68 



V. CELL-WALL, AND CELL CONTENTS. 



that nitrous salts do not occur in the living plant, and as the 

 other salts in question are at least very rare, we can fairly con- 

 clude that the reaction indicates the presence of a nitrate. If 

 instead of the somewhat dried section a fresh one is used for the 

 reaction, the colour which is formed is diffused far more rapidly 

 in the surrounding tissue, and the coloured zone is less sharply 

 delimited. If quite dry material is used, the reagent should be 

 in the form of a concentrated solution in concentrated sulphuric 

 ac id. While the diphenylamine reaction appears in such a 

 characteristic fashion in the sugar-beet, in other plants it may 

 fail, although nitrates are present ; so that if it fails we are not 

 justified in drawing a final conclusion as to the absence of 

 nitrates ; it is a positive, but not a negative, test. 



Structure of Cell-wall in Dahlia. As the next object of in- 

 vestigation we take the tubers of the Dahlia (D. variabilis). In 

 the tuber, halved longitudinally, we can readily recognise the 

 central pith, A longitudinal section prepared from this shows 

 under the microscope more or less rectangular cells, arranged in 

 longitudinal rows (Fig. 24) having a very reduced cytoplasmic 



sac, with nucleus, and colour- 

 less cell-sap. The inter- 

 cellular spaces are filled 

 with air ; the cell-walls 

 finely striate. The striae 

 are oblique, to the extent of 

 from 35 to 40. We appear 

 to see two diagonally op- 

 posed systems of striae in 

 the same plane ; this is ex- 

 plained by the comparatively 

 small thickness of the walls. 

 In fact, the two opposing 

 systems of striae belong to 

 the walls respectively of t wo 

 juxtaposed cells, as can be 

 determined especially at the free edges of the section. With chlor- 

 zinc iodine the striae soon colour violet ; the intermediate places, 

 where broad enough to see clearly, are manifestly uncoloured. A 

 cross-section of the same tissue shows on the end walls of the cells, 

 which we then see in surface view, no appearance of striation. It 

 shows only here and there larger rounded pits ; hence the neces- 



FIG. 24. -From the pith of Dahlia 



