THE REQUIREMENTS OF PLANTS 



45 



even of the stronger solution disappeared on adding one gram-molecule 

 of CaCl 2 for every 100 gram-molecules of NaCl. Magnesium chloride 

 and sulphate, potassium chloride and calcium chloride were also toxic 

 when used singly, but in admixture they formed a nutrient medium in 



which the plant grew normally and developed fruit even when i 



32 

 N NaCl was also present. 



It is also found that calcium and magnesium ions diffuse out from 

 the plant cell more rapidly into solutions of single sodium or potassium 

 salts than into pure water and very much more rapidly than into solu- 

 tions of calcium salts. Niklewski (214) found the amounts of CaO and 

 of MgO diffusing out of cut pieces of beet to be : 



These and many other experiments all indicate that a complex 

 equilibrium normally exists in the cell between colloids and electro- 

 lytes which can only be maintained when the external medium has 

 an appropriate composition. 



Other facts are less easy to explain, such as Grafe and Portheim's 

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

 much delayed, when sugar is supplied. 1 



Reed found that mitotic division proceeded normally in absence of 

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

 entirely i 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 



1 Bied. Zcntr., 1908, xxxvii., 571, 



