264 H. von Mohl on the Investigation of Vegetable Tissue 



middle line corresponds to one of the neutral axes of the sele- 

 nite-plate are either blue or green, the other two yellow or red. 

 If the selenite-plate is rotated so far that its neutral axes are 

 perpendicular to the Nicol, all the colours are lost, and they 

 reappear on the continuation of the rotation, but in reverse 

 order, the quadrants previously blue now appearing yellow, and 

 vice versd. This alternation is repeated at each quarter of a 

 revolution. 



When the object used is the transverse section of a cellular 

 tissue with rectilinear side- walls, all the cell- walls which stand 

 perpendicular to one of the Nicols will exhibit the colour of the 

 field ; all those which run parallel with one of the neutral axes 

 of the selenite-plate, or form no great angle with it, will be blue, 

 and those parallel with the other axis yellow. When we direct 

 our attention to the concentric lamination of the annular cell- 

 membrane, and the rectilinear of the membranes of polyhedral 

 cellular tissue, we find that in both objects the same colour 

 occurs in the lamellse of the same direction. 



But if we compare (of course without changing the position 

 of the selenite-plate ; keeping also constant attention to the 

 identical direction of the lamination of the organ) vegetable 

 elementary organs of various kinds, in reference to colour, we 

 find that they fall into two classes, which are contrasted in re- 

 gard to the colours which they exhibit under the given circum- 

 stances. In one class all the layers which lie obliquely in the 

 direction of a right- wound screw, are coloured blue (or green), 

 those lying in the direction of a left-wound screw-line, yellow 

 (or red) ; in the second class the colours are opposite for the 

 same directions ; the organs of one class are optically positive, 

 those of the other optically negative. 



To the optically negative class belong the membranes of all 

 elementary organs situated in the interior of a plant, whether 

 they be left in their natural condition, or cellulose be purified 

 from the infiltrated substances by the help of nitric acid and 

 chlorate of potash. In this respect agree not only ordinary 

 cells and vessels, but even structures whose substance is assumed 

 by many chemists to be essentially diff'erent from cellulose, for 

 instance the medullary cells of Sambucus nigra, the mucilaginous 

 secondary layers of the hairs of the seeds of Acanthodium spi- 

 catum, the cells of Lichens and Fungi, collenchyma- cells, the 

 horny endosperm of Phytelephas, the cells of the cotyledons of 

 Schotia speciosa, the gelatinous cells of the Algse, for instance of 

 Bangia atj-opurpurea ; finally, among the parts lying nearer to 

 the surface, the fibrous cells of the envelope of the roots of 

 Orchidcse and Aroidese. 



On the other hand, optically positive colours are exhibited by 



