434 PROTOPLASM 



Chlorine. — Chlorine is an element, usually present and often 

 abundant, that does not appear to be necessary for all kinds of 

 life, though as hydrochloric acid it is an important constituent 

 of the gastric juice. Its excessive presence may be objectionable. 

 Potatoes grown in soil rich in chlorine contain less starch than 

 those grown in soil deficient in this element. Chlorine fertilizers 

 are, therefore, to be avoided. 



Iodine, Bromine, Fluorine. — The presence of iodine, bromine, 

 and fluorine in algae (the brown algae are the commercial source 

 of iodine) leads one to assume that these elements are necessary 

 to certain plants ; of course, the case may as well be one of toler- 

 ance as one of need; the latter, however, seems the more probable. 

 In animals, the need of iodine is well recognized. It has of late 

 played a prominent part in the prevention of goiter. 



Silicon. — Silicon is present in plants, e.g., in grasses and the 

 scouring rush, Equisetum, but rarely has a prominent physiologi- 

 cal role been ascribed to it. However, there is apparently a need 

 for silica in plants as a protection against disease. 



Boron. — Boron is a rather unexpected element to be raised 

 to the importance in plant life that it has of late attained through 

 the work of Winifred Brenchley in England and of McHargue in 

 America. Both prove boron to be necessary in the nutrition of 

 plants and incapable of being replaced by any other element. 



The work of T. Schmucker on boron is interesting. He 

 attempted to germinate the pollen of the water lily in artificial 

 media as nearly as possible like the liquid occurring on the top 

 of the pistil where the pollen grains grow. Many attempts 

 failed, but the ash of the liquid, taken from the pistil and added 

 to 1 per cent glucose, was as effective as the original liquid. The 

 active element proved to be boron. Boric acid, H3BO3, occurs 

 in the pistil fluid. In artificial media, 0.01 mg. of boric acid in 

 1 cc. gives good results; even 0.0005 mg. acts favorably. 

 Schmucker regards boron as an inorganic hormone, or catalyst. 



Lithium. — Lithium is widely distributed in plants but in 

 greatly differing quantities. Haeddon finds that alfalfa stores 

 little lithium (and much strontium), while tobacco stores much 

 lithium (and little strontium). Nakamura finds lithium salts 

 to exert a stimulating action on plants (barley and peas). 



Barium and Strontium. — Barium and strontium are interesting 

 because of the controversy that has centered around the question, 



