202 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
of vessels which exhibit an exogenous arrangement, and have, passing outwards between 
them, thin vertical layers of cells which I believe to be early forms of medullary rays. 
Mr. Carruthers rejects this interpretation; but I think I shall be able to show, in the 
course of my descriptions, that they are what I have affirmed them to be. Mr. Carruthers 
objects to the idea of their being medullary rays, because “ the axis of the stem is not 
occupied with a cellular medullary tissue, but with scalariform vessels” *; and that con- 
sequently “ it cannot be interpreted as similar to that of the medullary system of Dico- 
tyledons.” Experienced as my friend and co-worker in the field of phytology is, I must 
venture to differ from him here. He recognizes, in his description, the existence of the 
scalariform cells which I have also described, though in phraseology different from my 
own. “ Some of those (vessels) in the centre of the axis are divided into chambers by 
horizontal septa, or rather they appear to be made up of a series of short, obtuse cells, 
whose transverse as well as longitudinal sides are marked with scalariform bars. Such 
interrupted vessels are scattered irregularly through the others. I can detect no trace 
of any other structure in the axis than scalariform vessels” f. I submit that such tissues 
as are here so correctly described cannot, in any accepted sense of the term, be called 
vessels ; they are cells, which it is true might by fusion become vessels. In their earliest 
state they were not barred or scalariform, but simply forms of parenchyma. 
In seeking an explanation of the philosophy of these medullary rays, we must not limit 
our attention to their matured state, but go back to the time when all the tissues asso- 
ciated with them existed as a cluster of undifferentiated parenchymatous cells. One of 
the first changes to be detected would be the development of a few vessels, and amongst 
others would ultimately appear those destined to constitute the incipient exogenous 
ring. The moment these made their appearance, they converted the few cells which 
separated them into incipient medullary rays. Thus much of a change might occur 
before the cells deposited in their interiors their bands of lignine which give them 
their barred or scalariform structure. It would not be necessary that, as growth 
advanced, all the cells should follow the same course of development. Such we know 
is never the case in the higher plants ; were it so, differentiation of tissues would be 
impossible. Further, I think Mr. Carruthers must differ from me as to the essential 
characters and functions of medullary rays. Though in their earliest state their purpose 
is doubtless to connect the medulla with the more external tissues, such is not their 
permanent function. As exogenous stems grow, the pith gradually contracts, and what 
cells remain do so in a final condition that rather represents effete structures than active 
cells filled with vigorous protoplasm. Yet though the medulla becomes thus altered, 
and its primary mission a thing of the past, the medullary rays continue to grow and 
actively fulfil their essential functions, which is to maintain free lateral communication 
between the inner and outer layers of the wood, and between both these and the bark. 
If this reasoning is sound, and I believe it to be so, the fact that a matured Lepidoden- 
droid stem has its medullary axis occupied in some cases with barred cells, and in others 
* Monthly Microscopical Journal, October 1869, p. 180. t Lac. cit. 
