OFFICE OF THE ASH CONSTITUENTS. 253 



element must be present in order to have any starch formed. 

 The transport of the starch from the leaves to other parts is, 

 however, dependent upon the form in which the potassium is 

 presented to the plant, and for this purpose the chloride is most 

 efficient." 



677. Calcium and magnesium,. These elements cannot re- 

 place one another in the plant, though it is not clear what office 

 they perform. Pfeffer regards it as possible that calcium may 

 play an important part in the formation of the cell-wall, inas- 

 much as it can always be detected there. Melnikoff is quoted by 

 Pfeffer * as stating that in the cell-wall calcium generally exists 

 as the carbonate. It is suggested by Sachs that this element 

 may enter into combination with cellulose, as it does with some 

 other carbohydrates. 



When seedlings ar.e grown in pure water their development 

 after a short time becomes completely checked, and the addition 

 of all necessary substances except calcium salts fails to stimulate 

 a normal growth ; but after the addition of a small amount of 

 any calcium salt the normal processes of the plant recommence 

 at once. 2 Regarding the almost universal occurrence of calcic 

 oxalate in plants, Sachs says: "The importance of calcium 

 must therefore be sought partly in its serving as a vehicle for 

 sulphuric and phosphoric acid in the absorption of food-material, 

 and parth' in its fixing the oxalic acid, which is poisonous to 

 the plant, and rendering it harmless." 3 



678. Phosphorus. The principal and perhaps the only com- 

 bination of this element available for plants is phosphoric acid 

 (the phosphates). The experiments by Ville upon the absorption 

 by plants of calcic phosphite and hypophosphite, although not 

 conclusive, make it appear probable that these salts cannot 

 replace the phosphate in absorption. 



It is not clear what the office of phosphorus is in the plant, 

 but in some of its compounds it is so often associated with the 

 soluble albuminoids that it is believed to assist in the transfer 

 of these matters. Schumacher holds that the chief work of the 

 alkaline phosphates is the acceleration of the diffusion of these 

 difficultly diffusible substances (the albuminoids). 4 (See 957.) 



1 Pflanzenphysiologie, i., 1881, p. 259. 



2 Boehm : Sitzungsb. d. Wien. Akad. Band Ixxi. Abth. i., 1875, p. 481. 

 8 Text-book, 2d ed., 1882, p. 699. 



* " If these [alkaline phosphates] substances are mixed with a solution of 

 albumin, or if a solution of them is permitted to diffuse against one of albumin, 

 a much greater amount of the latter will pass through the membrane than 



