32 SCLERENCHYMA 



if the application of iodine is followed by that of strong sul- 

 phuric acid. The acid causes swelling of the wall, and this 

 is followed by complete solution, the blue colour with iodine 

 being due to one of the products. Cellulose-walls are likewise 

 dissolved by ammoniated copper oxide (cuprammonia, see 

 Appendix II). A blue colouration and swelling also results from 

 the application of chlor-zinc-iodide (Schultze's solution, Appen- 

 dix II), but it is necessary to use the freshly made compound, 

 as it soon decomposes. For general purposes useful stains for 

 cellulose walls are methyl blue and haematoxylin (Appendix VI), 

 the latter giving a deep purple colour. 



The remaining mechanical elements form tissues spoken of 

 as sclerenchyma. They are distinguished from collenchyma by 

 a chemical change of the thickened walls known as lignification, 

 in consequence of which the cell-contents die. Such lignified 

 walls are much harder and stronger than those consisting of 

 cellulose, features which are the cause of the toughness of wood. 

 They possess relatively little elasticity, and when wet are not 

 readily permeable to water and practically impermeable to air. 

 The exact nature of the chemical change is not known, but it 

 is sufficiently profound to lead to marked differences in reaction. 

 Thus such walls are insoluble in cuprammonia, whilst the lignin 

 is dissolved by Eau de Javelle (Appendix II). Iodine solution 

 stains lignified walls brown, and a similar colouration is obtained 

 with chlor-zinc-iodide. Aniline chloride, or sulphate, which does 

 not affect cellulose-walls, yields a brilliant yellow colouration 

 and, as it does not stain starch, is particularly useful when 

 that substance is present in quantity. An alcoholic solution of 

 phloroglucin, followed by strong hydrochloric acid, produces a 

 scarlet colouration. Lignified walls stain yellow or brown with 

 haematoxylin, and are deeply stained by gentian violet. The 

 latter is most effective in conjunction with Bismarck brown, the 

 wood and other sclerenchyma becoming violet and the ordinary 

 cell-walls brown. 



There are two principal types of sclerenchymatous elements, 

 viz. stone-cells and fibres ; of these the former are more or less 

 parenchymatous, the latter prosenchymatous in shape. The 

 gritty character of the flesh of the Pear is due to stone-cells 

 which, in a thin section, will be found to occur as little clusters 



