THE REQUIREMENTS OF PLANTS 45 



observation that the toxic effects of a single salt fail to appear, or are 

 much delayed, when sugar is supplied.* 



Reed found that mitotic division proceeded normally in absence of 

 calcium, but the new transverse cell wall was either incomplete, or 

 entirelyi absent. 



Strontium salts not only have no nutritive value, but in Loew's 

 experiments on algae (182) they injuriously affected the chlorophyll 

 bodies, causing loss of starch-making power and finally death. 



Magnesium, like phosphorus, finally moves to the seed, and is thus 

 in contrast with calcium and potassium which remain behind in the leaf 

 or the straw. Willstatter has shown (310) chlorophyll to be a mag- 

 nesium compound, an observation that accounts for the unhealthy 

 condition of the chlorophyll bodies, and the final etiolation of mag- 

 nesium-starved plants. Further, magnesium seems to be necessary for 

 the formation of oil, the globules being absent from algae growing in 

 solutions free from magnesium salts ; oil seeds are richer in magnesium 

 than starch seeds. An excess of magnesium salts produces harmful 

 effects which, as we have seen, can be lessened by addition of calcium 



CaO 

 salts ; Loew indeed considers (180) that plants require a definite „ ^ 



ratio in their food, but neither Gossel ^ nor Lemmermann ' could 

 obtain evidence of any such necessity. 



Iron. — For some reason difficult to explain the formation of chloro- 

 phyll is absolutely dependent on the presence of a trace of some ferric 

 salt, although iron does not enter into the composition of chlorophyll. 

 So little is wanted that iron salts never need be used as manures, ex- 

 cepting for water or sand cultures. 



Manganese is considered by Bertrand to be a constituent of oxidases, 

 and, therefore, necessary to the plant ; minute traces only are required, 

 larger quantities being harmful. A number of field experiments * have 

 shown that manganese salts may act as manures. Bertrand classes 

 them as "engrais compl6mentaires " (35). 



Chlorine does not appear to be necessary to the plant, indeed Knop 

 grew even the halophytes without it Chlorides are always present in 

 rain water in ample amount to supply any trace that might be needed. 

 In small doses iodides and fluorides have been found, according to 

 Japanese experiments, to produce beneficial results (183 and 278). 



^Bied. Zentr., 1908, xxxvii., 571. ^Bied. Zentr., 1904, xxxiii., 226. 



^Landai. jfahrbuch, igii, xl., 175 and 255. 



* Numerous Japanese experiments are recorded in the Bull. Coll. Agric, Tokyo, igo6, 

 et seq. (210), and Italian experiments in the Studi e Ricerche di Chimica Agraria, Pisa, 

 igo6-8 ; pot experiments have also been made by J. A. Voelcker at the Woburn Experiment 

 Station. 



