80 CELL-CONTENTS AND CELL-WALLS. 



(a) Examine common resin, and note that it is a yellow trans- 

 lucent amorphous substance, insoluble in water, soluble in turpen- 

 tine, benzine, alcohol, and ether. It is coloured red by alkannin. 



(b) Cut transverse sections (wetting the razor with water) from 

 a young Ivy stem or Pine stem. (1) Mount in water, and note the 

 highly refractive resin drops, found chiefly in and around tl>e resin 

 ducts. (2) Irrigate with alcohol ; the drops are dissolved. (3) Test 

 a section with alkannin, using either the alcoholic solution or 

 (better) laying a section of dry alkanna root on the stem section ; 

 cover and leave for about an hour the resin drops are stained red. 

 (4) Test other sections with osmic acid. Compare with the re- 

 actions of oils. 



(c) Place a piece of Ivy or Pine stem in strong copper acetate 

 solution for about a week ; then wash the pieces in water or dilute 

 alcohol, cut sections, and note that the resin is stained green. 



89. Latex. This is a liquid found in many plants. Occasion- 

 ally it is watery and colourless (Banana, etc.), but it is generally a 

 milky emulsion owing to the presence of suspended particles, and 

 sometimes it is coloured (Chelidonium). In the fluid of latex there 

 occur dissolved salts, sugars, etc. ; the suspended particles consist 

 chiefly of rubber or caoutchouc, but sometimes there are starch 

 grains (Spurges) ; various nitrogenous organic substances also occur 

 in latex, e.g. proteins, enzymes, and alkaloids (opium, etc.). 



Cut across the stem of a Spurge freshly pulled up : the latex 

 escapes as a white juice, which was evidently under pressure 

 (exerted by the turgid parenchyma around the latex tubes), since it 

 escapes in considerable quantity. (1) Quickly cover and examine 

 fresh latex on a slide : at first it is like milk, containing numerous 

 suspended particles, but after a time a clot is formed and the 

 materials originally distributed uniformly in the liquid collect into 

 masses. (2) To another fresh portion of latex add alcohol ; clot- 

 ting occurs much more rapidly and completely. Hence it is neces- 

 sary, in the study of laticiferous tissue, to place an entire plant, or 

 pieces cut from it, at once in alcohol, in order to coagulate the con- 

 tents and prevent their escape. (3) To another portion add iodine ; 

 the granular masses stain brown (proteins), while here and there 

 are rod-like or dumbbell-like starch grains. (4) Treat another por- 

 tion with alkanna : the rubber particles are stained red. 



For the structure of laticiferous tissue see 139. 



IX. NON-NITROGENOUS ORGANIC ACIDS; MINERAL 

 DEPOSITS. 



9O. Non-nitrogenous Organic Acids are frequently 

 formed from carbohydrates, usually by processes of oxida- 

 tion, and are present in the cell-sap, either in the free 



