Monthly Microscopical! 
Journal, Aug. 1, 1869. J 
sonie Exogenous Stems, 
69 
were of the coniferous type — neither of us at that time being 
aware of the demonstration that the tangential section would afford. 
I think I now see my way to an explanation of the absence of 
the dot. The question has a sufficiently important bearing upon the 
hypothesis of development of one tissue out of another, and, pari 
jpassu, of one plant out of another, to give it importance. 
In studying the microscopic tissues of the Cycadese, I have for 
some time been convinced that the discigerous vessels in Cycas re- 
voluta, usually supposed to be of a coniferous type, were in some 
measure modifications of scalariform tissue. I have now found nume- 
rous vessels from the above plant, which renders the fact certain, 
since they exhibit discigerous tissue at one end of the vessel, whilst 
it becomes scalariform at the other. My views on this point, when 
promulgated in private correspondence with some botanical friends, 
were at once rejected by them ; but there is no reason for ques- 
tioning their correctness. I was not aware, however, when I came 
to this conclusion, that I had been anticipated by the late Mr. Don, 
The question is of some importance, since it affects the possibihty 
of a glandular coniferous fibre being developed out of a reticulated 
one. 
There exists amongst geologists some misconception respecting 
the true nature of a glandular disc, which consists of two very 
distinct elements, viz. the circular or hexagonal areola and the 
central dot. The outlines of the former vary chiefly according to 
the mutual pressure to which they are subjected. The latter 
variously appear as a small circular dot (Fig. 5, a), an oblong one 
(Fig 5, h), which is sometimes so hnear as to stretch across a great 
part of the disc (Fig. 5, c) ; and occasionally they assume a regular 
(Fig. 5, d) or irregular crucial shape (Fig. 6, h). The simple dot is 
due to a deficiency of the lignine lining the rest of the interior of 
the vessel, and is in no respect different from the pits of a pitted 
or porous vessel. But the circular disc or areola is external to the 
tube, consisting of a lenticular depression on the exterior of the wall 
of the fibre. That there is some connection between these two 
objects, when first formed, is obvious, from the constant presence of 
the dot in the centre of the areola, wherever the latter exists in 
recent plants. Thus the coniferous disc consists of two elements, 
one of which is internal to the primary wall of the vessel, whilst 
the other is external to it. The former may exist without the 
latter, as is the case in all porous or pitted vessels ; but, as I have 
just observed, the latter never exists without the former, in the 
fibres of recent stems. Fig. 6 represents part of a scalariform vessel 
from Cyeas revoluta, where the ligneous deposit has been so extended, 
that the oblique, transverse, thin spaces separating the bars of 
hgnine, are reduced to mere oblong " pits " (Fig. 6, a), but where 
in a paper which he read before 
