108 MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
secondary layers. Several years since, indeed, I described* the 
fibres in the ligneous cells of Taxus as a peculiar cell-membrane, 
since the spiral course which they take in the cells of some trees 
is wound in a direction opposite to that of the greatest breadth 
of the canals of the dots. Hartig apparently founded his theory 
of the ptychode chiefly upon the structure of the wood of Taxus, 
and differs from previous phytotomists, principally in the circum- 
stance that he assumes that the spiral fibres are not isolated in 
the interior of the ligneous cells, but connected together by a 
membrane, or rather are nothing else than inwardly projecting 
folds of this membrane (the ptychode). I examined the wood 
of Taxus baccata and cuspidata, Torreya taxifolia (fig. 10) and 
nucifera, with the application of sulphuric acid, in reference to 
these assertions, and agree perfectly with Hartig as to the pre- 
sence of an inner membrane closely connected with the spiral 
fibres; whether the fibres owe their origin to a folding inwards 
of this membrane, as quite appears to be the case when the cel- 
lular membrane is half-swollen in sulphuric acid, or whether 
they are mere thickenings of it, I will not yet venture to decide, 
as my microscope does not suffice for an accurate investigation 
of these points. The accuracy of the further assertion of Hartig, 
that the canals of the dots are clothed by a continuation of this 
membrane, may also hardly be doubted, since we see a clearly 
defined band of light, appearing to proceed from an immediate 
continuation of that membrane, running through the canal of the 
dot as far as its outer end, but the precise determination of the 
relation which this appendage bears to the surrounding parts is 
also impossible with my microscopes. That the lenticular cavity 
which lies between two dots is clothed by a continuation of the 
outer cellular membrane, and is shut off from the canals of the 
dots, may be seen most distinctly, and I do not hesitate to declare 
the representation given by Hartig, that the inner cellular mem- 
brane passes through one of the dots into the cavity and lines it, 
to be wholly erroneous ; but to make out whether the inner mem- 
brane ends in the form of a blind sac at the bottom of the canal 
of the dot, or, as in general, attaches itself to the external mem- 
brane, requires better microscopes than we possess at present. 
That we should distinguish this inner membrane from the ex- 
ternal secondary membranes, as a peculiar layer, sufficient 
grounds exist in its distinct spiral structure, in the variable 
* Dissertation tiber den Bau der vegetab. Zellmembran, p. 25. 
