CHAPTER IX. 
PLANT PHYSIOLOGY. 
84. Nutrition of Plants.—The problem of how and 
whence plants obtain the necessary materials for life 
and growth is one which was fora long timea puzzle 
to investigators. Until 1630 it was commonly believed 
that they received all their food from the soil, in a state 
ready for use, 1.e. that no further elaboration took place 
in the plant itself. But in 1630 Van Helmont made the 
first experiment in plant physiology, with the object of 
disproving this hypothesis. He thoroughly dried and 
weighed some soil, and put itin a pot, and in this pot he 
planted a Willow branch which he had also weighed. 
He watered it daily with rain water for five years, and 
at the end of that time he weighed both again, having 
previously dried the soil. He found that the plant had 
gained about 160 Ib. in weight, whereas the soil had 
only lost two oz. He concluded that all this gain in 
weight was due to water alone, ie. that the substance 
of the plant is almost entirely formed of water. Other 
investigators showed that this conclusion was not cor- 
rect, but many of the facts of plant nutrition were not 
discovered till much later, and even now there are 
stages in the process of assimilation which are still not 
certainly known, 
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