CARBOHYDRATES 159 



grain : examine with a low power and make drawings of the starch-grains 

 within the cells observed. 



Ex. 8B. — Make a strong solution of potassium iodide in water and add to 

 if a few crystals of iodine. Allow the mixture to stand for twelve hours, and 

 shake occasionally in order to facilitate the solution of the iodine. When the 

 latter is all dissolved, add more water until the whole is the colour of dark 

 sherry. 



When examining the starch-grains in Exs. 82 to 84, place a drop of this 

 solution near the edge of the cover-slip so that it may run under the latter 

 and come in contact with the starch-grains. Note the change in colour of the 

 starch-grains. 



Ex. 86. — Make an extract of malt diastase as follows : — Shake up five grains 

 of ground malt with 50 c.c. of cold water and after allowing it to stand for 

 four hours, filter so as to get a clear solution. 



Next grind some starch with water in a mortar and pour a little of the 

 mixture into a 200 c.c. flask of boiling water. When cool pour about 20 c.c. 

 of this thin starch paste into three test tubes : show the presence of starcl^ by 

 adding a few drops of the solution of iodine mentioned in Ex. 85 to one 

 tube, and to the other two tubes add 3 or 4 c.c. of the diastase extract, and 

 warm them to 60° C. Test for the presence of starch in one of these two tubes 

 by taking out at intervals of five minutes a few drops with a pipette and adding 

 them to weak solutions of iodine kept in a series of test tubes. 



After a time the starch is changed into sugar and dextrin : When this has 

 happened show the presence of the sugar by means of Fehling's solution. 



See if Fehling's solution is acted upon by the thin starch-paste when no 

 diastase is added. 



c. Celluloses. — The solid fabric of a plant consists mainly of 

 cell-walls which are produced by the protoplasm of the cells. 

 At first the walls are thin, but in many cases thickening takes 

 place by the deposition of layer after layer of substance on the 

 inside of the walls where they are in contact with the' cytoplasm. 

 Where cells are in a state of division and new walls are being 

 produced, the latter are first visible in the form of thin plates of 

 cytoplasmic substance stretched across the dividing cells, and in 

 the process of thickening the new layers appear to be produced 

 by a conversion of the outermost layers of the cytoplasm, for 

 where thickening of a cell-wall takes place there is always noticed 

 a gradual diminution of the protoplasmic cell-contents until at 

 last none remain within the cell-cavity. 



