68 THE MICROSCOPE AND CELL-STRUCTURE. 



Very slender or delicate tissues, or thin structures like leaves, 

 should be cut by embedding in pith or carrot. If carrot be used, a 

 piece 1 in. x in. X in. will be found convenient. 



101. Structure of Beau Cotyledon. 1 A thin section 

 across a Bean cotyledon shows an outer layer or skin (epider- 

 mis) of small cells containing only fine-grained protoplasm 

 and a nucleus. The inner cells contain, besides protoplasm 

 and nucleus, large starch-grains and small proteid- grains. 

 With iodine solution, the starch-grains are seen to turn blue, 

 while the protoplasm and proteid-grains turn brown and the 

 nucleus is deeply stained. The cotyledon has feebly - 

 developed veins, appearing in section as irregular streaks 

 and patches of small narrow cells. 



The dry Broad Bean seed contains about 60 per cent, of starch, 

 25 per cent, proteids, 3 per cent, oil, and 4 per cent, ash ingredients 

 (calcium, magnesium, potassium, sulphur, phosphorus, etc.) ; the 

 residue consists chiefly of cellulose (in cell- walls) and some silica (in 

 seed-coat) . 



102. Slide Mounting. Sections and other specimens may be 

 mounted permanently in glycerine jelly. The bottle of jelly is set 

 in a cup or dish of hot water until the jelly melts, then the superfluous 

 water is drawn from the specimen by using a bit of blotting-paper, the 

 specimen covered with a drop of melted jelly (taken out with a glass 

 rod), and a cover-glass put over it. The jelly is apt to liquefy by 

 absorbing moisture from the air, but this may be prevented by paint- 

 ing round the edge of the cover -glass with a cement, e.g. Canada- 

 balsam or gold-size. 



More lasting preparations are made by mounting in Canada-balsam, 

 but all water must be extracted first, by washing or steeping in 

 absolute alcohol, or in strong methylated spirit ; then the specimen is 

 treated with oil of cloves and (after removing excess of oil with 

 blotting-paper) mounted in a drop of balsam. 



Preparations of Spirogyra, Moss leaves, or other green parts will 

 often keep their green colour for a long time if the fresh specimens are 

 mounted in a drop of acetate of potash (strong solution) and ringed 

 with balsam or size. 



103. Staining 1 . Many staining substances are used in advanced 

 work, or to pick out the different tissues in sections of stems, roots, 

 etc. For instance, lignified (woody) cell-walls stain deep red with 

 safranin, which does not stain cellulose walls or only faintly ; hae- 

 matoxylm stains cellulose but not lignified walls. Instead of safranin 



1 See Slide No. 1 in Plant Biology Collection of Microscopic Slides. 



