BOTANY 167 



tome (or thin-section cutter) has made possible investiga- 

 tions that could have been prosecuted only with extreme dif- 

 ficulty, if at all, without it. 



The solution of the riddle of plant nutrition is another 

 milestone of botanical progress. Aristotle had taught that 

 the chemical compounds found in plants enter from without 

 ready formed. Van Helmont erroneously concluded from 

 his classic experiments, above referred to, that plants derive 

 all their substance from water. He grew a plant in soil, 

 furnished it with nothing but water, and found in time that 

 it gained in dry weight more pounds than the soil had lost 

 ounces. Therefore, he argued, its substance was derived from 

 water. It was the physicist Mariotte who first saw the signi- 

 ficance of the fact that, when plant tissue is burned, the pro- 

 ducts of combustion are approximately the same, no matter 

 what the species. Therefore, he reasoned, it is these mate- 

 rials which enter the plant from without, and are elaborated 

 into the organic compounds within the plant. The validity of 

 this hypothesis he established by referring to the fact that, if 

 a scion of a cultured pear is grafted on a wild pear, the same 

 sap that produces undesirable fruit on the natural branches 

 of the tree produces delicious fruit on the graft. 



It would require a separate lecture to go into the fasci- 

 nating accounts of how Priestley's discovery of oxygen and 

 its evolution by plants led to the establishment, by Jan Ingen- 

 Houss, of the fundamental fact that plants derive most of 

 their solid substance from the carbon dioxide of the air, and 

 how this discovery in turn led to the establishment of the 

 fact that the respiration of plants is the same as that of ani- 

 mals. The notion that this is not the case is one of the most 

 persistent of all common misconceptions about plants. 



But the clearing up of this mystery left the botanist face 

 to face with another equally great, namely, the source of the 

 plant's nitrogen. Though the air is four-fifths nitrogen and 

 though the plant must have this element, it became certain 



