I 50 BOTANICAL MICROTECHNIQUE. 



of compound ethers or of condensation- or polymerization- 

 products of various acids seems to have most in its favor. 



[Van Wisselingh has lately (I) found that most of the con- 

 stituents of suberin melt at a temperature below 100 C., 

 but are deposited in a substance that does not melt and 

 must first be removed. He finds that the suberin constitu- 

 ents are mostly soluble in cliloroform, and believes that they 

 consist of various fatty substances with glyceril or other 

 -compound ethers.] 



264. In this connection the optical relations of suberized 

 walls are worthy of attention, since they enable us to draw 

 some conclusions as to the form in which the substances in 

 question occur in the membranes. The suberized mem- 

 branes, as well as the cuticle, show a pretty strong double 

 refraction, and their optical axes are usually placed in the 

 reverse position to those of the pure cellulose wall. But 

 this double refraction disappears completely, as Ambronn 

 has shown (I), on heating to 100 C., reappearing as before 

 on cooling. This may be easily seen by heating cross- 

 sections of the leaf of Agave americana in glycerine until 

 the fluid boils and then examining them with a polarizing 

 microscope. 



The optical relations of cork clearly compel the view that 

 its double refraction is due to the presence of regularly 

 arranged particles of crystalline form, which melt on heating 

 and, on subsequent cooling, recrystallize in the same regular 

 arrangement. 



265. It remains to be determined by further studies 

 whether all suberized membranes have the same composi- 

 tion, and to what extent the external layers of the epider- 

 mal cells, the cuticle and the cuticular layers, agree in 

 material constitution with the suberin lamella of cork cells. 



266. As has been long known, suberized membranes, as 

 ivell as cuticularized ones, show the following relations to 

 chemical reagents : 



They are insoluble in cuprammonia, are never colored 

 blue or violet by iodine and sulphuric acid or by chloro- 



