304 MEMORANDA. 



Brewster also observed the remarkable fact that each particle 

 has a regular axis of double refraction. In the straw and chaff 

 of wheat, barley, oats and rye he noticed analogous pheno- 

 mena." (Quoted by Lindley from Grevill. Fl. Edinens., 214.) 



In Quekett's 'Treatise on the Microscope,' 3rd ed., p. 358, 

 directions are given for preparing the siliceous cuticle of 

 Equisetum hyemale for microscopic examination, by boiling in 

 strong nitric acid, and it is added that " in balsam it forms a 

 beautiful object for polarized light." Similar directions are 

 given for pi'eparing the silica in the chaff of wheat, oats, &c. 



As these statements are contained in the last editions of 

 each of the above-mentioned works, it is evident that no con- 

 tradiction of the error involved in them has been pointed out ; 

 yet, notwithstanding the high authority on which they rest, 

 the statements so far as the polarizing action of the silica is 

 concerned are wholly erroneous. If the cuticle of the above- 

 mentioned plants is completely deprived of its carbonaceous 

 tissues, it will be found wholly devoid of action on polarized 

 light, and any preparation of the cuticle which is found to 

 affect polarized light will also be found to blacken when 

 heated in concentrated sulphuric acid, and if then decar- 

 bonised by throwing into the hot acid solution a little chlorate 

 of potassa, the residual silica shows no signs of action under 

 the polariscope, either alone or with the selenite plate, 

 although it still retains the forms of the cells, stomata, &c. 



It is clear then that the error in the above statements has 

 been caused by the imperfect removal of the dense carbona- 

 ceous tissues which are deposited beneath the silica. I have 

 examined several species of Equisetum, and a large number of 

 plants of the grass tribe which are most remarkable for their 

 siliceous cuticles, but have found no trace of any action upon 

 polarized light, when the carbonaceous matter was removed. 

 But it is unnecessary to resort to artificial preparations to 

 prove the correctness of my statements. Nature has made 

 her own preparations, and deposited them by myriads beneath 

 every peat bog, where may be found not only the siliceous 

 shells of the Diatoms, and the spicules of the fresh-water 

 sponges, but also a large number of the siliceous parts of the 

 grasses, sedges, &c. Ehrenberg has shown, (Berlin Monthly 

 Reports, May, 1848,) and I can confirm his statements, that 

 the silica in these Fhytolitharia, as well as in the Diatomacece, 

 PolycistinecB and Spongiolites is not doubly refractive. He 

 makes an exception in the case of the shell of Arachnoidiscus, 

 but my own experiments prove that when properly cleaned 

 this shell forms no exception. As I have shown above that 

 the silica in the cuticle of the Equisetum and grasses, agrees 

 with that in the lower tribes in characters, I think the conclu- 



