34 SCIENCE AND SOIL 



sugars are converted into starch and stored away in roots, tubers, 

 or seeds, to supply the future needs of the same plant or of new 

 plants. At the proper time the plant reconverts the insoluble 

 starch into soluble sugar 1 and carries it through the circulation 

 to the point of consumption as food by the plant, either for 

 energy, repair, or growth. Food materials are thus consumed or 

 oxidized within the plant, and carbon dioxid is constantly given 

 off from all its living parts, including the roots. During the day 

 the fixation of carbon is commonly so great as to completely mask 

 the liberation of carbon dioxid in the green parts of the plant. 



Vegetable fats. Before leaving the subject of the fixation of 

 carbon, oxygen, and hydrogen, some further mention should be 

 made of the group of compounds called fats, whose importance is 

 exceeded only by that of carbohydrates and protein. 



The vegetable fats and oils show distinct relationship to a con- 

 densation process similar to the formation of sugars and other 

 carbohydrates from formic aldehyde, the photosynthetic product of 

 the reaction between carbon dioxid and water in the leaves of 

 plants. The following series of compounds will show this relation- 

 ship: 



HYDROCARBONS FATTY ACIDS 



HCH 3 Methane ........ HCOOH Formic acid. 



CH 3 CH 3 Ethane ........ CH 3 COOH Acetic acid. 



C 2 H S CH 3 Propane ....... C 2 H 5 COOH Propionic acid. 



C 3 H 7 CH 3 Butane ........ C 3 H 7 COOH Butyric acid. 



C 4 H 9 CH 3 Pentane ........ C 4 H 9 COOH Valeric acid. 



C 5 H U CH 3 Hexane ....... C 5 H U COOH Hexoic acid. 



C 6 H 13 CH 3 Heptane ....... C 6 H 13 COOH Heptoic acid. 



C 7 H 15 CH 3 Octane ........ C 7 H 15 COOH Octoic acid. 



C n H 2 3CH 3 Dodecane ....... CnH^COOH Laurie acid. 



C 15 H 31 CH 3 Hecdecane ...... C 15 H 31 COOH Palmitic acid. 



C 17 H 35 COOH Stearic acid. 



Oleic acid. 

 Unsaturated ........ C 17 H 31 COOH Linolic acid. 



Linolenic acid. 



The hydrocarbons, which constitute the simplest series of carbon 

 compounds, are shown for direct comparison with the fatty acid 



1 Glucose sugars and sirups are manufactured in large quantities by use of strong 

 acids for converting the starch into glucose. 



