298 



SCIENCE 



[Vol. LVI, No. 1446 



would undoubtedly follow an abstraction of 

 basic elements from the plant compounds by 

 the nutrient solution, and if the change were 

 great enough, the plant would be killed. 



In this connection it should be noted that 

 the soil solution of acid soils often contains 

 more calcium than the soil solution of less acid 

 or neutral soils, and yet plants like alfalfa may 

 suffer for lack of calcium in the former case 

 and not in the latter, due to the fact that the 

 acidity makes the calcium less available for 

 certain purposes even though it is in solution. 

 Availability is thus not only a question of 

 solubility. It also depends on the form in 

 ■\vhich an element exists in solution. 



SUMMARY 



1. Differences in the feeding power of com- 

 mon agi'iculturall plants for the essential ele- 

 ments of comparatively insoluble minerals are 

 not due primarily to differences in amounts or 

 kinds of acids excreted. The differences are 

 due to several factors, some of which are con- 

 cerned with external equilibrium conditions 

 around the feeding roots, and others with 

 internal equilibrium conditions inside the plant 

 where the elements are actually used. 



2. In case two soluble products are formed 

 in the feeding region of the roots due to the 

 action of carbonic acid on a mineral as is the 

 case with rock phosphate, the feeding power 

 follows the law of mass action and chemical 

 equilibrium, being dependent on the removal 

 of both of the soluble products either by the 

 plant or partly by the plant and partly in 

 other ways; thus plants with a high content of 

 calcium feed strongly on rock phosphate be- 

 cause they remove both the soluble phosphate 

 and soluble calcium bicarbonate in proper 

 proportion. 



3. If only one soluble product is formed as 

 is the case with feldspar, the feeding power of 

 the plant for the potassium depends on its 

 abUity to utilize potassium from a dilute solu- 

 tion which in turn depends largely on the 

 acidity of the plant sap; the less acid the sap 

 the greater the ability of the plant to utilize 

 potassium from this source due to the fact 

 that potassium is more easily and completely 

 precipitated in the form of plant compounds 

 in the less acid sap. 



4. The feeding power of a plant for calcium 

 which is used for the regulation of the reac- 

 tion of the plant sap and colloidal system, and 

 precipitation of acids, or for other elements 

 used for these purposes, is also dependent upon 

 the reaction of the plant sap but the relation 

 is opposite to that of potassium; the more acid 

 the plant sap the more easily can the plant 

 compete with another acid system — the soil 

 solution of an acid soil, for needed basic ma- 

 terial. 



5. In the case of base forming elements used 

 for other purposes than regulation of the re- 

 action and precipitation of acids, the relation 

 of the feeding power for these to the plant sap 

 is perhaps the same as for potassium. 



6. There are undoubtedly many other fac- 

 tors which affect the feeding power of a plant 

 but it seems that the ones given often exercise 

 a controlling influence. 



E. Teuog 

 Department of Soils, 



Agricultural Experiment Station, 



XTniversity of Wisconsin 



THE TEACHING OF EVOLUTION 



Every student, teacher and research worker 

 in various fields of science must find cause for 

 sincere regret in any attitude or movement that 

 would limit the search for knowledge, or the 

 presentation of scientific fact in the class room. 

 There certainly is such a menace in the sug- 

 gested limitation or elimination of the teach- 

 ing of "evolution." It seems rather strange 

 that such a conflict should be staged in a cen- 

 tury made notable by outstanding advance in 

 both pure and applied science. At no pre- 

 vious time have all men profited as much by 

 the efforts of scientific workers. Then why 

 such a hubbub about the teaching of what 

 many think a fundamental concept of bio- 

 logical science? 



The trouble seemingly was started by a 

 group of conscientious folk who saw a sharp 

 variance between their beliefs, religious or 

 otherwise, and the theories presented and 

 vigorously promulgated by many teachers. 

 Some prominent men, as Mr. W. J. Bryan, 

 made the matter one for public discussion, and 

 the controlling trustees of certain schools re- 

 quested or demanded that the doctrine of evo- 



