REACTIONS OF CELLULOSE. 65 



and stand out specially distinctly. These pit-surfaces are always 

 rounded, of variable size, and irregularly distributed, singly or in 

 groups. Large pit-surfaces are generally traversed by violet striae 

 of various breadth ; they are formed into compartments by them, 

 and give the impression of an irregular lattice. Bright granules, 

 coloured yellow-brown by the chlorzinc iodine, adhere in larger 

 or smaller quantity to the pit-surfaces. This violet coloration 

 with chlorzinc iodine is a specific, though often slow, reaction for 

 cellulose, which is not due to the pectin materials, and ceases 

 when the cellulose has been previously removed from the cell- 

 walls by prolonged action of ammonium cupric-oxide. (As to the 

 action of this reagent see posted, p. 72). 



For the purpose of comparison we now try the cellulose 

 reaction with iodine and sulphuric acid. The section is first 

 impregnated with iodine solution, best with potassium -iodide 

 iodine solution (1 per cent, iodine and 1J per cent, potassium 

 iodide), and afterwards transferred to dilute sulphuric acid (2 

 volumes acid to I volume water). The reaction commences at 

 once, from the edges inwards ; the section takes a beautiful blue 

 colour. The lesser pits here also remain uncoloured ; the larger 

 ones appear latticed with blue. 



Sclerenchyma in the Pear. We now prepare a section from a 

 ripening Pear. The pulpy flesh of the fruit appears here also to 

 be composed of regular thin-walled parenchyma, of large cells, 

 more or less rounded at the angles. These cells contain 

 colourless cell-sap, a very reduced plasma-sac, and a nucleus. 

 Scattered in the tissue are found nests of strongly-thickened cells 

 (Fig. 23). The number of the " stone cells " thus grouped varies 

 from part to part, according to the kind of pear ; they form the 

 so-called "grit" of the pear. The cells are distinguished by 

 the notable thickness of their walls, and by the numerous, fine, 

 branched deep pits or pore-canals. Their branching arises from 

 the diminution in the number of the pore-canals as the cavity of 

 the cell is reduced by the great increase in thickness of its walls, 

 so that several open into the cell-cavity by a common canal. 

 Where two thickened cells are in contact, it can be determined 

 that the pore-canals coincide in position with one another. In 

 their fully developed condition, in which we see them here, 

 these cells no longer contain living cell-contents, but only a 

 watery fluid. They represent, therefore, only dead cell-cases, 

 or cell-skeletons (exo-skeletons). After treatment with chlorzinc 



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