200 H. von Molil on the Inx-estii/ation of Vegetable Tissue 



that ])ortion of the membrane not covered by fibre was certainly 

 so weak, that it could not be detected without the interposition 

 of a doubly-refracting medium between the lower Nicol and the 

 object ; but by the aid of this, that is to say, by interposing a thin 

 plate of mica, it appeared most clearly. The assertion that the 

 primary cell-membrane forms an essential contrast to the layers 

 of thickening in respect to its behaviour towards polarized light, 

 is therefore decidedly incorrect. No better grounded did I find 

 Ehrenberg's statement, repeated by Schacht, that the scales of 

 the leaves of Olea, Rhododendron and Myrica do not act on polar- 

 ized light ; for the scales of Olca ewopcea, Rhododendron hirsutum, 

 and Myrica quercifolia, especially the first two, most decidedly 

 possess this power, although in a weak degree. 



It is not meant here, however, that the substance of all cell- 

 membranes acts with uniform force upon polarized light, and 

 that the brightness with which a membrane appears upon the 

 black field depends only on its thickness. On the contrary, 

 most important distinctions in this respect occur, according to 

 the modifications which the cellulose exhibits in diff'erent cells, 

 and indeed according to the diversity of the foreign matters im- 

 bedded in the cell-membranes. In general, a cell-membrane 

 acts the more strongly upon polarized light, and appears in the 

 greater brightness, the more solid its substance is, and vice versa; 

 hence, in general, wood-cells and liber-cells appear most illumi- 

 nated : the membranes of ordinary parenchyma likewise possess 

 this property in a high degree, for which reason a delicate cross 

 section of a stem, especially of a monocotyledonous plant, presents 

 a most elegant picture, through the contrast which is made by 

 the silvery lustre of its cell-membranes with the black ground 

 of the field. But when the cell- membranes swell up more or 

 less with water into a gelatinous condition, as in the Fucoidese, 

 or in the collenchyma-cells lying beneath the epidermis, e.g. of 

 Sambucus Ebulus, Beta, or Rheum, their capability of becoming 

 visible in polarized light diminishes to a more or less consider- 

 able extent. This is the case in a particularly high degree 

 when the disintegration of the outer layers of the cells goes so 

 far, that they are blended into what appears like a homogeneous 

 intercellular substance. Yet even in these cases the power of 

 acting upon polarized light is perhaps never totally lost ; at least 

 I found the intercellular substance of Fucus vesiculosus decidedly 

 doubly-refractive ; and the slimy substance of Nostoc commune, 

 of Collema nigrescens and C. flaccidum behaved in a similar 

 manner. The apparently pci-fectly structureless intercellular 

 substance in the endosperm of many Leguminosje also, for in- 

 stance of So])hora japonica and Schizolobium excelsum, acts de- 

 cidedly upon polarized light ; here, however, in the iiTegular 



