330 TEXTBOOK OF BOTANY 



phosphorus are constituents of protein molecules; that nitrogen and 

 magnesium are constituents of chlorophyll; and that calcium forms salts 

 with pectic acid in cell walls. Carbon, hydrogen, and oxygen are omitted 

 from this discussion, since, with the exception of oxygen in a few com- 

 pounds like carotene, they are constituents of the molecules of all the 

 substances of which cells are composed. 



We have also seen that the ions of some of these elements are catalysts 

 in cell processes. Others affect osmosis through their influence upon the 

 permeability of protoplasm and to a minor extent upon the concentration 

 of water in the vacuole. It was noted that sodium, which is not essential 

 to most plants, increases the permeability of protoplasm and that cal- 

 cium annuls this effect. This is but one example of numerous balancing 

 effects of the different ions in cells. Almost any ion may influence the 

 permeabilitv, viscosity, and water content of protoplasm. Some of them 

 even cause death of the cells in one or the other of these ways. The 

 whole complex of organic acids, bases, and inorganic salts, especially of 

 phosphorus, is capable of so many interrelated reactions that fluctuations 

 in the acidity of the cell seldom become fatal. 



Chemical elements essential to plants. If a botanist had been asked in 

 1920 to list the external conditions necessary to the complete develop- 

 ment of a green plant, his reply would have been about as follows: the 

 absence of destructive and toxic agencies; the presence of a suitable 

 temperature, length of day, and acidity; adequate light, water, free 

 oxygen, carbon dioxide, and certain soluble salts containing the elements 

 of nitrogen, phosphorus, potassium, sulfur, magnesium, calcium, and 

 iron. Today he would add that there must also be a trace of salts of 

 manganese, boron, copper, and zinc. He would also say that other ele- 

 ments mav be found essential to plants, at least to some plants. Perhaps 

 the last four elements named above are not essential to all plants. Even 

 calcium is not essential to certain algae and fungi. 



During the 19th century, botanists and agriculturists got along fairly 

 well on the conclusion that only 10 elements are essential to plants. Later 

 it was observed that copper-containing spravs sometimes improved the 

 growth of plants, and that molds would not grow and reproduce re- 

 peatedly in the same glass vessels unless a trace of zinc was added. The 

 first cultures of molds had obtained sufficient zinc from the new glass. 



These observations did not constitute a final proof that these elements 

 are essential. Final proof was obtained only when all the containers used 

 were of pyrex glass. The water used in the water cultures prepared for 



