by the aid of Polarized Light. 265 



the cell-membranes of the peridei'm and the cuticular layers 

 of the epidermal cells. This occurs, for instance, in common 

 cork, in the periderm of Quercus Cerris, Prunus virginiana, 

 jEscuIus Hippocastanum, Betula alba, and in the thin cork-layers 

 which divide the bark of old trunks of Pinus, e. g. P. nigricanSy 

 into scales. In the epidermis all those membranes which are 

 coloured blue by iodine and chloride of zinc, like an ordinary 

 cell-membrane, behave in optical respects also like a cellulose 

 membrane ; while all those layers which are coloured brown by 

 those reagents, like the cell-membranes of the periderm, are 

 optically positive. It here makes no difference whether the 

 cuticle forms but a thin pellicle on the outer surface of the 

 epidermis, as in most thin-walled epidermal cells [e. g. on the 

 rhizome of Polypodium aureum, in the stem of Impatiens, of 

 Euphorbia Caput Medusce, on the leaf of H'ellebo7'us foetidus, &c.), 

 or, as in thick- walled epidermal cells, a thick stratum of the 

 lamellse on the external wall of the epidermal cells, and parts of 

 their side-walls, possess the property of cuticle, — as, for instance, 

 in the leaves of Cycas revoluta, Phormium tenax, Hakea gibbosa, 

 and Aloe obliqua, and in the stems of Viscum album and Miso- 

 dendron. 



This contrast between the cuticular layer and the parts of the 

 cell-wall composed of unaltered cellulose, may be observed with 

 extreme clearness in the epidermis of the leaf of Aloe obliqua. 

 Seen from the surface, the epidermal cells appear as tolerably 

 regular hexagons, in whose side- walls may be distinguished the 

 primary membranes and a thick deposit of secondary layers. 

 Among the latter, even without the application of polarized 

 light, the innermost, rather thick layer is distinguishable by 

 being separated from the rest by a sharp line of separation. On 

 the application of a plate of selenite, the primary membranes, 

 and, with the exception of the said innermost, likewise the 

 secondary layers which lie parallel with the neutral axis of the 

 selenite-plate, appear, according to the thickness of the plate 

 used, red or yellow ; those lying parallel with the other neutral 

 axis green or blue ; while the innermost layer exhibits, with the 

 same regularity, the complementary colours of the other mem- 

 brane lying parallel with it. The same contrast in the colour of 

 this innermost layer and the rest of the layers, is exhibited in 

 the transverse section of the epidermis, in which it is further 

 observed, that this innermost layer is continuous with the un- 

 altered cellulose membranes of the posterior inner half of the 

 epidermal cells, and in its colour obeys the rule followed by these 

 membranes and the subjacent parenchymatous cells. 



I have demonstrated, on a former occasion*, that the chemical 

 * Scientific Memoirs, 2nd series, Nat. Hist. i. p. 95. 



