332 PHYSIOLOGY OF NUTRITION. 



in 10, Fig. 11. We dip the lower end of the eudiometer in water, 

 close the apparatus, and place it in the dark. After some time 

 (e.y. twelve hours) we find that the water has risen considerably 

 in the tube, while this is not the case in parallel experiments 

 in which young stems of non-succulent plants (e.g. Helianthus) 

 were introduced into a second eudiometer. The pieces of Rochea 

 leaf carry on not only normal respiration, but at the same time 

 vinculatory respiration. They absorb, in fact, much Oxygen, 

 without corresponding production of Carbon dioxide, for the 

 conversion of carbohydrates into organic acids. 



In exact quantitative investigations concerning the inspiration 

 of Oxygen by succulent plants we must naturally employ mercury 

 for closing the eudiometer. The method of procedure will be 

 obvious from what was said in 13. 



1 Literature : A. Mayer, Landwirthschl. Versuchastationen, Bd. 18 and Bd. 

 21 ; Detmer, Pringsbeim's Jahrbiicher, Bd. 12, and Lehrbuch tier Pftanzcn- 

 physiologic, 1883 ; H. de Vries, Verslagen en Mededeelingen der Koninkl. 

 Ahadem. van Wetenscliappen, 1884; G. Kraus, Abliandlungen der Naturfor- 

 schenden (resellschaft .~a Halle, Bd. 16; Warburg, Uiitersuchun(/en a. d. botan. 

 Institut zu Tiibirif/eii, Bd. 2. The views whicb I have expressed in my cited 

 papers on tbe subject before us are essentially different from those here 

 advocated by me. 



132. Gums and Mucilages. 



Gum arabic (the product of various sorts of Acacias) consists 

 chiefly of Arabic acid. If we treat some gum arabic in a watch- 

 glass with iodised Potassium iodide solution, and then add 

 Sulphuric acid, the mass only assumes a brown coloration. All 

 the true gums behave in this manner, while mucilages take on a 

 violet or blue coloration on treatment with Iodine and Sulphuric 

 acid. According to the researches of Von Mohl, gum tragacanth is 

 produced by the disorganisation of the cells of the pith and medul- 

 lary rays in different kinds of Astragalus. Gum tragacanth is not 

 a homogeneous mass, as we can see at once if we treat the com- 

 mercial and powdered substance with a large quantity of water. 

 A solution is produced which on evaporation yields a colourless 

 glassy mass, gum tragacanth proper, and a sediment which under 

 the microscope is seen to consist of starch-grains and fragments of 

 cell-walls. The amount of cell-membrane incompletely converted 

 into gum varies in different kinds of tragacanth. 



Gum reservoirs are readily detected if we examine sections 



