i62 PLANT PHYSIOLOGY 



water-culture have already been described, and the student may 



profitably apply them to the present subject after the following : 



Suggested Experiment. Using the water-culture methods earlier 

 described (page 116), determine what effect is produced upon the growth 

 of Barley or Oats by a complete nutrient solution in comparison with a series 

 of solutions, each of which lacks respectively one of the indispensable ele- 

 ments, while one lacks them all. Or the student may employ more elaborate 

 methods permitting a more extended observation, and he may profitably 

 use for comparison the fine water-culture diagram of Errera and Laurent 

 (page 23, note). After the plants have grown to near their limit, they 

 should be examined for any anatomical differences they exhibit. 



The student should now inform himself as to our present 

 knowledge of the selective power of the roots for particular min- 

 erals, and of the uses or significance of each of these minerals 

 to the plant. Upon the latter subject he will find a very valuable 

 paper by Loew on the " Physiological R61e of Mineral Nutrients 

 in Plants," in a Bulletin of the United States Department of Agri- 

 culture, 1903. This will lead naturally to a consideration of 

 the chemistry of soils, which is, however, a subject impracti- 

 cable of experimental study in this course. But the student 

 should inform himself upon its important phases through the 

 literature, which is comprised in the same works as those men- 

 tioned above under Soil Physics; and he should especially note 

 the origin, in the soil, of the minerals used by plants, the mode 

 of renewal of supply, and the comparative amounts dissolved by 

 water from different soils. Another important phase of the 

 subject concerns the possible secretion of acids from the roots 

 for dissolving mineral matters otherwise insoluble; this involves 

 the study of corrosion phenomena, upon the experimental study 

 of which there are directions in Detmer, 241. Allied to this is 

 the matter of the oxidizing power of roots through enzymes, and 

 its significance, which is fully discussed in the Journal of Bio- 

 logical Chemistry, 3, 1907, by SCHREINER and REED.by whom, 

 also, valuable and striking experimental methods of demonstrat- 

 ing this power, through use of phenolphthalin and other indi- 

 cators in water-culture solutions, have been worked out and will 

 later be published. The possible excretion of other organic sub- 

 stances by roots will be noted later under Excretion. 



