306 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
must not be forgotten that the homologue of the vascular bundles of the living Lycopods, 
i. e. the vascular medullary cylinder, is not encased within a ring of prosenchymatous 
cells, or Phloem, as in the recent plants : hence they remain throughout their entire 
development open and not closed bundles, which is a very important distinction. So far 
as I can judge, the appearance of this exogenous growth possibly corresponded with the 
period at which the leaves ceased directly or indirectly to produce further increase in the 
number of the vessels of the medullary cylinder. I can discover no reason for supposing 
that the number of the vessels of that cylinder subsequently received further additions, 
or that any further enlargement took place in the diameter of the cylinder. I can only 
account for the development of the exogenous layers by supposing the existence of some 
equivalent of a cambium-layer surrounding and parallel to the cylinder. The fact of 
these growths taking place as I have already described is beyond all question. The only 
debatable points refer to the source whence these exogenous layers were derived, and 
to the relations which they bear to the similar structures of other plants. 
Professor M‘Nab, who objects to my views on this point, lays much stress upon the 
distinction between a layer of “ meristem ” tissue and a cambium-layer. The distinction 
between these structures was made by Nageli and further illustrated by Sachs (Lehrbuch 
der Botanik, p. 75). The characteristic feature of a meristem structure is that all or 
most of its cells are capable of spontaneous division or multiplication by fission, as is 
the case with the first-formed elements of every plant ; whilst such cells as are no longer 
capable of undergoing such divisions become permanent tissues. Sachs points out that 
these meristem tissues were formerly comprehended in what was generally termed cam- 
bium ; but he urges the advisability of limiting this latter expression “to that meri- 
stemic (merismatische) layer in the tissues of the older parts of the plant by which is 
effected the lateral growth ( Dlclcenwaclistlium ) of the Dicotyledons and Coniferse.” 
Hence, as cell-fission occurs in the true cambium-layer as well as in meristem layers, 
one chief peculiarity of the former lies in its position relative to the older parts of 
the stem — or, in other words, to its location, in the case of Dicotyledonous plants, 
between the wood and the bark. Henfrey describes some of the peculiarities of the 
Dicotyledonous stem as follows : — “ When the buds open to produce new shoots, cell- 
division recommences in the cambium-region of the old bundles, and an additional layer 
of wood is added gradually during the season to that formed the year before. Season 
after season this process is repeated” (Elementary Course of Botany, p. 521, 2nd edition). 
“ The medullary rays which separate the primary bundles are developed in the cambium- 
region with the yearly layers of wood, and always extend to the cortical parenchyma ” 
{loe. cit. p. 523). Henfrey also points out, as other writers have also done, that one 
chief peculiarity in an exogenous stem resides in the parallelism of the cambium-layer 
to the previously formed fibro-vascular bundles ( loc . cit. p. 518), and in the periodic 
resumption of activity in the bundles {icl. p. 519). If all these conditions are not fulfilled 
in the specimens which I have described and especially illustrated by Plate XLIII. 
fig. 20, I know not where to seek for such a fulfilment in any living plant. But Sachs 
