Chapter i 



The Origin of Organic Matter and the Cosmical 

 Function of Green Plants 



1. Historical Introduction 



It is the history of almost every science that in the course of its de- 

 velopment the carefully executed and accurately recorded experiments have 

 stood out as bright beacons to guide workers in later generations. So 

 does a perusal of the early attempts to understand the phenomena asso- 

 ciated with the nutrition of plants clearly reveal this fact. 



Prior to 1779, the date of the publication of "Experiments upon Vege- 

 tables," by Ingen-Housz, the subject was in a highly confused condition. 

 Practically all of the work before this time was influenced by the Aris- 

 totelian dictum that plants derive their nutrition from the soil. Against 

 this mass of incongruous speculation there stand a few clear and beautiful 

 observations. 



The great iatro chemist van Helmont (1577-1644), endowed with an 

 experimental attitude of mind and extraordinary clearness of perception, 

 denied the Aristotelian doctrine of the composition of organic matter and 

 of plant nutrition. One of his experiments has become a classic. In a 

 pot he placed 200 pounds of thoroughly desiccated soil and planted therein 

 a willow twig weighing 5 pounds. This was protected from dust and 

 watered daily with rain-water. The twig took root, and after five years 

 had grown into a small tree. It was then carefully removed from the soil 

 and the latter was again thoroughly dried. The willow showed an increase 

 of 164 pounds while the soil had lost only 2 ounces. 



On the basis of this experiment van Helmont concluded that the great 

 increase in the weight of the plant had not been taken from the soil. His 

 further deductions, however, were not so fortunate. He concluded that 

 the soil contributed nothing to the growth of the plant, that water was the 

 only true element and that the substance of the plant had been formed 

 entirely from the water with which it had been irrigated. Later Mariotte 

 (1621-1684) showed that the mineral constituents of a plant which are 

 found in the ash thereof are taken up from the soil by the plant. 



That the leaves are the organs which produce the substances necessary 

 for the development of the plant was suggested by the Italian anatomist 

 Malpighi in the seventeenth century. By removing the cotyledons (which 

 he regarded as true leaves) he demonstrated the importance of the leaves 



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