PLANT PRODUCTS 573 



cellulose in the membranes of embryonic tissues, where it is distributed throughout 

 the entire thickness of the membrane. Pectose also occurs in most lignified, 

 suberized, and cutinized membranes. The middle portion of cell-walls the so- 

 called middle lamella consists, for the most part of calcium pectate. When thin 

 sections of plant tissues are treated for several hours with a mixture of i part of 

 hydrochloric acid and 4 parts of alcohol, the calcium pectate becomes changed, so 

 that pectic acid is liberated and calcium chloride is formed. The pectic acid is 

 insoluble in water, but is soluble in a 10 per cent, solution of ammonia, so that 

 after rinsing the sections in water and treating with the ammonia solution, the cells 

 may be separated from each other by a slight pressure on the cover-glass. When 

 the sections are placed for a considerable time in cold alkaline solutions, a double 

 pectate is formed which swells in cold water and finally dissolves in it. After the 

 calcium pectate of the middle lamella has been removed, the pectose which per- 

 meates the cell-wall still remains, but by treatment with cuprammonia it may 

 be removed from sections which have already been acted on by dilute hydrochloric 

 acid. The pectic substances may be stained only in neutral or slightly acid solu- 

 tions. For this reason it is a good plan to place sections for a short time in a 

 3 per cent, solution of acetic acid, and then to wash them in water before trans- 

 ferring then to the staining solutions. Safranin, methylene-blue, bleu de nuit, 

 and ruthenium-red are excellent stains for pectic substances. Safranin stains the 

 protoplasts and the lignified, suberized, and cutinized cell- membranes a cherry-red, 

 while the pectic compounds are stained orange-yellow. Methylene-blue and bleu 

 de nuit stain the protoplasts and the lignified membranes blue, and the pectic 

 substances a violet color. See also in the last chapter under Ruthenium-red. 



Pezizin. Pezizin is an orange-red coloring matter which occurs in solution 

 within the paraphyses of Peziza aurania and P. convexula. It is soluble in alcohol 

 and ether, and is not altered by alkalies and organic acids. It dissolves without 

 color in hydrochloric acid and is colored bright green by nitric acid. 



Phloridzin, C2iH24Oio. A glucoside occurring in, the leaves and in the cortex 

 of the roots and stems of the Pomaceae. When tissues of Pirns mains containing 

 phloridzin are treated with ferric chloride, a dark brown solution is formed, while 

 treatment with ferrous sulphate causes a yellowish-brown precipitate. The tissues 

 of the pear, cherry, and plum are apt to contain large amounts of tannins which 

 produce a green color with salts of iron, and so mask the phloridzin reaction. 



Phloroglucin, C6H 3 (OH) 3 . This occurs in solution in the cell-sap. To demon- 

 strate its presence treat previously dried sections with a solution prepared by 

 dissolving 0.005 Gm. of vanillin in 0.5 Gm. of alcohol, and adding 0.5 Gm. of water 

 and 3 Gm. of concentrated hydrochloric acid. When phloroglucin is present, 

 this solution produces a light red color. 



Phosphoric Acid, H 3 PO4. This can be best demonstrated in the ash. The ash 

 is dissolved in hydrochloric acid and the solution is evaporated to dryness; then the 

 residue is treated with ammonium molybdate, which, if phosphoric acid is present, 

 produces a precipitate of ammonium phospho- molybdate, the crystals of which 

 have a greenish-yellow color under the microscope. If the presence of phosphoric 

 acid is to be sought for in fresh tissues, sections should be heated in a drop of am- 

 monium molybdate on the glass slide. This method also produces a precipitate 

 of crystals of ammonium phospho-molybdate in the presence of phosphoric acid. 

 If ammonium tartrate is present in the tissues, ammonium molybdate does not 

 serve so well as a test for phosphoric acid. In such a case a solution should be 

 used, consisting of 25 volumes of a concentrated aqueous solution of magnesium 

 sulphate, 2 volumes of a concentrated aqueous solution of ammonium chloride, and 

 15 volumes of water. With phosphoric acid this solution produces a precipitate 



