26 THE CARBOHYDRATE ECONOMY OF CACTI. 



III. EXPERIMENTAL METHODS. 

 SELECTION AND PREPARATION OF MATERIAL. 



The greater part of the work here recorded was done with Opuntia pJicea- 

 cantha, known locally as Opuntia blakeana. 1 Other species were also used, 

 but the results do not differ materially from those obtained with this plant. 

 In working with the cacti considerable difficulty is encountered on account 

 of the large amount of mucilaginous substances therein. However, this in 

 itself is a subject of great interest, the nature, source, and function of these 

 slimes being still unknown and to a greater or less extent forming a char- 

 acteristic property of all succulent plants. Furthermore, the joints of 

 Opuntia phceacantlia are of a size that can be most easily handled for the 

 various experimental procedures, and also grow in abundance on the labora- 

 tory domain. On account of the smaller amount of mucilaginous substances, 

 Opuntia versicolor 2 was used where it was necessary to press out the juice. 

 Such a procedure is quite impossible with Opuntia phceacantlia, especially 

 during the dry seasons. 



Each series of determinations or experiments was carried out with 

 material from one plant. Although there are naturally differences among 

 individual plants, the changes which these undergo with season and under 

 experimental conditions are entirely parallel. Large, healthy plants have 

 a sufficient number of joints of the same age, so that extended experiments 

 can be conducted with material from the same plant and a number of joints 

 can be used for each determination. For the work on sugar estimation the 

 material was collected between 10 and 11 o'clock in the morning. The 

 joints were removed by cutting at the base with a sharp knife. The cut 

 surfaces were 2 to 3 square centimeters, and no loss of juice or other injury 

 was ever observed therefrom. The joints were immediately taken to the 

 laboratory, and the spines were removed with sharp pruning shears, care 

 being taken not to cause any injury. 



The cacti offer splendid material for investigations of this nature. All 

 the year fresh and normal material can be obtained in abundance; it is 

 notably resistant to lack of water, as well as to variations in temperature, 

 and can be kept in the dark for months without fatal results. Nevertheless 

 all these conditions effect decided changes in the plants. Furthermore, they 

 can be easily grown, even in nutrient solutions, and if necessary a single 

 joint offers ample material for complete analysis. 



PREPARATION OF MATERIAL FOR ESTIMATION OF SUGARS. 



All material, whether it had been subjected to experimental conditions 

 or taken immediately from out of doors, was prepared alike for analysis. 

 The joints, freed from the spines, were cut into three or four strips and 



1 BRITTON, N. L., and J. N. ROSE. The Cactacese: Description and illustration of 

 plants of the cactus family. Carnegie Inst. Wash. Pub. No. 248, vol. i, p. 144. 

 ' Ibid., p. 62. 



