40 



THE CARBOHYDRATE ECONOMY OF CACTI. 



The polysaccharides are present mainly in the form of starch and also 

 as a mucilaginous substance of pentosan nature. As was explained in the 

 section on methods of analysis, it was unfortunately impossible to determine 

 the starch quantitatively. However, by means of the iodine reaction it 

 became evident that the starch is present in the form of very numerous 

 small starch grains. The Schimper chloral hydrate-iodine reagent proved 

 the most satisfactory for the detection of these. From the variation in size 

 and number of the starch grains a general idea could be gained as to the 

 amount of this substance, though, of course, this is not an accurate measure 

 and reveals only wide differences. It is, nevertheless, a reliable method for 

 the detection of the presence of starch and can be used (to a measure) to 

 indicate the fate of this material under the various experimental conditions 

 hereinafter discussed. 



TABLE 8. Results of analyses of 0. versicolor and 0. phwacantha in 

 percentages of the fresh and dry material. 



The method adopted for the hydrolysis of the polysaccharides has already 

 been described. In order to establish the nature of the carbohydrates in 

 this hydrolyzed portion, the following procedure was carried out: The 

 neutralized solution from the hydrolysis was evaporated down at reduced 

 pressure. The residue, which contained a great deal of salt, was extracted 

 with hot alcohol ; this alcohol solution was evaporated at reduced pressure ; 

 the residue was dissolved in a very little water and a large quantity of 

 alcohol added. Hereby more salt, as well as some gum, was thrown out of 

 solution. This was repeated twice, after which the sugar residue from the 

 evaporation of the alcohol showed but a trace of inorganic ash on ignition. 

 This residue contained all the sugars present in the cactus, hydrolyzed to 

 monosaccharides. It consisted of a clear brown gum. The aqueous solution 

 was treated with a small quantity of lead acetate; the insoluble portion was 

 filtered off; the lead in solution was precipitated with a slight excess of 

 sulphuric acid, and the lead sulphate was filtered off. The sulphuric acid 

 was then carefully removed by precipitation with barium hydroxide. After 



