METABOLIC PROCESSES IN THE PLANT. 301 



The filtrate is easily cleared, however, by passing into it for some 

 time washed Carbon dioxide. The combined nitrates are precipi- 

 tated with Lead acetate, filtered and made up to a known volume, 

 say 200 c.c. In the resulting fluid we determine the sugar by 

 Fehling's solution (see 111). 



It is not to be forgotten in working out the results that 100 

 parts of maltose, which is the sugar especially found in malt 

 extracts, only decompose as much Copper oxide as 61 parts of 

 dextrose (Brown and Heron). 



To determine microscopically the presence of glucose in tissues, 

 we first cut sections of the objects, e.g. from pears or apples, 

 which, however, must not be too thin, so that not all the cells are 

 opened. It is best for the sections to include three layers of 

 uninjured cells. The sections are placed in a concentrated 

 solution of Copper sulphate at the ordinary room temperature, 

 removed with the forceps after a short time, and washed super- 

 ficially by dipping in clean water. We now at once place the 

 sections in boiling potash solution, 1 or better 2 in a boiling solution 

 of 10 gr. of Sodium Potassium tartrate, and 10 gr. of caustic 

 potash in 10 gr. of water. If glucose is present, a beautiful red 

 precipitate of Cuprous oxide is produced after a few seconds in 

 the cells which contain it. Microscopical examination of the 

 sections gives information concerning the distribution of the sugar 

 in the tissue. 



The following method for detecting glucose is very convenient. 

 The sections are rinsed with water, laid on a slide in a drop of 

 Fehling's solution (see 111), and covered with a cover-glass. We 

 now heat till small bubbles appear, but no longer. In presence of 

 sugar, Cuprous oxide separates out in the cells. 



1 See Sachs, Pringsheim's Jahrbiicher, Bd. 3, p. 187. 



- See Arthur Meyer, Berichte d. Deutschen botan. Gesellschaft, Bd. 3, p. 332. 



116. Dextrin. 



One hundred c.c. of water are mixed with 1 gr. of potato starch, 

 and heated to boiling. To the cooled paste are added a few drops 

 of Sulphuric or Hydrochloric acid, and it is then again warmed. 

 The fluid rapidly clears, and a small sample of it, after it has 

 been allowed to cool, still gives a blue coloration with Iodine. 

 If we continue to boil the acidified solution, remove from it from 

 time to time (say every five minutes) small samples, and treat them 



