208 H. von Mohl on the Investigation of Vegetable Tissue 



cases a means of rendering visible, lamellse which are difficult 

 of detection with the ordinary microscope. In this respect it 

 is a pretty common phsenomenon for the primary membrane, 

 and likewise the tertiary layer immediately lining the cavity 

 of the cell, to act more powerfully upon polarized light 

 than the secondary layers ; they therefore appear in far 

 brighter white light than the latter. In particular cases, for 

 instance in the parenchyma- cells of the cotyledons of Lupinus 

 hirsutus, the secondary layers act so weakly, that when an ob- 

 jective which does not admit a great deal of light is used, they 

 are almost invisible without the interposition of a doubly-re- 

 fractive medium in the illuminating apparatus, while the primary 

 and tertiary membranes appear in the form of delicate luminous 

 lines. In like manner, in the coUenchyma-cells of the stem of 

 Sambucus Ebulus and Cucurbit a Pepo, the" primary membrane is 

 most remarkably distinguished from the disintegrated secondary 

 layers, which I formerly erroneously regarded as intercellular 

 substance, by a far greater brilliancy, so that one may make 

 most absolutely certain that the so-called intercellular substance 

 is here not deposited between the cells, but is traversed by the 

 primary cell-membranes. A similar, although not so clearly 

 marked contrast between the primary and tertiary membranes, 

 occurs also in many thick-walled wood- and liber-cells ; for in- 

 stance, in a high degree in the thick-walled cells of the vascular 

 bundles of the outer layers of the stem of Aletris fragrans and 

 in the liber-cells of Rosa canina ; and in a less degree in the 

 wood-cells of many Coniferse, for instance of Abies pectinata and 

 Torreya taccifolia. In other, but rarer cases, the primary mem- 

 brane acts less strongly upon polarized light than the secondary 

 layers, so that the latter appear separated from each other by a 

 darkish line, for instance in the endosperm of Phytelephas. 



Cellulose membranes undergo no essential alteration in their 

 behaviour to polarized light when they are freed from the com- 

 pounds deposited in their substance, by boiling in Schulze^s mix- 

 ture of nitric acid and chlorate of potash. We must conclude 

 from this, that the action exerted by the cell-membranes is at- 

 tributable to the cellulose itself of which they are formed, and 

 depends upon the arrangement of their molecules connected with 

 their organic structure. Ehrenberg, from the behaviour of the 

 scales of certain plants, promulgated the opposite opinion. He 

 thought he had found that many scales, such as those of the 

 leaves of the Olive, do not act upon polarized light, while others, 

 such as those of Elrsagnus and Tillandsia usneoides, exert this 

 influence in a high degree. Believing, further, that he had dis- 

 covered that the said property might be abstracted from the 

 latter scales by the aid of acids, he came to the conclusion that 



