by the aid of Polarized Light. 205 



Wc will examine, in the first place, whether the power of clII- 

 meiubrane to doubly-retVact light is connected with its lamella- 

 tion. Schacht asserts this most decidedly, for he states that 

 only thickened cell-membranes are doubly-refractive ; that very 

 delicate-walled vegetable cells (such as the cambium-cells of the 

 vascular bundle of Ahicajjcctindtttjthv. parenchyma of young struc- 

 tures, the tissue of Fungi and Lichens) are completely invisible 

 upon the dark field, and therefore simply refract light; and that 

 consequently it may be determined by the polarizing microscope 

 whether or not a cell has already deposited layers of thickening. 

 These statements are false. Even from theoretical reasons, it 

 was not to be assumed that a lamellated membrane which is 

 traversed by light parallel with the planes of lamcllation, acts 

 as a doubly-refracting body on account of its lamcllation, — the 

 property of double refraction must depend upon the arrangement 

 of the molecules in each of the separate layers. It is without 

 doubt within the limits of possibility that primary membrane 

 should be distinguished in this respect fi*om the secondary 

 layers ; but observation proves that this is not the case. It is 

 a known fact that a doubly-refracting body acts the more weakly 

 upon polarized light the thinner it is, wherefore in very thin 

 lamellsc of crystals, as well as in organic membranes, the effect 

 may be reduced below the last degree of which the detection is pos- 

 sible. Hence in examining young vegetable structures we cannot 

 expect to see their thin membranes upon the black field with the 

 same brightness as in thick- walled cells. Nevertheless, in almost 

 all the cases examined by me, the membranes which from their 

 youth and organization we are accustomed to regard as primary, 

 behaved to polarized light most clearly in the same way as those 

 of thickened cells, that is, as doubly- refractive. In particular, 

 the membranes of the cambium-cells of all the Dicotyledons I 

 examined, e. g. of Pinus sylvestris, P. nigricans, Impatiens Bal- 

 samina, Sam/jucus Ebulus, and especially clearly that of Viscum 

 album, appear in bright light when placed at an angle of 45° to 

 the Nicol; still more clearly was this the case in the membranes 

 of the clathrate cells {gitter-zellen, Mohl) erroneously referred to 

 the cambium, both in Monocotyledons, for instance in Musa, 

 Cannn, Renealmia nutans, and Dicotyledons, e. g. in Bignonia ; 

 which membranes, it is true, are already, as their pits show, com- 

 posed of a number of lamella. The same appearances were 

 equally clear in the cell-membranes of the embryo of Pinus Pinea, 

 in the cambial tissue of the apex of the stem of Cocus coronata, 

 and in the primary coat of the spiral cells of Echinocadus multi- 

 plex. In certain very thin-walled librous cells, as in those of the 

 leaf of Sphagnum cynibifulium, r.nd those of the wing of the seed 

 of Swietenia Mahoguni, the action exerted upon polarized light by 



