186 PLANT SOCIOLOGY 



The gypsum soils of the Harz mountains are remarkable for the 

 poverty of their vegetation. The Festuca glauca association with 

 Gypsophila fastigiata, Alyssum montanum, Teucrium montanum, and a 

 few other species is spread over the dry slopes on soil containing little 

 fine material. More sharply distinguished from the lime vegetation is 

 the gypsum vegetation of the Spanish mesa. This is emphasized by the 

 fact that Del Villar (1925, p. 18) describes a special successional series 

 for the gypsum area. Indeed, the G. struthium-Lepidium suhulatum 

 association which he describes appears to be unique. Del Villar names 

 as characteristic gypsum plants G. struthium, L. suhulatum, Herniaria 

 fruticosa, Frankenia reuteri, Vella pseudocytisus, Agropyrum curvifolium, 

 Sedum gypsicolum, Narduretia gypsacea, Centaurium gypsicolum, and 

 others. 



On the high Algerian plateau the gypsophilous plants form an upper 

 zone of scanty vegetation bordering some shotts at Le Kreider. The 

 dominant plant is Frankenia thymifolia with which are also found L. 

 suhulatum, Schismus marginatus, H. mauritanica, and Er odium 

 glaucophyllum. 



2. Magnesium. — Magnesium occurs in numerous compounds but 

 usually in small quantities in rocks and soils. There are two important 

 magnesian rocks. Dolomite, MgCOa + CaCOs, with 54 per cent Ca 

 and 46 per cent Mg, forms great white shining mountain chains (the 

 Dolomites of southern Tirol). Serpentine, 2Si02Mg0 2H20, has a 

 dark greenish luster, weathers into sheets and jagged pieces, and forms 

 an unusually sterile residual soil. The serpentine ridges of the Alps, in 

 their dark, deathly hardness, are among the most depressingly lonely 

 phenomena of nature, and the popular reference tote Alp (dead Alp) is 

 quite appropriate. 



Saline soils of dry regions and large salt-water basins have their 

 magnesium mostly in the form of chloride and sulphate. 



Physiological Effects of Magnesium. — Willstatter and his students 

 have discussed the importance of magnesium in the molecular structure 

 of chlorophyll. Canals (1920, p. 33) has given a good digest of our 

 knowledge of the physiological effects of magnesium. By a number of 

 experiments he confirmed Andre's view that the Mg ion, in small 

 doses, favors the development of plants. Concentrations of less than 

 0.0001 mg. of Mg were beneficial to the plants used, while at higher 

 concentrations a toxic effect appeared. The addition of calcium, as 

 mentioned, reduces the toxic effect of magnesium, and considerable 

 amounts of magnesium may accumulate in plant tissues. The ashes 

 of the leaves of Ilex aquifolium contain 12.34 per cent Mg; those of 

 Stellaria media, 13.08 per cent; those of Solanum tuherosum, 17.08 

 per cent. 



