QUANTITTAIVE MICROSCOPIC DETERMINATIONS 167 



Rhamnus purshiana bark as the admixture. The rhamnus bark 

 must be reduced to the same fineness as the cinchona bark and the 

 mixing must be thorough. Make three admixtures. One of fifty 

 parts of cinchona bark and fifty parts of rhamnus bark; the second 

 mixture of seventy-five parts cinchona and twenty-five parts rhamnus; 

 the third mixture of ninety parts cinchona and ten parts rhamnus. The 

 usual suspensions of those several mixtures are made and the counts 

 made as for part one of this exercise. The results should be compared 

 and discussed. Keep the admixtures for problem two. 



2. Rhamnus Purshiana Bark. Prepare an absolutely pure average 

 sample of Rhamnus purshiana bark as for cinchona and make a count 

 of the bast tissue, counting each group of bast cells as one. The scle- 

 renchyma cells (individuals and groups) are not counted as they are 

 rather difficult to recognize and the counts based thereon would only 

 cause confusion and would give widely varying results. 



Use the mixtures prepared for the cinchona counts and from the 

 counts obtained therefrom determine the percentages of rhamnus bark 

 present. Do not ascertain the rhamnus percentages by merely 

 utilizing the counts of the cinchona bast cells as obtained from problem 

 one . Compare the errors in the two problems from the same admixtures 

 and it will be found that the variation in results will be somewhat 

 greater for problem two, although the error should not exceed 3 per 

 cent., even for beginners in this kind of work. 



3. Senna Leaflets. Prepare two samples. One of pure whole or 

 broken leaflets of Cassia angustifolia and one of pure whole or broken 

 leaflets of Cassia acutifolia. Make the 1-5 suspensions and determine 

 the number and the length of the simple single-ceiled trichomes 

 present. In the C. angustifolia the trichomes are fewer and longer 

 as compared with those of C. acutifolia. Also search for epidermal 

 tissue fragments and note the number and size of the neighboring 

 cells of the stomata in the two varieties of sennas. 



Make a fifty to fifty admixture of the two sennas and determine the 

 amount of each kind of senna based upon the count of the trichomes, 

 utilizing the following suggestion. The count of those trichomes 

 which exceed in length the longest cells of the C. acutifolia represents 

 C. angustifolia (100 per cent.). In this exercise the differences in 

 result will be found to be considerable. 



Make admixtures of each of the two samples with wheat flour and 

 determine the amount of such admixture from the trichomatic counts. 

 In this case the difference in the results should be slight. This par- 

 ticular problem will also serve as a preparation for problem ten. 



Make admixtures of pure senna leaflets and of senna stems and 



