164 H. von Mohl on the Formation of Gum-Tragacanth, 



my materials were so far imperfect that I possessed no thick 

 stems, only such as were at most as thick as a little-finger, such 

 as are found in herbarium specimens, on the bark of which no 

 exudation of tragacanth was visible. The sequel will show, 

 however, that this material was sufficient for the investigation of 

 the mode in which the gum is formed. 



The examination of excreted tragacanth is most instructively 

 made in thin flat pieces. A transverse section of such a plate, 

 swollen in water, exhibits abundance of thick- walled cells lying 

 in an amorphous slimy mass. The walls of these cells are 

 colourless, gelatinous, and formed of thick layers in part clearly 

 separated from each other, so that in this respect they exhibit 

 much resemblance to the laminated substance of a starch-grain. 

 In the cavity of the cells lies a more or less considerable quan- 

 tity of small starch-granules. Application of iodine is necessary 

 for the exact investigation of these cells. It acts upon them 

 very slowly, the iodized chloride of zinc solution producing no 

 other alteration for several hours beyond colouring the starch - 

 grains blue, and producing a slight yellow colour in the cell- 

 walls. Distinct coloration of the cell-walls only commences 

 after the solution has been in operation twenty-four hours or 

 more. The thin innermost layer of the cell-walls is then found 

 coloured bright violet, and isolated thin layers, of a more or less 

 bright violet colour, are observed in the thick swollen cell-mem- 

 brane, their thin, coloured laminae being separated from each 

 other by thick, colourless, gelatinous layers. The outermost of 

 these coloured laminae are frequently torn, in which case the 

 colourless gelatinous substance has partly exuded through the 

 rents, and become blended with the slimy substance in which 

 the cells lie imbedded. From this partial solution of the outer 

 layers, the magnitude of the cells, which are about '07 milli- 

 metre in diameter, cannot be determined accurately, and many 

 detached fragments of the blue iodine-coloured laminae occur, 

 scattered irregularly in the amorphous mucilaginous mass. 



In tragacanth of the form of vermicular filaments, the cells 

 were less perfectly preserved, and the amorphous mucilage in 

 which the lighter and darker violet membranes and starch- 

 granules were diffused, formed a relatively large portion of the 

 entire mass. 



In still smaller quantity did well-preserved remains of cells 

 occur in Syrian tragacanth, having the form of nodular, yellowish 

 fragments, in which, moreover, the quantity of starch-grains 

 was far greater and their size more considerable, the granules 

 being also frequently conglomerated in compound granules. 



From the investigation of tragacanth gum I proceeded to the 

 examination of the stems, which extended to all the following 



