30 
The fact that neither renewing the cut nor killing the lower ends of 
the pinne prevented the practical cessation of the transpiration would 
suggest that this cessation is due to some reaction on the part of the 
stomata, but this can hardly be true, for, as already noted, the cut pinnz 
lose weight during the experiments. The only explanation I can suggest 
for the persistent refusal of the cut pinne to absorb water at least as 
readily as they normally secure it from the rachis of the leaf is that a 
pressure of less than one atmosphere within the trachee is a condition 
for the ready movement of water through them, and that offering water 
to the pinne at a higher pressure than the usual one, instead of making 
them absorb more, is in itself the cause of their absorbing less. I am not 
ready to support this suggestion here, and know that it is contrary to the 
generally accepted opinion that water travels through wood with equal 
readiness regardless of whether the motive force is applied as a pressure 
(above one atmosphere) or as a suction (less than one atmosphere). It 
seems to me that this may be true in some cases and not in others, depend- 
ing, for one thing, on the amount of air in the conducting elements. 
Determining the water given off by leaves by absorbing and weighing 
it is a method which has long been in use. A decade ago, in a paper not 
accessible to me at the time I carried out this work, Stahl introduced 
the use of anhydrous chloride of cobalt, the rapidity of transpiration 
being estimated by its change in color from blue to pink as the salt absorbs 
water, because cobalt salts are blue when anhydrous, red when hydrated. 
As standards I used pieces of absorbent paper saturated with cobalt- 
chloride solution, one set not quite as blue as it would be if entirely anhy- 
drous, the other not as red as if entirely hydrated; these sets were 
separately sealed in glass vials. While changing from the color of one of 
these to that of the other, a piece having an area of 100 square centimeters 
absorbed 0.46 gram of water. By the use of this cobalt paper the trans- 
piration of pinne in their natural positions on the tree could be tested, the 
evil effects of cutting being entirely obviated. However, the method has 
compensating disadvantages. 
The cobalt paper must be directly applied to the transpiring surface, 
and it must be protected against the possibility of absorbing water from 
the atmosphere. This is accomplished by holding it in place with glass 
(microscope slides serve the purpose well), the latter in turn being held 
by clamps. This method is likely to make the transpiration abnormal 
by interfering with the wind, by cutting off some of the illumination, and 
by placing a portion of the leaf, at least for a part of the time, in an 
abnormally dry atmosphere. 
Transpiration is exceedingly sensitive to changes in the illumination, 
so much so that if a slide which is locally corroded be used over either 
surface, the paper under the etched spot will be noticeably slower to turn 
red; therefore clear and perfectly clean glass must be used. However, 
