2 The American Naturalist. [January, 
set of organs to the trachee of insects, and therefore believed 
them to have the same office. His views were accepted by the 
Englishmen, Grew and Ray, who wrote about the same time, 
but found no supporters in Germany or France. The views 
of Malpighi, who in many respects was far in advance of the 
other botanists of his age, fell into obscurity, insomuch that 
even the existence of ducts was finally denied. 
The subject was not revived until 1715, when Nieuwentyt 
demonstrated the presence of air in plants by placing sections 
of stems in water under an air pump. The demonstration 
was better performed by Christian Wolff, who was a philo- 
-= sophical naturalist of much insight. He placed leaves, wood, 
and other parts of plants, in water, freed from air, under the 
air pump, and after seeing the bubbles rise from the tissues as 
the air was exhausted from the receiver, he allowed the air to 
re-enter the receiver, and found that the tissues were at once 
filled with water, and that some kinds were thereby made so 
heavy as to sink. 
In England, a few years afterward (1727), Stephen Hales, 
the real founder of experimental vegetable physiology, 
repeated and improved upon the air pump experiments, but 
used his knowledge to explain the life processes in a different 
manner from his predecessors. He combined the fact that 
gases are recovered from plants by dry distillation and fer- 
mentation to support a well arranged theory of the use of 
gases in forming the solid parts of plants. The use of gases 
by plants was, therefore, according to Hales, a part of the sub- 
ject of the nutrition of plants. 
But this small advance led to no further developments, and 
was again lost sight of for many years. After nearly half a 
century, in 1771, Priestley hit upon a discovery which, coming 
as it did only three years before the discovery of oxygen, and 
only shortly before the re-organization of chemical theories by 
Lavoisier and others, proved very fruitful. 
Priestley’s discovery was simple enough, amounting only to 
the fact that plants give off oxygen. He tells of his discovery 
in an interesting way, and I, therefore, quote a few paragraphs 
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