ORGANIC SUBSTANCES AS PLANT FOOD 233 



ing animal refuse into inorganic form, they make it adaptable to 

 the nutrition of higher green plants, and through them for animals. 

 Organisms of this kind are found in tremendous numbers in the soil, 

 in drainage water, in rivers and lakes, everywhere displaying their 

 life activity. The other group is composed of microorganisms 

 capable of feeding only upon some sharply limited group of organic 

 substances. Some of them — like the vinegar bacteria, thrive only 

 in alcoholic solutions; others feed upon cellulose or upon dextrin 

 or pentoses; still others are capable of assimilating even such inert 

 substances as paraffin, which is entirely unfit for other organisms. 

 Because of their adaptability to certain substances, such sapro- 

 phytes remind one of the parasites, many of which are likewise 

 adapted only to certain hosts which produce specific substances 

 necessary for their development. 



For the nutrition of the omnivorous saprophytes different 

 organic substances are used with unequal readiness, and hence 

 their development proceeds at varying rates. It is impossible to 

 determine the exact needs of nutrition of these plants since even 

 of the omnivorous saprophytes not all have like requirements. Yet 

 a certain order and preference in the consumption of different 

 substances seems to exist. The sugars, especially glucose, are the 

 best sources of carbon, and substances with straight carbon chains 

 are usually preferred to compounds with side chains. The lower 

 members of a homologous series are less utilized by these organ- 

 isms than the higher ones, and the most readily absorbed com- 

 pounds seem to be those with five or six carbon atoms. The intro- 

 duction of hydroxyl groups into the carbon chain likewise raises 

 the nutritive value of a compound. All these interrelationships 

 have their explanation in the fact that before they are used these 

 substances are transformed by the saprophytes into sugars, which 

 are the main nutritive substances for all plants. In general then, 

 the closer the structure of the organic substances is to that of sugars 

 or the easier their transformation into sugars, the more readily are 

 they assimilated. 



Highly interesting is the relationship between the nutritive 

 value and the optical structure of some of these compounds. Of 

 the sugars only a certain group is readily transformed; namely, 

 d-glucose, d-fructose, and d-mannose. All these sugars show a 

 related configuration of atomic groups and with intramolecular 

 migration of the hydroxyl they exhibit the same enolic form. 



