66 



V. CELL-WALL, AND CELL CONTENTS. 



iodine, the thin parenchyma-cells take on gradually, but even 

 more slowly than in the beetroot, a violet colour ; the strongly 



thickened cells ra- 

 pidly become yel- 

 low-brown. These 

 latter are therefore 

 lignified, and be- 

 long, on account 

 of their strong 

 thickening and lig- 

 nification, to the 

 sclerenchyma or 

 mechanical tis- 

 sue, The struct- 

 ural characters of 

 the thickened cells 

 become especially 

 clear under treat- 



FIG. 23. From the flesh of the fruit of the Pear. Strongly- l^ent with chlor- 

 thickened cells with branched pore-canals, surrounded by zmc i o di ne . 

 thin-walled parenchyma ( x 240). 



Reactions of 



Sugars. We will use the flesh of the pear in order to test the 

 micro-chemical reactions for sugar, Most commonly used is 

 that obtained with Fehling's solution. For this purpose we 

 prepare three solutions, of which one contains 35 grammes of 

 sulphate of copper, the second 173 grammes of potassio-sodic 

 tartrate (Kochelle salt), the third 120 grammes caustic soda, 

 each dissolved in a litre of water ; or smaller quantities in like 

 proportions. One volume of each of these solutions must 

 be mixed, for use, with two volumes of water. The separate 

 solutions can be kept ready prepared, while when mixed they 

 in time undergo change. The reaction can be carried out upon 

 an object-slide. For this purpose we place upon it a large drop 

 of distilled water, and three small drops of the ready-made 

 solutions, and mix them with a glass rod. The section on which 

 the reaction is to be tried must not be too thin, should contain 

 at least two layers of uninjured cells, and naturally should not 

 previously have been laid in water. The section should not 

 therefore be cut till the Fehling's solution is ready mixed, and 

 should then be laid immediately in it and covered with a cover- 

 glass. The object-slide can then be warmed over a spirit lamp or 



