







' 









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IN YEGETABLE BIOLOGY. 



531 











































tannin side by side in the same tissue, viz. the epiderm of Sedum 



■Lelephi; 



turn. 



her 







thro 











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Group III.— The use of Nessler's fluid is especially valuable 

 •e, inasmuch as other reagents may not act within a reason- 

 able time. Strictly speaking, however, it is the caustic potash 

 which produces the effect ; indeed any alkali will give the yellow 

 colour with this form of tannin, as has been known for many 

 years. So far as I am aware, Wiesner * was the first to draw 

 attention to this matter, and he also noticed that this alkali- 

 yellowing tannin gives a green precipitate with iron salts. I 

 can confirm Wiesner on this point, as in every case in which the 

 yellow colour has been given with Nessler's fluid, iron salts have 

 vvn down a green or greenish-yellow precipitate. The chief 

 point to note is the light yellow colour immediately given with 

 fessler's flu id, as also with alkalies, and (what has not, it is believed, 

 been observed before) the diffusion of this coloured matter through 

 e cell- walls, so that after a few minutes the piece of tissue under 

 examination is surrounded by a yellow halo easily visible to the 

 naked eye. Take, as an example of the occurrence of this tannin, 

 the scales of the scape of JPetasites vulgaris; the following re- 

 marks apply to the epiderm alone, but tannin occurs elsewhere 

 as well. Nessler's fluid immediately brings out the yellow colour 

 in all the cells except the guard-cells of the stomata, and the tissue 

 soon rendered colourless for the reason just given. Osmic 

 acid within three minutes stains almost all the cells a pale slaty 

 grey, a colour which osmic acid always gives, at least as far as 

 one s observation goes, with the iron-greening tannin. Ferric 

 chloride immediately gives a dark green precipitate in all the cells, 

 and potassium bichromate acts pretty fairly after some time, half 

 an hour at least. Many cells are at once stained the beautiful 

 and characteristic colour with ammonium molybdate j but even 

 after two hours only groups of cells are coloured. Again, the 

 large stellate hairs closely set upon the axis and appendages at 

 th e growing-points of the Ivy— the ordinary reagents are not 

 nearly so satisfactory in this case as in the last, for, whereas the 

 action with Nessler's fluid is immediate, with ferric chloride a 

 couple of hours must elapse before the green precipitate is thrown 

 down. Ammonium molybdate will colour a few of the hairs 

 within ten minutes, but many remain unaffected a much longer 





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* Bou Zeit. 1862. 



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