224 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
‘ Lehrbuch der Botanik ’ altered the position of this question. Sachs showed ( loc . cit. 
p. 25) that what is essential in the formation of these pits is primarily due to a mere 
variation in the deposition of the lignine in the interior of the cell or fibre, corresponding 
exactly in this respect with what takes place in the formation of a scalariform or spiral 
vessel, the only material difference being found in the location of these internal deposits 
and their greater or less close adhesion to the cell-wall within which they are formed. 
This is a very different thing from what it was believed to be when it was supposed that 
the lignine was deposited inside the cells or vessels, whilst the disks depended upon 
something external to their walls. In the latter case the gradual transition from the 
one form to the other was unintelligible. Now, on the contrary, nothing is more simple. 
This physiological inquiry is not foreign to the subject of this memoir, but forms an 
essential part of it, since it gives us some clue, helping us to determine what may and 
what may not have been Coniferous plants. It affects the position in which special 
genera should he located, besides having a very important bearing upon the general 
question of evolution. Though at present unable to see my way to the acceptance of 
this doctrine in connexion with the origin of extinct plant forms, I am anxious to place 
on record all the facts which my studies in paleeo-botany supply which appear to 
support that doctrine. Hitherto the difficulty of explaining the transition from the 
ordinary types of fibro-vascular cells and vessels to that of the discigerous types has 
been one very great hindrance to my accepting the doctrines of the evolutionists in 
reference to fossil plants ; this one difficulty, at all events, has now been removed. But 
its removal is also an important fact bearing upon another branch of the present inquiry. 
M. Brongniart, Dr. Dawson, and Professor Newberry have long held the view that 
many of the Sigillarioe and a large group of allied stems were those of Gymnospermous 
plants. The two first distinguished palseo-botanists also believe that many of the forms 
of Calamites, which they recognize under the name of Calamodendron, are also members 
of the same Gymnospermic family. But these observers have arrived at their conclusions 
on somewhat different grounds. M. Brongniart has done so from his accurate recog- 
nition of their exogenous structure and mode of growth — features which, as I think I have 
demonstrated, are to be found in many indisputably Lepidodendroid plants. Hence, I 
cannot accept M. Brongniart’s conclusions as a legitimate deduction from the evidence 
upon which the distinguished Frenchman made them rest. Dr. Dawson has told us, in a 
succession of memoirs, that he has arrived at very similar conclusions to those adopted 
by M. Brongniart, but through a somewhat different line of argument, the chief points 
of which are prominently set forth in his memoir “ On the Structure and Affinities of 
Sigillaria, Calamites , and Calamodendron ,” published in the Quarterly Journal of the 
Geological Society of London for May, 1871. In that memoir he describes a stem in 
which the pith was Sternbergian, whilst the inner part of the vascular cylinder “ consists 
of scalariform tissue, passing towards the outer surface into pseudo-scalariform, reticulated 
with pores, and discigerous.” The accuracy of this description of a Sigiilarian stem 
appeared to most British botanists so little probable and so little in accordance with 
