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



perfectly colourless or tinged with a very light violet colour. 

 A similar condition is well known to occur frequently in other 

 thick - walled parenchyma-cells which become softened and 

 greatly swollen when placed in water ; for instance, in those of 

 Schotia. 



The further the dissolution of the cells and their conversion 

 into gum-tragacanth proceeds, the lighter is the violet colour 

 assumed by the entire mass of them, since the uncoloured or 

 slightly tinted laminae more and more exceed in proportionate 

 dimensions; and even the coloured laminae, especially the outer, 

 perhaps simply as a result of greater mechanical expansion, 

 exhibit a lighter colouring. 



The described observations will leave no doubt of the fact 

 that gum-tragacanth is neither a secreted sap dried by exposure 

 to the atmosphere, nor an independent Cryptogamic organism, 

 but that its formation depends upon a more or less perfect 

 transformation of the cells of the pith and medullary rays into 

 a gelatinous mass, which swells up to many hundred times the 

 original size of the cells when placed in water. 



Whether the production and expulsion of the gum occur but 

 once in one and the same part of the stem, or are repeated during 

 many years, can of course be determined only in the native 

 country of the tragacanth-plants ; perhaps, however, the con- 

 jecture that it is a process persisting through an extended period 

 is not too bold. The transformation of the pith can of course 

 only occur once in any given part of the stem, and this source 

 will be extinguished with the earlier or later expulsion of the 

 gum formed. But the case may be otherwise with the medul- 

 lary rays, since all the medullary rays of any particular part of 

 the stem do not undergo their metamorphosis at the same time. 

 In the younger of the stems examined by me, at least only a 

 portion of the medullary rays had undergone this change, while 

 the rest still displayed the usual constitution of thin-walled cells. 

 It may be indeed assumed, from the great firmness of the peri- 

 derm clothing the stem, that the breaking-out of the gum through 

 the bark takes place annually from only a small portion of the 

 medullary rays, and thus perhaps goes on for many years, until 

 all the medullary rays of a length of the stem have been 

 emptied. 



Surveying the vegetable kingdom, with a view to ascertain 

 whether analogous conversions of cells into mucilage occur 

 elsewhere, we find similar processes to be anything but rare 

 Attention has been especially directed by Alex. Braun* to the 

 fact of an exactly corresponding softening of the cell- membranes, 



* Vcrjungimg, p. 203. Ray Society's Memoirs, 1853, p. 18J>. 



