PLANT PRODUCTS 569 



double stains may be achieved with algae which have been growing in a dilute 

 solution of congo-red (see under this head in the preceding chapter), which stains 

 the cell-walls, but not the gelatinous sheaths. See also in the preceding chapter 

 under India ink. 



Globoids. The globoids found in aleurone grains consist of a double phosphate 

 of calcium and magnesium, which is insoluble in alcohol and dilute potassium hy- 

 drate but is soluble in dilute mineral acids and in acetic, oxalic, and tartaric acids. 

 In an ammoniacal solution of ammonium phosphate the globoids are replaced by 

 groups of crystals of ammonium-magnesium phosphate. Treated with ammonium 

 oxalate, they become replaced by crystals of calcium oxalate. The globoids may 

 be isolated to a certain extent by extracting the oil from sections of endosperm 

 containing them by means of alcohol or alcohol and ether, and then dissolving the 

 ground substance and crystalloid by means of i per cent, potassium hydrate. If 

 crystals of calcium oxalate are present along with the globoids, they may be dis- 

 tinguished by means of the polarizer, since they are doubly refractive, while the 

 globoids are not. 



Glucose, CeH^Oe. This occurs in sweet fruits and in the leaves and other mem- 

 bers of plants, being one of the most common forms in which carbohydrates circu- 

 late within the plant. The warty crystals of glucose which are deposited from 

 aqueous and alcoholic solutions at low temperatures melt at 86, and become free 

 from water at I ioC. At from 30 to 35C. glucose crystallizes from concentrated 

 solutions in water, ethyl- and methyl-alcohol in the form of hard crusts, which 

 melt at I46C. The presence of glucose may be easily demonstrated in the fruit 

 of the pear, for instance, and in the leaves of balsamina, or other rather translucent 

 leaves which have been cut from the parent plant and kept fresh under a bell-jar 

 for several days. Pieces of the flesh of a ripe pear may be put into a test-tube 

 with Fehling's solution and brought to a boil, when a reddish precipitate of cuprous 

 oxide will be thrown down. This reaction is characteristic of dextrose, maltose, 

 lactose, laevulose, and many glucosides. In this instance, however, we are dealing 

 with dextrose. This reaction may also be carried out on the microscopic slide. 

 Sections from the pear three or four cell-layers thick should be placed on the slide 

 in a few drops of the solution, the cover-glass should then be put on, and the solu- 

 tion heated until bubbles begin to arise. The microscope will then reveal the granu- 

 lar precipitate of cuprous oxide within the cells. Portions of the leaf of the bal- 

 samina may be treated on the slide as directed for the sections from the pear. 

 See under Fehling's Solution in the preceding chapter. 



Glycogen, CeHioOj. This is a colorless, amorphous, highly refractive substance 

 occurring quite commonly in the cells of fungi. It is soluble in water, but within 

 the cells it may be stained a reddish-brown by means of iodine. 



Gums. These are amorphous, transparent substances which dissolve in water 

 more or less completely and form a sticky solution. They may be precipitated 

 from their aqueous solutions by alcohol. Those gums which dissolve in water 

 completely, such as the gum of the cherry, apricot, peach, and gum arabic, are 

 called true gums, while those which contain cellulose and are not completely solu- 

 ble in water are known as mixed gums. Gum tragacanth is an example. One 

 of the most striking characteristics of gums which may be used in their identifica- 

 tion is their great capacity to swell in water. To follow the process of swelling 

 with the microscope, sections should be cut from dry material with a razor which 

 may be wetted with alcohol, but not with water. The sections should be placed 

 on a slide in a drop of strong alcohol, the cover-glass should be put on, and a drop 

 of water placed on the slide so that it just touches the edge of the cover-glass. 

 As the water mixes with the alcohol and comes in contact with the section a slow 

 swelling of the gum will begin, which may be followed very accurately through the 



