104 PLANT PHYSIOLOGY 



since this latter element is necessary for fat formation, perhaps 

 the magnesium affects the formation of fats only indirectly. 



Phosphorus. — This element is generally supplied to the plant as 

 phosphate and enters into the formation of many proteins, es- 

 pecially those of the nucleus. When young, the presence of phos- 

 phorus stimulates healthy root growth, and when the plant is 

 mature the presence of phosphorus hastens the ripening process. 

 It is stored in seeds, as evidenced by the common expression that 

 " phosphorus makes seeds." Photosynthesis occurs without it, 

 but the breaking down of the insoluble carbohydrate into a soluble 

 form for translocation generally occurs only in its presence. Ac- 

 cording to Turner (1929), the favorable effect upon root growth 

 is due primarily to this favorable effect upon translocation. Loew 

 thought it also made fats assimilable. He demonstrated that 

 there was no cell division in the absence of phosphorus and at- 

 tributed this to the absence of phospholipoids, for the formation 

 of which the phosphorus is necessary. Phosphorus aids respira- 

 tion because it is a coenzyme of zymase (Chap. XXV), and Ecker- 

 son found that when phosphorus is deficient the enzyme, reductase, 

 which reduces nitrates, is diminished so that the plant is unable to 

 reduce the nitrates present and manufacture proteins (Chap. XIV). 

 Although nitrates are present they cannot be used. The result 

 is a disintegration of the protoplasm, which is checked when phos- 

 phates are again supplied. 



Potassium. — This element is needed for the formation of car- 

 bohydrates and their translocation. In its absence starch is pro- 

 duced in noticeably reduced quantities and no reserves are stored, 

 possibly owing to the fact that potassium may aid in the forma- 

 tion of invertase, which decomposes cane sugar into glucose and 

 fructose (Chap. XII). Plants lacking this element are incapable 

 of synthesizing proteins properly. If potassium is absent, cell 

 division is markedly affected; the cells elongate but fail to divide 

 and the plants are weak and very susceptible to cold and disease. 

 The mechanical tissue is not well developed in the stems. In the 

 absence of potassium, plants seem unable to control respiration 

 as do normal plants, and fruit trees consequently often present 

 a " scorched" appearance where portions of the leaves have dried 

 up. The "bronzing" of Citrus leaves under similar conditions is 

 probably related to this. Young parts are always richer in potas- 

 sium than older ones, and James has concluded that the lack of 



