e) 
PLANT FOOD re 
the composition of organic matter is indicated by the 
use of such terms as “ spirits of nitre’’ and “ spirits 
of hartshorn,”’ 
During the seventeenth century, when the so-called 
“nature philosophers ’’ were endeavouring to trace the 
origin of living things to a “‘ first principle,”’ the search 
for a principle of vegetation to account for the then 
known facts about soil fertility and plant growth led 
Van Helmont, about 1630, to state that he had found 
this principle in water. From his experiments with 
willow plants he concluded that water is transmuted in 
some mysterious manner directly into plant tissue. 
In 1650 Glauber suggested that saltpetre is the prin- 
ciple of vegetation. Some years later Kulbel stated 
that a magma unguinosum which is obtained from soil 
humus must be regarded as the principle sought for. 
Then in 1726 appeared the celebrated textbook of 
chemistry by Boerhaave, in which he says that plants 
absorb certain juices of the soil—the humus—and work 
them up into food: The raw material, the prime 
radical juice of vegetables, is a compound of fossil bodies 
and putrefied parts of animals and vegetables. ‘* This,” 
he says, ‘‘ we look upon as the chyle of the plant, being 
found chiefly in the first order of vessels, viz. in the roots 
and the body of the plant, which answers to the stomach 
and intestines of an animal.” 
Thus arose the old humus theory that plants derive 
all their nourishment from the humus of the soil, and 
this when exhausted was replenished by manuring with 
dung and other organic matter. 
This theory held the field for the next 150 years, 
and it was not until the middle of last century that it 
