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TEXTBOOK OF PLANT PHYSIOLOGY 



Galactose, showing a somewhat different configuration, is utilized 

 with greater difficulty. 



Therefore, substances which are analogous to this group, such 

 as dextrotartaric acid, are assimilated much better than their 

 laevo-rotating optical isomers, which are often left untouched by 

 microorganisms. Of the great variety of organic compounds of 

 different degree of availability plants will always completely 

 utilize the more available compounds and only then begin to use 

 the less assimilable ones. On the different nutritive value of the 

 I- and d-isomers is founded the biological method of differentiation 

 of racemic compounds, first used by Pasteur (1850) in his classical 

 work on tartaric acid. 



Substances of the aromatic series, in general, are utilized with 

 difficulty by saprophytes, but some hydroaromatic ring compounds, 

 as quinic acid, inositol, quercitol, are good nutritive material for 

 molds, since they readily break down and give rise to sugars. 



The nitrogen requirements of saprophytes are varied. The 

 interesting and important group of bacteria which are able to 

 assimilate nitrogen of the atmosphere were discussed in Chap. II 

 (Arts. 19 and 20). The majority of saprophytes do not have 

 this capacity, but they can build their protein substances from 

 nitrates, nitrites, and ammonia, of which the latter is a better 

 source of nitrogen for them than either the nitrates or nitrites, 

 since for its conversion into proteins and amino acids considerably 

 less chemical transformation is required. In this connection, it is 

 interesting to note that the nutritive value of various nitrogenous 

 substances to a large degree depends upon the nature of the avail- 

 able carbon compounds. In the presence of sugars, nitrates are 

 utilized more readily than in the presence of mannite or glycerine. 



