124 



When starch grains are treated for a time witli a small amount of 

 the annuoniacal silver solution to allow the silver salt to he evenly dis- 

 trilinted throughout the grain, and then an equal amount of the tartrate 

 sohithm added, the siher salt is reduced by the grauulose present in the 

 grain, giving stains varying in color from purple through shades of brown 

 to black, depending upon the state of reduction. Keduction occurs in a 

 few hours at mdinary temperatuie, or may be tiuickly accomplished by 

 warming very slightly although tito strongly heating causes complete reduc- 

 tion of the siilution and total black<>ning of the grain. 



If it is desired to make permanent mounts by tlie above method, tlie 

 starch should be stained in bulk until the proper depth of color is reached, 

 and then the soluble and unreduced silver salt washed out with water to 

 prevent further reduction. The grains may then be (leliydrate<l and 

 mounted permanently in balsam. 



When starch grains are treat»'d according t<i the above method, alter- 

 nate layers and the cciitral jiortion i educe the silver solution most rapidly, 

 showing the presence of reducing substances which cause a dei>osit of 

 silvei*. Fig. 2. The alternate layers which st.-iin most deeply with iodine 

 sohitiou take on a gr.innjai' appearance, and show tlie presence of crystals. 

 The location and si/c oT these ci'vstals become more distinct uiion furtlier 

 treatment ol the monnt with ]'/, chromic acid solution. 'I'jien they are 

 seen ti> lie of considei able size in some grains, and often to be ai'ran'.ied in 

 a group with their longer .i.xes radiating- frcm the hilnni. (Jnite l.-irge 

 crystals occin- towai'd the outer jiortion of each layer, and their piesence 

 accounts lor the dilTerence in rel'racti<)n of the layers at this point, which 

 produces the apiieai'.-ince of concentric rings. Fig. .*!. 



jU (lonlile or coinpnund grains a crystalline mass is seen al the center 

 of each part, indicating that com]iound gi'ains are formed by the (le])osi- 

 lion of material around a number of points ol crystallization, and the sub- 

 s(N]uent growth of each part until fusion occurs. In the outer layer of the 

 giMin no cr.\stnls ajipear. and this poition seems to be made u)! for the 

 greater part of starcli cellulose, which exiilains its lesser solubility in dia- 

 static solutions. Fig. 3. 



\u :ipplic;ition of the differences in the structure of starch grains is of 

 value in tlie examination of such closely related sjiecies as the conunon 

 sarsajvarillas of the genus Smilax. All the members of this grouii, whieli 

 are connnonly met with, have similar histo](tgic;il structures. All show the 

 ]ii-esence of raphides ci'ystals of calcium oxalate and parenchyma cells 



