H. von Mohl on the Formation of Gum-TVagacanth. 167 



transitory, for in the course of a few hours tlie blue colour had 

 entirely vanished, and a yellowish tint remained. This phaBno- 

 menon again reminds us of the cotyledons of Schotia, from which 

 in like manner exudes a slimy substance coloured bright blue 

 by iodine and coagulable by alcohol. In other cases, in parti- 

 cular in A. ci/Uenius, the metamorphosed cells discharged into 

 the water a slimy substance coloured yellow by iodine; both 

 this, however, and the slime coloured blue by iodine, are excep- 

 tional. 



AVhen the transformation of the cells into gum-tragacanth 

 has advanced farther, the membrane which swells up strongly in 

 water is no longer distinctly seen to be composed of numerous 

 thin laminae. This conversion into a mass, of homogeneous 

 aspect, advances from without inwards in the cell-membrane ; for 

 I saw (in Astragalus murinus) cells in which this affection had 

 attacked the outer half of the cell-wall, whde the inner half^ 

 separated from the outer homogeneous mass by a sharp line, 

 still displayed the fine lamination. 



The final conversion into perfect tragacanth results when the 

 cells lose their external solid boundaries, and their outer laminse 

 coalesce into a more or less uniform slimy mass ; in which, as is 

 frequent in exuded gum, the inner lamiuse may still exist in 

 perfect integrity. 



The cells metamorphosed in the above-described manner ex- 

 hibit, at least when wetted, a considerably greater diameter than 

 the thin-walled cells from which they originate : thus, one of the 

 large unaltered cells of the medullary rays of Astragalus denu- 

 datus is '0064 of a millimetre in diameter, while a metamor- 

 phosed but still distinctly defined cell from the interior of the 

 same medullary ray measures "OSo millim., that is, about five 

 times the size ; in A. Echinus the size of the metamorphosed 

 cells of the pith amounted to "06 of a millimetre, and had thus 

 reached about the magnitude of the cells contained in tragacanth 

 which had exuded from the stem. 



The behaviour of the cells with iodine alters in proportion to the 

 degree of metamorphosis they have undergone. The unchanged 

 cells of the pith and medullary rays assume a deep violet colour 

 with the iodized chloride of zinc solution in the space of twenty- 

 four hours. The cells which have been only slightly altered, 

 and have still the form of angular but thick-walled prosenchy- 

 matous cells, are likewise coloured deep \'iolet. But this colour 

 is not uniform throughout the whole thickness of the cell-wall, 

 the innermost and outermost laminae being especially brightly 

 coloui'ed, while soUtary thin lamellae among the secondary 

 layers exhibit a violet tint. I could not determine whether 

 the uncoloiu'ed laminae between these coloured layers were 



