286 THE EFFECT OF FOOD 



late flowers (the r being respectively .51 and .75 in the 

 two cases). As Professor Weldon remarks, these re- 

 sults " provide a most valuable lesson as to the possible 

 danger of asserting that such differences are significant 

 of local races." 



By observations upon the growth of seedlings placed 

 in various solutions, it has long been known that normal 

 growth is possible only if various inorganic salts are 

 present. There must be nitrogen in the form of 

 nitrates or ammonium salts, sulphur in the form of sul- 

 phates, phosphorus as phosphates, chlorine as chlorides, 

 and the metals sodium, potassium, magnesium, calcium, 

 and iron in solution as salts. The absence of any one 

 of these substances speedily inhibits normal growth; as 

 soon, in fact, as the seedling has exhausted the small 

 quantity of it stored up within itself. For instance, 

 plants grown in solutions containing no iron soon show 

 a sickly appearance; "the leaves are no longer green, 

 but white, and microscopic examination of them shows 

 that abnormal chlorophyll bodies, or none at all, are 

 present in their cells. If we add to the food solution 

 a few drops of dilute ferric chloride solution, the pre- 

 viously white leaves become green in two or three days, 

 and the growth of the plants now proceeds normally." * 

 It follows, therefore, that if the absence of these vari- 

 ous substances stops growth altogether, a deficiency in 

 them must produce diminished or abnormal growth, 

 and so lead to the production of variations. 



With members of the Animal Kingdom, variations in 

 the inorganic salts of the food may also be a source of 



* Quoted from Detmer's " Practical Plant Physiology," p. 84. 



