564 ELEMENTS OF PLANT HISTOLOGY AND MICROTECHNIC 



to boiling; then to allow the water to evaporate slowly and to treat the residue with 

 a drop of benzol. On the evaporation of the benzol, caffeine appears in the form 

 of^fine needle-crystals. 



Calcium. When the ash of plants is treated with sulphuric acid, this unites 

 with the calcium present to form crystals of gypsum. If calcium sulphate is al- 

 ready present in the ash, its characteristic crystals may be detected when an aqueous 

 solution of the ash is allowed to dry slowly. If calcium is present in sections, it 

 may be deposited in the form of crystals of calcium oxalate if the sections are 

 treated with a solution of ammonium oxalate. 



Calcium Carbonate, CaCOa. This rarely occurs in the crystalline form within 

 the cells. It may, however, be found imbedded in, or incrusted on, the cell-walls. 

 Calcium carbonate dissolves with effervescence when treated with dilute 

 acetic acid. When treated with concentrated hydrochloric acid, it dissolves with 

 the evolution of carbon dioxide gas. The ingrowths from the walls of certain cells 

 of the leaves of Ficus elastica, known as cystoliths, are thickly incrusted with cal- 

 cium carbonate, and afford excellent material for the demonstration of this salt 

 within plant tissues. 



Calcium Phosphate, Ca 3 (PO)2. This salt of calcium occurs usually, if not 

 always, in solution in the' cell-sap. It may be deposited in the form of sphaero- 

 crystals when plant tissues containing it are kept for a long time in strong alcohol. 

 When treated with sulphuric acid, the sphaerocrystals are dissolved and crystals 

 of calcium sulphate are formed in their stead. When sections containing calcium 

 phosphate are heated on a slide in a drop of ammonium molybdate acidulated with 

 nitric acid, a yellow precipitate is produced. This reaction may be hindered by 

 the presence of certain organic compounds, such as potassium tartrate, in which 

 case the sections should be treated with a mixture of 25 volumes of a concentrated 

 aqueous solution of magnesium sulphate with 2 volumes of a concentrated aqueous 

 solution of ammonium chloride and 15 volumes of water. In this case a crystalline 

 precipitate of ammonio-magnesium phosphate is formed. 



Calcium Oxalate, CaCzO^ Crystals of calcium oxalate occur so commonly in 

 plants that it is safe to assume that any crystals observed in fresh tissues are of this 

 substance until the contrary is demonstrated. The crystals may occur singly in the 

 cells, in which case their definite crystalline form can be made out, or in the form 

 of agglomerated star-shaped clusters of crystals, or in bundles of parallel needle- 

 shaped crystals, or they may occur very numerously in cells in the form of very 

 minute crystals. The crystals are insoluble in water and acetic acid, but dissolve 

 without effervescence in hydrochloric acid. When they are treated with sulphuric 

 acid, crystals of calcium sulphate are formed in their place. Calcium oxalate 

 appears to be an excretion formed by the union of salts of calcium, which have 

 been absorbed from the soil, with oxalic acid which is formed by the plant. 



Calcium Sulphate, CaSO4. Minute crystals of calcium sulphate occur in many 

 desmids. They are insoluble in concentrated sulphuric acid. A solution of barium 

 chloride dissolves them with the formation of barium sulphate. 



Callose. Callose occurs in sieve tubes, where it may close up the sieve pores. 

 It also occurs commonly in cystoliths, and in the membranes of pollen-grains and 

 various fungi. Callose is insoluble in water, alcohol, and cuprammonia, but it is 

 readily soluble in cold sulphuric acid, calcium chloride, and concentrated chloride 

 of zinc. It is insoluble in cold alkaline carbonates, but swells up without dissolving 

 in ammonium. Corallin, aniline blue, and a mixture of soluble' blue and vesuvin, 

 or of vesuvin and orseillin, are suitable stains for callose. The corallin should be dis- 

 solved in a saturated solution of sodium carbonate. After remaining in this solu- 

 tion for a time, the sections should be examined in glycerine. If the sections are 

 over-stained, the intensity of the stain may be reduced in a 4 per cent, solution of 



