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PLANT PHYSIOLOGY 



seed is then passed through this cotton by means of special 

 devices. The seed used for this purpose is thoroughly washed 

 with disinfectants previous to germination, and then germination 

 is allowed to proceed in a sterile medium. As a result, the roots 

 of the plant develop beneath the cotton with a complete absence 

 of bacteria, while its leaves spread freely in the air (Fig. 65). 



Fig. 65. — Sterile water culture of a corn plant {redrawn after Knudson). 



Experiments with sterile cultures have definitely shown that 

 many of the organic compounds of nitrogen, such as amino acids, 

 lecithin, etc., may be directly assimilated by plants. Their utili- 

 zation, however, proceeds at a very slow rate. Plants developing 

 in such mediums are, therefore, considerably checked in growth 

 as compared with plants receiving nitrates or ammonium salts. 

 Hence, it may be concluded that mineral compounds of nitrogen 

 are better sources of this element for plants than the organic 

 forms, and that nitrogen in humus is utilized only after it has 



