I7 2 BOTANICAL MICROTECHNIQUE. 



as Canada balsam), a part dry and a part moist. But it 

 should be observed that the complete removal of water can 

 only be accomplished by drying at a temperature of 50 to 

 100 C., according to the nature of the object. The dehy- 

 drating media used by various authors for the same purpose, 

 especially absolute alcohol, do not give demonstrative results 

 (cf. Zimmermann I, 87). 



297!. It is also to be noticed that, where the water-content 

 is unequal, changes in form must occcur on drying, and 

 therefore, as Correns (III, 262) has specially observed, a cer- 

 tain distinction between differentiations due to sculpturing 

 of the wall and those due to unequal water-content cannot 

 always be drawn from the comparison of dried and moist 

 membranes. Concerning the possibilities in this respect, 

 Correns' work (III) may be consulted. 



297111. The presence of differences in water-content was 

 demonstrated by Correns (III, 294) by impregnating the 

 membranes with a salt-solution (NaCl), which is not recog- 

 nizably accumulated. The conclusion is then justified that 

 where the salt occurs in greater quantity this is in conse- 

 quence of greater water-content. Correns used for this 

 purpose potassium ferrocyanide and silver nitrate, and made 

 the salt contained in the walls visible by conversion into 

 .a colored precipitate. 



Concerning the method of using the potassium ferrocy- 

 anidC) it may be observed that Correns placed the mem- 

 branes, previously washed in water and then dried by 

 warming to 50 to 100 C., for a few minutes in a 10$ 

 solution of the substance and then placed them, after super- 

 ficial drying on filter-paper, but without washing, in a dilute 

 solution of ferric chloride, in which the formation of Berlin 

 blue at once takes place. In the bast-cells of Nerium or 

 Vinca, which are especially suited to these experiments, the 

 striation becomes clearly visible with this treatment, even 

 after drying and mounting in cedar-oil and Canada balsam. 



Correns (III, 295) was able to show that the potassium 

 ferrocyanide is merely absorbed, but not accumulated, by 

 placing dry starch-grains in a solution of this salt, and testing 



