hy the aid of Polarized Light. 207 



masses which these substances form in the swollen- up condition, 

 the position of the neutral axes can no longer be discovered, but 

 certain not clearly defined parts of the substance appear light, 

 others dark, and on horizontal rotation of the object become 

 alternately light and dark. In the swollen cells of gum-traga- 

 canth likewise, traces of double refraction may still be detected. 



Besides these more or less disorganized membranes, cell-walls 

 occur in many plants which exert only a very weak effect upon 

 polarized light. To these belong, for example, the parenchy- 

 matous cells of the cotyledons of Lw/?mMS hij^sutus, in spite of the 

 considerable thickness they possess. The same is the case in 

 most of the cells of the Lichens and Fungi. That these are sim- 

 ply-refracting bodies, as Ehrenberg asserted of the membranes 

 of Fungi, and Schacht of these and of those of the Lichens, is 

 incorrect. In the Lichens the looser filamentous cells of the so- 

 called medullary substance exhibit an action upon light at least 

 evident in all cases, for example in Roccella fusiformis and 

 Parmelia ciliarisj while the firmer central substance of Usnea, 

 and, above all, the cortical layer of all the Lichens I have exa- 

 mined, act with tolerable strength. In the Fungi the facts differ 

 a good deal according to the species. While in many species 

 of very delicate structure, for instance in the mycelium of 

 Erisyphe, the effect, though always evident, is but weakly ex- 

 hibited ; in other species, the first glance removes all doubt that 

 the thin cell-membranes behave towards polarized light in ex- 

 actly the same way as the cellulose membranes of the Phanero- 

 gamia. Thus the threads of Mucor Mucedo and Ascophora Mu- 

 cedo, and the filamentous Cells of Merulius lacrymans, presented 

 themselves, notwithstanding their delicacy, in very bright light : 

 still more evidently was this the case in Fungi of firmer consist- 

 ence; for instance, in the cells of the outer peridium of Geaster ru- 

 fescens, and more especially in the cells of the pileus of the harder 

 Polyporei, e.g. of Polyporus hirsutus, in which, notwithstanding 

 the small diameter of the cross section of the filaments, I re- 

 peatedly saw most distinctly the black cross corresponding to 

 the neutral axes. These appeared on the surface of transverse 

 sections of the rather thick coat of the unripe spores of Tuber 

 cibarium, with a clearness equal to that in the cross section of 

 a vessel of the Phanerogamia. The spiral cells of the capillitium 

 of the Trichice also are among the membranes acting powerfully 

 upon polarized light. 



Not only do the cells of different plants, or the cells of dif- 

 ferent organs of the same plant, thus exhibit great distinctions 

 in their effect upon polarized light, but not unfrequently a 

 similar distinction occurs between the different layers of one 

 and the same cell, whence the polarizing microscope is in many 



