CELL-CONTENTS AND CELL-WALLS. 71 



or glycerine), mount in strong glycerine, and note that the epi- 

 dermis of the seed-coat consists of cells which have thick walls and 

 are covered externally by a distinct cuticle. (2) Irrigate with water 

 (or lift off the cover-glass and place a water drop on the section), 

 and note that the walls of these cells become swollen, the stratified 

 structure of the outer walls becoming more marked ; the cuticle is 

 ruptured as the swollen mass bulges out ; the middle lamella of the 

 walls between adjacent cells does not swell up, but remains dis- 

 tinct. The swelling of the walls may be hastened by warming the 

 slide. (3) Treat the section, which has been soaked in water, with 

 iodine solution the gummy walls are not stained or only slightly. 

 (4) Treat a section with iodine and sulphuric acid a bluish 

 colouration is produced. (5) Treat a section with Hoffman's blue 

 the gummy walls are not stained, or very slightly. (6) Treat a 

 section with corallin-soda the gummy walls turn pink. (7) Treat 

 a section with potash the swelling of the walls occurs much more 

 rapidly than with water. 



(c) Examine commercial salep, or make it by drying and crushing 

 the tubers of Orchis mascula (or O. maculata, or O. latifolia) ; treat 

 the salep, or the pounded tubers, with cold water, and filter. 

 (1) To the clear filtrate add alcohol the white flocculent pre- 

 cipitate consists of Orchid mucilage, insoluble in alcohol. (2) 

 Evaporate the liquid, and treat the residue with iodine and sul- 

 phuric acid the blue or violet colour produced is distinctive of the 

 so-called "true vegetable mucilages." 



(d) Cut transverse sections of the tuber of an Orchis. (1) Mount 

 in alcohol, and note that the ground tissue (parenchyma), in which 

 the vascular bundles are embedded, consists of small starch-con- 

 taining cells, together with larger cells each of which contains a 

 bundle of needle-like crystals (raphides) of calcium oxalate em- 

 bedded in mucilage. (2) Treat a section with corallin-soda, mount 

 in glycerine, and note that the large cells have their mucilaginous 

 contents stained pink. (3) Treat a section with iodine and sul- 

 phuric acid, and (4) another with Hoffman's blue, and note the 

 results. 



VII. G-LUCOSIDES AND TANNINS. 



83. Glucosides are combinations of glucose, or more 

 rarely of other sugars, with various classes of organic 

 compounds, especially those of the aromatic series. In 

 general chemical properties they resemble cane sugar and 

 the polysaccharides, and various glucosides have been pre- 

 pared synthetically from glucose. The glucosides yield 

 glucose 011 being hydrolysed by means of acids or of special 

 enzymes (glucosidases or glucoside- splitting enzymes). 



