl, E., Einige Versuche tiber Transpiration und Assimilation, Bot. Zeit., 111, p. 117, 
1894. 
