4 GENERAL BIOCHEMISTRY 



standpoint. It should be remembered that water was one of the recog- 

 nized chemical elements at the time of van Helmont's work, and as a 

 result of his studies he concluded that water must be the "principle 

 of vegetation," citing the following experiment as proof of his theory 

 that water could be transformed into plant tissue: 



I took an earthen vessel in which I put 200 pounds o£ soil, dried it in an 

 oven, then I moistened it with rain water, and pressed hard into it a shoot 

 of a willow weighing 5 pounds. After exactly 5 years the tree that had grown 

 up weighed 169 pounds and about 3 ounces. But the vessel had never re- 

 ceived anything but rain water or distilled water, to moisten the soil when 

 this was necessary, and it remained full of soil which was tightly packed, and 

 lest any dust from the outside should get into the soil, it was covered with a 

 sheet of iron coated with tin, but perforated with many holes. I did not take 

 the weight of the leaves that fell in the autumn. In the end I dried the 

 soil once more and got the same 200 pounds that I started with, less about 

 2 ounces. Therefore, the 164 pounds of wood, bark, and roots arose from the 

 water alone. 



This experiment is thoroughly typical of much of the early investi- 

 gational work in agricultural chemistry, as well as of other sciences. In 

 this, as in other branches of science, it is very easy to fail to consider 

 a vital factor and, as a result, to draw from perfectly good experiments 

 a conclusion which appears to be correct but which is in reality en- 

 tirely wrong. In the work cited above, van Helmont failed to take 

 into consideration two most important factors, namely, the role played 

 by the constituents of the atmosphere, and the small amount of soil 

 which had disappeared. 



Some years after van Helmont reported his result, Glauber proposed 

 the hypothesis that saltpeter is really the "principle of vegetation." 

 This conclusion was reached by Glauber because he secured such large 

 increases in the yield of crops by applying this material as a ferti- 

 lizer. For many years his view was widely accepted by agricultural 

 writers. The only prominent opponent was Jethro Tull, who believed 

 that the fineness of the soil particles had a beneficial infltience on 

 plant growth. According to this latter view, it was "the very minute 

 particles of soil loosened by the action of moisture that constituted the 

 proper 'pabulum' of plants. The pressure catised by the swelling of 

 the growing roots forced these particles into the lacteal mouths of roots 

 where they entered the circulatory system. All plants live on these 

 particles, that is, on the same kind of food." Various other ideas re- 

 garding the "principle of vegetation" were proposed. The general 

 view held at the close of this period cannot be better summed up than 

 in Tull's own words: "It is agreed that all the following materials con- 

 tribute in some manner to the increase of plants, but it is disputed 



