METABOLIC PROCESSES IN THE PLANT. 307 



again evaporated. There remains behind a syrupy fluid of 

 sweetish taste, which is glycerine. If this residue is dissolved in 

 AY ;iter, and a part of the fluid mixed with a dilute solution of 

 Copper sulphate, to which potash solution has been added, so that 

 it holds in suspension a precipitate of Copper hydrate, then the 

 Copper hydrate will be dissolved (glycerine reaction). 



121. Reactions of Fatty Oils. 



With a glass rod we place a drop of any fatty oil on a glass 

 slide, add to it a mixture of alcohol and ether, which dissolves the 

 fat, cover with a cover-glass, and observe tinder the microscope. 

 When the alcohol and ether have evaporated, we shall see in our 

 preparation large and small drops which consist of fat. In optical 

 section these appear light grey, and are bounded by a narrow 

 black ring. If the tube of the microscope is lowered, each oil 

 drop will 'appear bounded no longer by a black ring, but by a 

 bright one. The oil drops cannot well be confused with bubbles 

 of air, for if we focus these under the microscope, and lower the 

 tube, their margin does not become bright, but, on the contrary, 

 the dark band already present increases in breadth. 



In the cells of the endosperm of Ricinus, or in those of the 

 cotyledons of Brassica, there are present, besides the proteids, 

 large quantities of fat, and to prove this we need only treat thin 

 sections of the seeds on the slide with a mixture of alcohol and 

 ether. The drops of fat at once separating out are readily recog- 

 nised as such. 



Tincture of alkanna also (a deeply coloured extract prepared by 

 treating alkanna root with 70 to 80 per cent, alcohol) may be 

 used for the detection of fats. If we examine, e.gr., sections from 

 the endosperm of Ricinus, whose cells contain a fat differing from 

 others in being soluble in alcohol, we treat the alkanna tincture 

 with an equal volume of glycerine, pass the sections a few times to 

 and fro in the mixture, wash in alcohol, and mount in glycerine. 

 The aleurone grains are stained lightly or not at all, while the 

 ground-mass, in consequence of its fatty contents, is stained deeply 

 reel. 



A further reagent for fatty oils is an aqueous 1 per cent, solution 

 of Osmic acid. If we lay sections from the endosperm of Ricinus 

 in the solution, they assume after some time a dark colour owing 

 to the blackening of the fats by the Osmic acid. It is to be 



