432 THE FOOD OF PLANTS 



Molisch found that it cannot be replaced by barium, strontium, beryllium, zinc, 

 or cadmium, and none of these elements can replace calcium in tho^e plants 

 to which it is essential '. 



The physiological difference between calcium and magnesium is shown in 

 the higher plants, for magnesium has a distribution and importance somewhat 

 similar to potassium, whereas but little calcium is found in young tissues, or in 

 storage organs where the ash constituents are present in reduced amount. In adult 

 organs however the percentage of calcium usually increases both relatively and 

 absolutely. This calcium is for the most part not utilized further, and remains in 

 the organ when it dies, commonly in the form of crystals of calcium oxalate, but 

 also, though more rarely, impregnating the cell-wall either in amorphous form, or as 

 cystoliths of calcium carbonate (Sect. 23). Calcium salts may also be present in 

 solution in the cell-sap, and it is possible that they may unite with the organic 

 substances which build up the protoplast ~. 



Calcium cannot have any general importance since fungi and certain algae 

 may grow without it ; in other plants, however, it may acquire such importance 

 for special functions that these cease or are performed abnormally in its absence, 

 and this may so affect the entire organism as to render further development 

 impossible. It is uncertain whether the union of calcium salts with the constituents 

 of the cell-wall may not exercise some influence upon the secondary growth of the 

 latter, for the fact that calcium is not essential to certain plants simply indicates that 

 it is not necessary for the formation and growth of the cell-w r all in the primary 

 meristem. In the absence of calcium, only the stalk of the cystolith can be 

 formed, and according to Mangin ?1 the chalk in the body of this structure is 

 derived from calcium pectate. It is probable that similar calcium-compounds 

 may be present in most cell-walls. 



Calcium may be necessary in certain plants in order to prevent a poisonous 

 accumulation of soluble oxalates by removing oxalic acid in an insoluble form, 

 but this can hardly be its general importance, as Schimper supposes, for in many 

 plants calcium oxalate is either absent or present in very small amount. This 

 may be the case even in those which produce an abundance of oxalic acid and 

 which are injured when supplied with potassium oxalate as readily as other plants 



1 Ba: Knop, Versuchsst., iS'66, Bd. vm, p. 143. Sr and Be: Benecke, Bot. Centralbl., 1894, 

 Bd. LX, p. 195. and Jahrb. f. wiss. Bot., 1895, Bd. XXVIII, p. 519. Sestini erroneously supposed 

 that Be could replace Mg (Bot. Jahresb., 1891, p. 27'. For algae, cf. Molisch, Sitzungsb. d. Wien. 

 Akad., 1895, Bd. CIV, Abtli. i, p. 783. On partial replacement, cf. Sect. 73. Ba and Sr are generally 

 absent from plants in nature, but both have been found. Cf. Forchhammer, Ann. d. Phys. u. 

 Chem., 1855, Bd. xcv, p. 84; Boedecker u. Eckhardt, Ann. d. Chem. u. Tharm., 1856, Bd. C, 

 p. 294; Dworzak, Versuchsst., 1874, Bd. XVII, p. 398. 



' 2 On the distribution of Ca, cf. Kohl, Kalksalze u. Kieselsaure, 1889; also Zimmermann, 

 Mikrotechnik, 1892, p. 56. Hansen (Mitth. a. d. zool. Station zu Neapel, 1893, Bd. xr, p. 258) 

 found only traces of Ca and no Mg in the cell-sap of I'alo7iia, but in other cell-saps both elements 

 are undoubtedly present. 



* Melnikoff, Unters. u'ber d. Vorkommen d. kohlens. Kalkes, 1877, p. 32; Kohl, I.e., p. 14. 

 For further literature see Sect. 23. Mangin, Compt. rend., 1892, T. cxv, p. 260; Rech. anat. s. 1. 

 composes pectiques, 1893, p. 48. 



