166 H. von Mohl on the Formation of Gum-Tragacanth. 



walled parenchyma-cells, the membrane of which is coloured 

 bright violet by the iodized chloride of zinc solution. These 

 unaltered cells form, however, usually only a very thin stratum, 

 composed of but a few cells, while all the rest of the cells con- 

 stituting the central mass of the pith and of the medullary rays 

 are metamorphosed. 



That the peculiar character of these cells is a result of the 

 transformation of ordinary parenchymatous cells, and not an 

 original peculiarity of the cells of these parts, is shown by the 

 fact, that the pith and medullary rays of the extremities of the 

 shoots exhibit no unusual appearance. 



The metamorphosed cells are distinguished from ordinary 

 cellular tissue, in respect to their physical character, in that in 

 a dry condition they form a very hard, transparent, gum-like 

 mass, and in a wet condition a swollen slimy substance. Under 

 the microscope these cells display, when their transformation 

 has not advanced far, the angular forms and the close cohesion 

 of parenchymatous cells, but their walls are very thick, and 

 distinctly composed of many very thin laminae; their primary 

 membrane may be readily distinguished from the secondary 

 layers of thickening, and is not thickened, as is seen in particular 

 in cross-sections of the ' pits,' in which the primary membrane 

 lies free. The whole form of these cells, the distinct lamination 

 of their membrane, and the gelatinous softness of the latter in 

 a wet condition, impart to them a great resemblance to the 

 well-known cells of the cotyledons of i:ichotia. 



In this stage of transformation into gum-tragacanth were, 

 according to circumstances, either only those cells next in con- 

 tact with the unaltered layers, or those also which formed the 

 centre of the pith and the medidlary rays ; as I saw in young 

 stems of A. cyllenius, Boiss., which, according to Orphanides, is 

 one of those from which tragacanth is collected in Greece. 



If the metamorphosis has advanced a step further, the indivi- 

 dual cells swell up in a conical form in watei", and become more 

 or less completely isolated, retaining however their full integrity, 

 no slime exuding from them into the water being rendered visible 

 by the application of iodine. 



In the last respect, however, I found a striking exception in 

 some species {A. aureus, W. ; Pseudo-tragacantha, M.B, ; com- 

 pactus, W.; pycnocephalus, Fisch.),for the cells were surrounded 

 by an exuded slimy substance, apparently perfectly soluble in 

 water, which immediately assumed a beautiful indigo-blue colour 

 on the application of the iodized chloride of zinc. The same 

 colour was produced in the amorphous contents of the cells, as 

 also in the contents of the unchanged cells of the medullary 

 rays, and of a part of the cortical cells. But this coloration was 



