DIV. i MORPHOLOGY 29 



in the leaves of succulents, and also outside the protoplasts in the cell wall 

 (cf. p. 38). 



AMIDES, especially Asparagin and also ALBUMINOUS SUBSTANCES, 

 occur in the cell sap as reserve materials or as intermediate products 

 of the metabolism (cf. p. 1 4 for reactions). 



Highly refractive vacuoles filled with a concentrated solution of 

 TANNIN ( 25 ) are of frequent occurrence in the cytoplasm of cortical 

 cells, and may often grow to a considerable size. ALKALOIDS, 

 GLUCOSIDES, and BITTER PRINCIPLES allied to these are also not 

 infrequent in the cell sap. All these are usually end products of 

 metabolism. 



The dark-blue or green, colour reaction obtained on treatment with a solution 

 of ferric chloride or ferric sulphate, and the reddish-brown precipitate formed 

 with an aqueous solution of potassium bichromate, are usually accepted as tests 

 for the recognition of tannin, although equally applicable for a whole group of 

 similar substances. The tannins are not further utilised in the plant. They 

 often impregnate cell walls, which then persist and resist decay. 



The cell sap is often coloured, principally by the so-called ANTHO- 

 CYANINS, a group of non-nitrogenous glucosides. They are blue in 

 an alkaline, and red in an acid- reacting cell sap, and, under certain 

 conditions, also dark-red, violet, dark-blue, and even blackish-blue. 

 Alkalies frequently change the colour to green. Anthocyanin can be 

 obtained from the cell sap of a number of deeply coloured parts of 

 plants in a crystalline or amorphous form. Less commonly yellow 

 substances, ANTHOCLORE and ANTHOXANTHINE, are found dissolved in 

 the cell sap as in the yellow floral leaves of the Primrose and the 

 yellow .Foxglove. A brown pigment called ANTHOPHAEINE occurs in 

 the cells of the blackish -brown spots of some flowers. 



The researches of WILLSTATTER and his pupils ( a ) have advanced our knowledge 

 of the chemical constitution of the anthocyanins. They are glucosides in which 

 cyanidins (aromatic pigment components, hydroxyl compounds of phenylbenzo- 

 pyrilium, and apparently related to the flavones), are combined with sugar, e.g. 

 in the Cornflower cyanidin (C 15 H 10 6 ) and in the flower of the Larkspur delphinidin 

 (C 15 H 10 7 ). In red flowers the cyanes are united with acids and in blue flowers 

 with alkalies-, while the pigments in violet flowers are neutral. The anthoxanthins 

 also are glucosides with aromatic pigment components which belong to the flavones. 



Blood-coloured leaves, such as those of the Copper Beech, owe 

 their characteristic appearance to the united presence of green 

 chlorophyll and anthocyanin. The autumnal colouring of leaves 

 also depends on the formation of anthocyanin. The different colours 

 of flowers and fruits which often serve to attract animals are due to 

 the varying colour of the cell sap, to the different distribution of the 

 cells containing the coloured cell sap, and also to the different com- 

 binations of dissolved colouring matter with the yellow, orange, or 

 red chromoplasts and the green chloroplasts. 



