Pioneer Investigators of Photosynthesis. 197 
and Observations,” it is impossible to avoid the conclusion that 
Sachs’s estimate of Priestley’s contributions to Vegetable Physiology 
is at least misleading if not actually unjust. 
Between the date of Hales’s “ Vegetable Staticks ” and that of 
Priestley’s “ Experiments on Air,” nothing was published worthy 
of being regarded as a serious contribution to our knowledge of 
plant nutrition—unless we include Bonnet’s treatise “ Recherches 
sur l’usage des feuilles dans les plantes,” published in 1757. The 
central idea of this pretentious work is one that Bonnet adopted 
from the mathematician Calandrini of Geneva, viz., that the under 
side of the leaf absorbed the dew that rose from the soil 1 Bonnet’s 
rank among the founders of plant physiology cannot be more 
tersely and accurately defined than in Hansen’s scathing words :— 
“ Erst wenn eine Geschichte der falschen Propheten in der Wissen- 
schaft geschrieben werden wird, dann wird Bonnet zu Ehren 
gelangen, denn er wird in erster Reihe mit aufmarschiren.” 
Ingenhousz’s contributions to the subject of gaseous exchange 
are expounded in his “ Experiments on Vegetables discovering their 
great power of purifying common air in the sunshine and of injuring 
it in the shade and at night,” published in 1779. His discoveries 
are abstracted in the preface to his volume, but as the statement 
is diffuse and couched in old-fashioned terminology it may be 
convenient to condense and translate it into rather more modern 
phraseology. “ I observed that plants were able to purify bad air 
in a few hours if subjected to sunlight; that they could transform 
air absorbed from the exterior into oxygen ; that this oxygen is 
exhaled into the atmosphere thus rendering it more fit for animal 
life ; that the exhalation of oxygen by plants begins after sunrise, is 
the more active the brighter the day and the more the plants are 
exposed to solar radiation, and the less active the more they are 
shaded by buildings or by other plants, when, so far from purifying 
the air, they contaminate it as animals do ; that they cease to 
exhale oxygen as darkness comes on ; that only leaves and petioles 
carry out this function ; that the exhalation of oxygen in sunlight 
is independent of the poisonous or other quality of the plant; that 
the oxygen is given off chiefly by the under surfaces of leaves and 
that the mature leaves, caeteris paribus, give off more oxygen than 
young leaves; that some plants, especially aquatics, give off more 
oxygen than others; that, on the other hand, all plants in darkness 
render the air impure, especially such parts as flowers, fruits and 
roots, no matter how economically useful they may be; that the 
