100 PLANT PHYSIOLOGY 



as the wood, calcium salts are less abundant. These facts 

 are indicated by the following table * 



Potato leaves have 2.90% of dry weight as calcium salt. 



tubers 0.10% 



Pea straw 1.88% 



" seeds 0.13% 



Tobacco leaves 6.18% 



Hop " 7.38% 



Wood of trees 0.02-0.10% 



From such figures, the result of gross analyses confirmed 

 by microchemical tests, it has been inferred that calcium 

 is concerned in the formation of cell-wall and in neutralizing 

 the oxalic acid set free in various chemical changes ( notably 

 respiration) taking place in plant cells. Proof that these 

 inferences are correct is still lacking, however. 



Loew's hypothesis! that the framework of nucleus and 

 plastids is a double organic salt of calcium and magnesium, 

 or a complex union of calcium and magnesium compounds, 

 is a hypothesis only, and hence need only be mentioned. 



The value of calcium to higher plants is beyond question, 

 but how it is used is still unknown. It is found in plants 

 usually as the oxalate (crystallized out as needles or as 

 polyhedra of more symmetrical dimensions), as carbonate 

 ( deposited in the peculiar outgrowths of cell-wall known as 

 cystoliths), much less frequently as sulphate and phos- 

 phate. Calcium salts are abundant enough in the soil to be 

 taken up by all plants, and although the sulphate and 

 phosphate are only slightly soluble, the large volumes of 

 water absorbed will still carry adequate amounts, even from 

 soils which contain little or no other calcium compounds, 

 to the cells using them. 



MAGNESIUM, absolutely indispensable to all plants, $ in 

 spite of the assertions of earlier authors to the contrary, 



* Frank's Lehrbuch, I., p. 590. 



t Loew, 0. Uber die physiologischen Functionen der Calcium-und Mag- 

 nesiumsalze im Pflanzenorganismus. Flora, 1892. Uber das Mineralstoff- 

 bediir f. niss von Pflanzenzellen. Bot. Centralblatt, Bd. 63, 1895. 



t Molisch, H. L. c. under calcium. Benecke, W. L. c. under calcium. 



Nageli. Botanische Mittheilungen, Bd. 3, 1881 and others. 



