102 MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
gin, but also affirms the constant existence of a third inner mem- 
brane (the ptychode) in all full-grown cells and vessels, which 
clothes the cells and is imbedded in all the canals of pores. 
This latter point first merits consideration. Hartig founds 
his views of the presence of the ptychode more particularly on 
the structure of the elementary organs of the wood of Taxus, 
which I believe I demonstrated years ago to be composed of at 
least three layers. I cannot allow this conclusion from the 
structure of one formation, not indeed altogether without paral- 
lel, but up to this time always regarded as an exception, to be 
extended to the structure of all other elementary organs. 
In reference to Hartig’s assertion, that all full-grown cells 
and vessels possess an inner membrane, the substance of which 
is essentially distinguished from that of other secondary mem- 
branes by its behaviour towards iodine and sulphuric acid, I 
have examined anew the cells of a tolerable quantity of plants 
and obtained a negative result. It is generally admitted that 
the secondary cellular membrane consists of many superposed 
layers; in a great number of plants any one may convince him- 
self of this in every transverse section of the cellular membrane, 
I will merely adduce as very striking instances the ligneous cells 
of Clematis Vitalba and the cells of the vascular bundle of the cane 
(Calamus — ?) (figs. 24—26. Plate II.). In many cases, on the 
contrary, the cellular membrane appears perfectly homogeneous 
under the best microscopes, even when it has attained some 
thickness ; this is especially the case in the uncoloured, horn-like 
cells of many seeds, for instance, the albumen of Phytelephas, 
many Palms, Liliacee, Rubiacee, &c., and also very frequently in 
the gelatinous, soft cells which lie under the epidermis of the stem 
in many plants, as in Sambucus, Spinacia, and many Labiate. 
The application of acids, however, proves that in these cases the 
cellular membrane is not in reality homogeneous, but equally 
composed of numerous superposed layers. The softer and 
more gelatinous the membrane, the sooner does the lamellar 
structure show itself on the application of nitric or hydrochloric 
acid, for instance in Spinacia; the firmer and more horny its 
substance, as in many Palms, the more necessary becomes the 
application of sulphuric acid. No universal directions can be 
given in reference to the degree of concentration of the acid to 
be employed, as this must be found by experience in each indi- 
vidual case; but a too weak and a too powerful acid are equally 
