MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 109 
direction of its spiral, and in its continuation through the canals 
of the dots; but in my opinion there is not the slightest reason 
for placing it, with Hartig, in direct opposition to the secondary 
layers and regarding it as the primary membrane to which all 
the others owe their origin. No such extraordinary importance 
should be attached to the deviations in its structure from that of 
the outer secondary layers, for these differences are not nearly 
so significant as those we find between the different secondary 
layers in many other cells, especially in the spiral cells of the 
seeds of Rwellia, &c. MHartig’s view could solely be deduced 
from the history of the development of these cells, but he does 
not appear to have traced this, neither would it favour his theory, 
for no spiral membrane is found in young cells. I myself pre- 
maturely hazarded a conjecture that it makes its appearance 
earlier than the outer secondary layers in the young cells. I 
must retract this in consequence of more recent observations, 
from which it appears to be the last formed membrane. 
An equally striking distinction occurs between the outer and 
inner secondary layers in the cells of the wood of Pinus sylvestris, 
and still more strongly marked in the wood of Taxodium dis- 
tichum. If a transverse section of the latter is allowed to swell 
in sulphuric acid (figs. 29, 34. Plate II.), the secondary mem- 
brane appears to be composed of two layers. The outer layer is 
thin, homogeneous, and the canals of the dots have a roundish 
form ; the inner layer, on the other hand, is split into many 
lamellz, which are perpendicular to the outer layer, and run in 
a spiral direction through the cell. The canals of the dots in 
this inner layer are formed by the mutual retraction of two such 
lamellz, and consequently present the appearance of narrow 
oblique clefts (fig. 31). This layer appears perfectly homo- 
geneous (fig. 30) before it swells in acid, and a multitude of de- 
licate spiral furrows are seen to traverse it, only when it is 
examined from the cavity of the cell. Even during the tume- 
faction the division between the contiguous lamellz is invisible, 
but the inner margin appears bordered by a band of light which 
seems to indicate the existence of an inner membrane ; on com- 
plete distention however no trace of this is seen. The spiral 
striation of the inner cell-wall structure of Pinus sylvestris was 
one of the chief grounds on which Meyen founded his theory, 
that the cell-wall was formed by the growing together of distinct 
fibres. The presence of distinctly separated lamellz in the tu- 
