H. von Mohl on the Formation of Gum-Trag acanthi 165 



species of the section TragacantfuE : Astragalus angustifolius, Lam.; 

 aristatus, L'Herit. ; Anacantha, M. B. ; aureus, Willd. ; Barba 

 JoviSjjy.C'y brevifloruSf J). C.'j bunophilus, Boiss. ; campylanthus, 

 Boiss. ; caucasicus, M. B. ; cephalanthus, D.C. ; chromolepis, 

 Bohs.; compactuSjW,; creiicits,Ij3im.; c^/Z/emW, Boiss. et Heldr. ; 

 denudatus, Stev. ; echinoides, L^Herit. ; Echinus, D.C. ; erianthus, 

 W. ; gossypinus,Yi^c\i.', lagopodioides, Yahl; leiocladus, Boiss.; 

 massiliensis, L. ; microphysa, Boiss. ; murinus, Boiss. ; persicus, 

 Fisch. et Mey. ; plumosus, W. ; Pseudo-tragacantha, M. B. ; 

 ptychophyllus, Boiss.', pychocephalus, Fisch.; pycnophyllus, Stev.; 

 sciureus, Boiss. ; siculus, Biv. ; susianus, Boiss. ; tumidus, W. 

 Among these occurred only four species in which no tragacanth- 

 formation could be found in the stems, namely A. aristatus, 

 L'Herit. (from the Pyrenees), massiliensis, L., angustifolius, Lam., 

 and echinoides, L'Herit. In all the rest, tragacanth had made 

 its appearance in more or less abundance*. 



The structure of the stem in general is as follows. The wood 

 is composed of thin annual layers, extremely tough, tearing 

 easily in a longitudinal direction into slender fibres, enclosing a 

 small pith, and traversed by pretty numerous medullary rays, 

 exhibiting nothing unusual, as is the case with the bark also, 

 which contains a well- developed liber and is clothed by a dense, 

 tough periderm. The pith, on the other hand, and a large por- 

 tion of the medullary rays, have a most striking characteristic, 

 for, instead of presenting a thin-walled parenchymatous tissue, 

 they appear to the naked eye in the form of a hard, transparent, 

 gummy mass, and swell up into a jelly in water. On cut sur- 

 faces of the stem we frequently find a projecting mass of dry 

 gum, which has exuded from the pith-cavity. 



When we have recourse to the microscope, we perceive im- 

 mediately that the gum-like mass which fills up the pith-cavity 

 and the medullary rays, or has exuded from cut surfaces, does 

 not consist of dried mucilage, but of the cells of the pith and 

 medullary rays themselves, which have undergone a more or 

 less complete transformation into gum-tragacanth. 



Ordinarily this transformation has not affected all the cells of 

 the pith and the medullary rays, but the outermost layers of 

 the medullary rays, next to the wood-cells, regularly, and not 

 unfrequently, in like manner, the outer part of the pith, lying 

 against the woody bundles, are composed of ordinary thiii- 



* How far the subsequently described conversion of the cells into gum- 

 tragacanth occurs in the species of Astragalus belonging to the other 

 sections of the genus, I have not specially investigated ; I may remark, 

 however, that I observed the same transformation also in a couple of spe- 

 cies of the section Incani which I took out at random, viz. A. br achy carpus ^ 

 M.B., and A. angulosus, D.C. 



