THE CARBOHYDRATES OF THE CACTI. 



47 



Of importance for the present consideration is the fact that the cells of 

 the cactus under normal conditions contain many minute starch grains. 

 However, in these mucilage cells the starch grains are either very few in 

 number or absent entirely. When the microscopic sections or pieces of 

 cactus 1 c. c. in size are washed, and then placed in water, the starch in the 

 mucilage cells disappears within 18 hours. The starch in the other cells 

 also disappears, though more slowly, 2 to 6 days, according to the material. 

 During the process there is no loss of cane-sugar or reducing sugar from 

 the plant into the water. In similar material, kept so that the water-content 

 of the plant does not change greatly, the starch remains for several weeks, 

 while, on the other hand, in the pieces of plant which are dehydrated, as, for 

 instance, partially drying in the air or over calcium chloride, there is an 

 increase in the starch in all cells. These facts are of importance in con- 

 sideration of the carbohydrate equilibrium to be discussed later. The 

 immediate source of formation of this mucilage has not been absolutely 

 established, though from Lloyd's observations it would appear that the 

 mucilage in some cases is a product of the TABLE 10 ._ L(m Bl/ evaporation 

 cell wall. This is important in connection 

 with the observations of MacDougal, Long, 

 and Brown on the formation of lacunae in 

 the starved Bisnaga already referred to. The 

 theory of the chemistry of mucilage and 

 pentosan formation will be taken up under 

 the chapter on respiration. 



Finally, it may be of interest to note some 

 experiments on the water-holding power of 

 the mucilage, as this has often been con- 

 sidered the chief function of these substances 

 in the succulent xerophytes. The mucilage had a specific gravity of 1.017 

 and was placed in wide-mouth weighing-bottles, of as nearly the same size 

 as could be obtained. Comparisons were made with tap-water. The bottles 

 were placed on a table 1 meter in diameter, which revolved once every 2 

 minutes. Weighings were made every hour. The difference, it will be 

 noticed, is very slight until, of course, the evaporation is retarded through 

 the formation of a slight film on the mucilage. 



of mucilage and water from 

 equal surfaces, in grams. 



