’ 
104 MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
frequently in such a manner, that a thick one, and a thinner and 
somewhat paler one situated on its inner side, always remain 
attached together, which would make it appear that these two 
layers formed together a connected whole. The bright and the 
dark parts of each division show no perceptible difference in 
their behaviour to sulphuric acid and iodine, hence we are well 
entitled to consider them rather as slight modifications of the 
same membrane than as essentially distinct. On the other 
hand, slight distinctions occur in some cases between the differ- 
ent layers of the same cell in reference to their behaviour to 
iodine and sulphuric acid. Thus the innermost or even several 
of the more internal layers of the liber-cells of Calamus (see 
figs. 25, 26) are usually coloured of a darker yellow by iodine 
than the outer, and the inner swells somewhat more slowly than 
the outer layers in sulphuric acid; so likewise the inner layer 
of the parenchymatous cells of the stem of Polypodium incanum 
(fig. 18) is coloured a deeper yellow by iodine. If dilute 
sulphuric acid be allowed to act upon the cells treated with 
iodine, they will usually, though not always, take a blue colour 
at a certain stage of disintegration. If, then, as in Calamus and 
Polypodium incanum, the various layers exhibit a marked dif- 
ference in respect to the rapidity with which they swell in sul- 
phuric acid, the layer which first becomes swelled (consequently 
in this case the outer) will be coloured blue sooner than the 
inner, which remains yellow for some time, till the longer con- 
tinued action of the acid renders it also blue. On this blue 
colouring, however, 1 can place no value whatever; it presents 
itself in all membranes swelling easily on the application of a 
weak acid, in all membranes swelling slowly by a strong sul- 
phuric acid; and if two layers of the same cell show any dif- 
ference in this respect, this is by no means a proof that we have 
different membranes before us, since even in the same layer dif- 
ferences of age in the cells suffice either to allow of or hinder 
the production of the blue colour on the application of acid. 
For instance, the primary membrane of all young organs is very 
readily coloured blue; the primary membrane of many full- 
grown cells is coloured blue, as in the spiral cells of Melocactus, 
and in the horny cells of very many plants ; while the primary 
membrane of full-grown ligneous cells, as a rule, obstinately re- 
sists the action of sulphuric acid, and is not coloured blue; 
further, the secondary membrane of spiral vessels, dotted ducts, 
lt Pe el a 
