V] HYDROLYZING ENZYMES 55 



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

 in leaves (Davis, Daish and Sawyer, 15). 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, 15.) 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 oft' the 

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

 grind up the leaf residue. Perform the test for pentoses (Expt. 38 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, bran and leaves, etc. 

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

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

 gauze and distil into a solution of phloroglucin in 12% hydrochloric acid. A green 

 precipitate of furfural phloroglucide is formed which eventually becomes almost 

 black. P'or accurate estimations of pentosans this is filtered off" and weighed on a 

 Gooch crucible. The same method may be used with leaf residue 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 Monocotyle- 

 dons, e.g. Iris, Snowdrop (Galanthus), Hyacinthus, etc. 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 progress, 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 and 

 soluble sugar, which is translocated and used as a basis for meta- 

 bolism. During the night in leaves there is also a similar hydrolysis 

 of the starch which has been temporarily stored from the excess of 

 sugar synthesized during the day. 



Starch has a very large molecule and thus a high molecular weight. It 

 is insoluble in cold water. When heated with a little water it gives starch 

 paste, but on boiling with water it gives an opalescent "solution" which 

 really contains starch in the colloidal state as an emulsoid. In this con- 

 dition it does not diffuse through dialyzing membranes and does not 

 depress the freezing point of water. The " solution " cannot, strictly 



