74 CELL-CONTENTS AND CELL-WALLS. 



use young oak galls ; acorns (cut across the cotyledons, which also 

 contain abundant starch) ; twigs of Hazel, cut in winter ; young 

 Rose stems. Rose leaves contain abundant tannin ; fold up several 

 leaves and crush with the fingers between folded white paper to 

 press out the sap, then touch the moistened portions of the paper 

 with ferrio chloride solution, and note the dark blue colour. 



(c) Cut thin sections of any of the above. (1) Irrigate with 

 ferric chloride, or with ferrous sulphate note that at first a deep 

 blue precipitate is formed, which soon dissolves and imparts its 

 colour to the surrounding liquid. (2) Place sections in 10 per cent, 

 potassium dichromate solution a reddish brown precipitate is 

 formed in the tannin-containing cells. (3) Place sections in strong 

 solution of ammonium molybdate in strong ammonium chloride a 

 brown or yellow precipitate. (4) Place sections in lead acetate 

 solution a white precipitate. (5) Place sections in strong copper 

 acetate solution for a week ; then place them on a slide in a drop of 

 1 per cent, solution of ferrous sulphate ; after a few minutes, wash 

 with water, transfer to a watch-glass of alcohol (to remove air-bubbles 

 and to extract chlorophyll if present), and mount in glycerine. An 

 insoluble brown precipitate is found in the cells containing tannin. 

 If the sections are taken from the alcohol and placed in iron acetate 

 solution, a blue or green colour is produced, according to the kind 

 of tannin present. 



(d) It can be shown that in various plants tannins are produced in 

 green leaves exposed to light and supplied with carbon dioxide, but 

 not in darkness or in absence of carbon dioxide. They are probably 

 formed as bye-products in the process of proteid-formation, rather 

 than as primary products of photosynthesis. They probably mi- 

 grate from the leaf during the night and are ultimately deposited 

 in the stem tissues, but it is doubtful whether the primary tan- 

 nins thus formed enter largely again into metabolism. So-called 

 ' ' secondary " tannins are, however, formed in many plants when 

 kept in darkness. Seeds of Broad Bean and Scarlet Runner con- 

 tain no tannin ; but seedlings grown in darkness are rich in tannin. 

 Apply the tannin tests to sections of (1) dry seeds, (2) the stems of 

 darkened seedlings of these plants, and note the results. 



VIII. OILS, RESIN, LATEX. 



85. Oils. Under this name are included two series of 

 substances, which give certain reactions in common but 

 differ considerably in chemical properties, and in their 

 functions. 



The fatty oils, or fats, occur in many seeds, and less 

 frequently in other parts of plants (e.g. stems of Lime, 



