Y] CARBOHYDRATES 57 



Expt. 51. Hydrolysis of xylan. Put the xylan obtained in the last experiment 

 in a round-bottomed flask fitted with an air condenser (see p. 46). Add 100 c.c. of 

 4 % sulphuric acid and heat on a water-bath for 4 hrs. NeutraHze the solution with 

 calcium carbonate, filter from calcium sulphate and concentrate on a water-bath. 

 Test a portion for pentoses (see Expt. 39) and a positive reaction will be obtained. 

 To a small quantity add also a few drops of Fehling's solution and boil. Reduction 

 will take place. 



To the remainder of the xylose solution add bromine (see p. 47) gradually until 

 there is excess. Then remove the excess of bromine by warming on a water- bath. 

 Neutralize the solution, which contains xylonic acid, with cadmium carbonate and 

 evaporate on a water-bath. Extract the residue with alcohol and filter. On concen- 

 trating the alcoholic extract, white prismatic needles of cadmium xylonate separate 

 out. 



It has been shown that pentosans, xylan and probably araban, occur 

 in leaves (Davis, Daish and Sawyer, 17). It is likely that the xylan is 

 widely distributed in all tissues since it forms a constituent of lignified 

 cell-walls. 



Expt. 52. Detection of pentoses from pentosans in leaves. (Davis, Daish and 

 Sawyer, 17.) Take two large leaves of the Sunflower {Helianthus annuus). Tear into 

 small pieces and drop into boiling 98 % alcohol in a flask. Boil well and filter off" the 

 alcohol. Repeat until all the green colour is removed. Then dry oflf the alcohol and 

 grind up the leaf residue. Perform the test for pentoses (Expt. 39 a and c) on the 

 dry leaf tissue. It should give the above tests showing the presence of pentosans. 



Leaves of the Violet ( Viola odorata) and Nasturtium ( Tropaeolum majus) may 

 •also be used. 



Expt. 53. Method for determination of pentosans in tissues, hraii and leaves, etc. 

 Weigh, out 2 gms. of bran, put it into a round-bottomed flask, add 100 c.c. of 12 % 

 hydrochloric acid and fit the flask with a water condenser. Heat gently over wire 

 gauze and distil into a solution of phloroglucinol in 12 ^/^ hj'^drochloric acid. A green 

 precipitate of furfural phloroglucide is formed which eventually becomes almost black. 

 For accurate estimations of pentosans this is filtered off" and weighed on a Gooch 

 crucible. The same method may be used with leaf residue prepared as in Expt. 52. 



Starches. 



Starch. This is a very widely distributed substance in plants. It 

 occurs as solid grains throughout the tissues, in leaves, stems, roots, fruits 

 and seeds. It is absent, however, from a number of Monocotyledons, 

 e.g. Iris, Snowdrop (Galanthus), Hyacinthus, etc. (Blackman, 5). It forms 

 one of the chief reserve materials of plants, that is, it is synthesized from 

 sugar when carbon assimilation and carbohydrate synthesis are in pro- 

 gress, and is stored in the solid form in tissues as grains. In other 

 circumstances of the plant's existence, when material for metabolism is 

 not available from carbon assimilation, as for instance in germinating 

 seeds or growing bulbs or rhizomes, the starch is hydrolyzed into dextrin 



