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



rayon), and as it was gradually driven out on the surface of the 

 stem by fresh sap taken up by the roots, hardened in the air 

 in the shape of worms. He adds further, the conjecture that 

 contraction of the fibres of the stem assists in pressing out the 

 gum, since the fibres, broken down into stringy masses, where 

 exposed to be trampled on by sheep and horses, were contracted 

 by heat, which would favour the emission of the sap. 



These statements agree in many points with those of Olivier*, 

 who observed the secretion of tragacanth from Astragalus verus 

 in Persia. 



In this species, also, the stem attained a thickness of an inch 

 or more, and the gum exuded at the time of the greatest heat 

 of summer, partly where the bark was split by the pressure of 

 the sap, partly where the stem was injured by the tread of 

 animals. 



Further confirmation as to the season at which the tragacanth 

 exudes from the stem is furnished by the statements of Labil- 

 lardiere and Landerer, the former of whom saw the gum ex- 

 creted from Astragalus gummiferj Labili., in August, on Mount 

 Lebanon; the latter, from Astragalus aristatus, in Greece, in 

 August and September. In like manner, the statement that 

 wounding the plants favours excretion of tragacanth, derives 

 confirmation from the circumstance that in the district of Bitlis 

 it is customary to make incisions in the plants for this purpose f. 



LabillardiereJ mentions as an additional external circum- 

 stance favouring the exudation of tragacanth, moisture of the 

 atmosphere, stating that, in Lebanon, cloudy nights and abun- 

 dant dew are necessary for the excretion of the gum, and that 

 this only exudes in quantity during the night, and for a short 

 time after sunrise, — whence it happens that the shrubs growing 

 on the lower part of Lebanon, which are exposed to very great 

 heat by day, but receive little moisture at night, yield only a 

 small quantity of tragacanth. 



These statements of Labillardiere are confirmed by the ob- 

 servations of Fraas§ in Greece, who states that no gum exudes 

 (from Astragalus aristatus and creticus) on the higher mountains 

 of the Peloponnesus, on Parnassus, or on dry mountains in ge- 

 neral, while the gum is collected in Achaia. He regards the 

 excretion of the gum as dependent on atmospheric influences, 

 and ascribes it to the abundant cold rain with alternations of 

 great heat, in the mountains of Calaryta, &c. 



Labillardiere drew, from the facts he observed, the conclu- 



* Voyage dans TEmpire Othoman, 1807- 



t Ritter, Erdkunde, x. 689. 



X Rozier, Observations sur la Physique, &c., 1790? xxxvi. p. 48. 



§ Synops. Plant. Florae Classicse, p. 39. 



