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MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 105 
coniferous woods, &c., is coloured blue only by the action of con- 
centrated sulphuric acid, while the secondary membrane of most 
parenchymatous cells is coloured with the utmost ease. The 
blue colour, therefore, only shows the degree of laxity of the mem- 
brane of plants ; it occurs in many vegetable cells, as in the coty- 
ledon-cells of Schotia (fig. 17. Plate I.), on the simple application 
of iodine, in others it requires a weak acid, and in many cases, 
as in the wood of Taxus, it comes out only when the texture of 
the membrane has been wholly destroyed by a very strong acid; 
it may therefore frequently be a convenient means of enabling us 
to distinguish more readily two layers of a cell from each other, 
but it can never admit of any conclusion being drawn from it as 
to the primary or secondary nature of a membrane. As we have 
already seen, Hartig distinguishes the asthate from the ptychode 
and the eusthate, by the circumstance that the former acquires 
a blue colour by the action of iodine and sulphuric acid, while 
the other membranes remain yellow; this is totally false, as 
every one must find who extends his investigations over a greater 
number of plants. 
A regular alternation of dark and pale layers, as above de- 
scribed, is not the condition which thick-walled cells usually 
exhibit, but in the majority of cases only one thin pale layer is 
to be remarked immediately bounding the cavity of the cell. 
This presents itself equally in thick-walled parenchymatous 
cells, as in the cells of the cotyledons of Schotia speciosa (fig. 
17), and in prosenchymatous cells, especially in the ligneous 
cells of almost all our trees, which latter will be more specially 
considered hereafter. This inner pale layer appears more di- 
stinctly than would be the case but for the imperfection of our 
optical instruments, in consequence of a band of light which 
microscopes exhibit, on the employment of the high magnifying 
powers necessary in these investigations, at the edge and within 
the margin of all semi-opaque bodies. Since this band of light 
is a great obstacle in researches of this nature, I took every 
pains, by alterations of the illumination, by the employment of 
Dujardin’s illuminating apparatus, &c., to remove it, but in vain. 
In the gelatinous cells lying under the epidermis of the stem 
of Spinacia, it will presently be shown how easily an illusion 
might arise from such a band of light, how, consequently, we 
might be led to think we could perceive, with great certainty, a 
peculiar membrane lining the cavity of the cell. In fig. 22. 
