124 



When .starch .L'lains are li-»\iti'<l lor a time with a small amount of 

 tlio ammoniafal silver soluticii to alUtw the silver salt to he evenly dis- 

 trilmted throu.nhont the grain, and tlien an equal amount of the tartrate 

 solution added, the siher salt is reduced l).v the granulose present in tlie 

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

 to l)lack, depending upon the stiite of reduction. Reduction occurs in a 

 few iiours at ordinary temperature, or may be (piickly accompli.slied by 

 warming very slightly although too strongly heating causes complete reduc- 

 tion of the solution and total blackening of tlie grain. 



If it i.s desired to make permanent mounts by the above metlnuk the 

 starch sliould be stained in bullc until the proper depth of color is reached, 

 and then the soluble and unreduced silver salt washed out with water t<> 

 prevent further reduction. The grains may then be dehydrated and 

 mounted permanently in lialsani. 



When starch grains are treated according to the above method, alter- 

 nate layers and the central portii)n reduce the silver solution most rapidly, 

 showing the presence of reducing substances wliicli cause a deposit nf 

 silver". Fig. 2. The alternate layers which stain most deeply with iodine 

 solution take on a granular appearance, and show the presence of cryst<ils. 

 The location and size of these crystals become more distinct upon further 

 treatment of the tnount with I'/o chromic acid solution. Then they are 

 seen to be of considerable size in some grains, and often tn lie arraiezed in 

 ;i group with their longer axes radiating IriiUi the liiluni. (Juite large- 

 crystals occur toward the outer portion of each layer, and their presence 

 accounts for the difference in refraction of the layers at tliis point, which 

 produces the aiipeai'aiice of concentric rings. Fig. .'{. 



In (bmble or compound grains a crystalline mass is seen at the center 

 of each ]>art, indicating that compound grains are formed by the deposi- 

 tiim of material aremnd a number of jioints ol crystalli/.ntinn. and tlu' sub- 

 scnpient growth of each part until fusion occurs. In the outer \;\\rv of the 

 grain no crystals appear, and this ])ortion .seems to be made up for the 

 greater pai't of starch cellulose, which e.xidains its It^ssi-r solubility in dia- 

 sfatic S(>lutions. Fig. 3. 



An aiiplieation of the differences in tlH> structure of starch grains is of 

 value ill the ex;iniiiiat ion nf such clos<'ly I'clated species as the cmiiinoii 

 sarsa|»arillas of the giMuis Smilax. .\11 the members ol' this Lcrouii. which 

 arc; C(Mnmonly met with, have similar histological structures. .Vll show the 

 lu-cseiicc of i-M|iliiiles crystals of calciinii oxalate and parenchyma cells 



