SPECIAL METHODS, I/I 



from true differentiations of the membrane without much 

 difficulty, and often accompanies these. 



Striation due to sculpturing of the membrane is, plainly, 

 only visible when the membrane and the mounting fluid 

 have different refractive indices. And it becomes the 

 plainer as this difference is the greater, disappearing en- 

 tirely as the refractive- indices become equal. For example, 

 Canada balsam has almost the same optical density as cell- 

 walls ; on the other hand, Correns used methyl-alcohol with 

 good results, on account of its low refractive index. This 

 is only 1.321 and therefore less than that of water (1.336). 



2971. Further, it is evident that it is unimportant in case 

 of differentiations depending wholly on sculpturing of the 

 wall, in opposition to those which are to be referred exclu- 

 sively to unequal water-content, whether the membranes 

 are placed in a mounting fluid of similar refractive index in 

 a dry or swollen state. But the use of this criterion en- 

 counters difficulties, as Correns has shown (III, 260), if rifts, 

 or canals in the interior of the membrane are involved, as in 

 the bast-cells of Nerium, where the different layers have 

 different systems of striation. In this case it does not seem 

 practicable to fill these capillary spaces with ethereal oils or 

 with balsam without the removal of imbibed water. But 

 even in this case, the behavior of the dried membranes on. 

 being imbedded in Canada balsam or the like may permit 

 positive conclusions as to the nature of the differentiations, 

 in question, since only a slow expulsion of the enclosed air 

 from capillary spaces in the interiors of membranes can take- 

 place. 



For distinguishing water-bearing clefts from substances 

 rich in water, chloroiodide of zinc and various dyes may be 

 used. Clearly, the capillary spaces must always stand out as 

 colorless streaks on suitable sections, while the parts richer 

 in water may show a more or less deep stain. 



2Q7k. Differentiations due to unequal ivater-content must, 

 plainly, disappear on drying, as a rule. The presence of 

 such differentiations may therefore be recognized by exam- 

 ining the objects in the same anhydrous mounting fluid (such 



