46 GROWTH OF THE PLANT 



(d) It is also supposed to be a necessary constituent of the 

 proteins of the cell nuclei. 



46. Iron. — This element is necessary for the manufacture 

 of chlorophyl, although not a constituent part of the chloro- 

 phyl compound. Plants which fail to absorb iron, or which 

 do not absorb enough to keep pace with growth, produce 

 white leaves, a diseased condition known as chlorosis. 



47. Magnesium. — (a) The production of chlorophyl is 

 dependent on magnesium, which is a necessary constituent. 

 (b) Protein formation is helped by this element, and par- 

 ticularly in the assimilation of phosphorus, (c) The forma- 

 tion of seeds is also dependent on the activity of magnesium, 

 possibly as a carrier of phosphorus in the plant. 



48. General Distribution in Seed Crops. — The staple crops 

 in agriculture are the grains, typical seed plants, which 

 are of value both for their seeds and for their stems and 

 leaves. The distribution of the essential elements, and 

 particularly of the fertilizing elements, in such plants is of 

 importance, not only in connection with their physiological 

 functions, but also with respect to their use on the soil and 

 their economic value. Carbon, hydrogen, and oxygen need 

 not be considered. 



Taking the mature plant as harvested, it is to be noted 

 that phosphorus and nitrogen move to the seeds during 

 ripening to such an extent that there are two to three times 

 as much phosphorus and nitrogen in the seeds as in the 

 leaves and stems. Potassium and calcium, on the other 

 hand, being necessary parts of the assimilating mechanism, 

 remain in the stems and leaves where two to three times as 

 much potassium, and nearly eight times as much calcium, 

 are found as in the seeds. Sulphur is more evenly dis- 

 tributed, being a trifle more in the stems and leaves than 

 in the seeds, except in the corn plant. Iron, being necessary 

 for the formation of chlorophyl, is found in the leaves and 

 stems to a greater extent than in the seeds. Magnesium 

 is more evenly distributed, with a trifle more in the seeds 

 than in the stems and leaves of most grains except corn. 



For the distribution of fertilizing constituents in fruits and 

 vegetables the reader is referred to Table III, Chapter IV. 



