OZ CELL-CONTENTS AND CELL-WALLS. 



with ammonia and heated, but gives a black deposit after boiling 

 for some time. 



Try these tests with citric acid or a soluble citrate. Repeat the 

 tests with juice of Orange or Lemon. 



94. Malic Acid, usually combined with lime, is abundant in 

 various Crassulaceae, etc. The calcium malate, which may form 

 nearly half the dry weight of the sap in Sempervivum, Echeveria, 

 and other plants of this family, can be extracted by bruising or 

 pounding up some fresh leaves, filtering the pulp, and adding to 

 the filtered sap four or five times its volume of strong alcohol the 

 malate is precipitated as a white powder. 



See Text-books on Organic Chemistry for detailed reactions of 

 these acids, methods of detecting each in mixtures, etc. 



95. Mineral Deposits may occur either in the cell-contents 

 or in the cell-walls. The commonest of these deposits consist of 

 calcium oxalate, calcium carbonate, and silica, which are easily dis- 

 tinguished from each other. 



(a) Calcium oxalate is chiefly found in the cell-sap as crystals 

 of various forms, of which the chief are (1) single prismatic, flat, 

 or diamond-shaped crystals ; (2) more or less spherical aggregates 

 (sphaero-raphides) with numerous small pyramidal crystals on the 

 free surface ; (3) needle-like crystals (raphides) arranged in bundles 

 and generally embedded in mucilage. Calcium carbonate often 

 occurs on cell-walls as an incrustation, the most striking of which 

 are those called cystoliths. Silica occurs chiefly as incrustations on 

 the cell-wall. 



(6) Calcium carbonate is soluble in acetic acid, and in weak 

 nitric acid, with evolution of gas-bubbles (carbon dioxide). Cal- 

 cium oxalate is insoluble in acetic acid ; soluble in dilute nitric 

 acid, but without evolution of bubbles ; soluble in sulphuric acid, 

 with formation of a crystalline precipitate of calcium sulphate. 

 Silica is insoluble in acetic or nitric acid, and remains as a flinty 

 residue after strongly igniting the tissue on a cover-glass or on 

 platinum-foil and treating the ash with nitric acid. 



(c) Cut transverse sections of the leaf of India-rubber Plant 

 (Ficus elastica), mount in water, and note the large pear-shaped 

 cystoliths, each occupying one of the large cells below the upper 

 epidermis. Add a drop of acetic acid : the cystoliths become 

 transparent and dissolve, bubbles of gas being given off. When 

 the carbonate is dissolved, a mass of cellulose (on which the car- 

 bonate is deposited) is left, showing concentric stratification and 

 radial striation. 



(d) Since calcium oxalate crystals are so abundant in plants, they 

 will be frequently found in sections of stems, leaves, etc., often 

 occupying special cells. On testing the sections, note that the 



