492 
lular pith, an inner ring of vessels belonging to the medul- 
lary portion of the axis, and an external vascular cylinder, 
which grows by additions to its exterior, and which no more 
belongs to the central medulla than do the ordinary wood 
layers of an exogenous phanerogam, It has unquestion- 
ably been the product, as Prof. Dyer admits to be pro- 
bable, of a cambium layer. 
Speaking cf the Lycopodiaceous stems of the coal 
measures, Prof. Dyer says, “Iam inclined to think, with 
Prof, Williamson, that the stem increased in thickness.” 
This point is not one to be thought about as if it was 
uncertain. We have in our museum accurate casts of the 
Dixonfold trees, and the base of the stem of the largest of 
these, above the point whence the huge roots are given off, 
is twelve feet incircumference! Higher up it is eight feet. 
There is surely no room for questioning an increase of 
thickness here, and this instance is but one example of what 
is sufficiently common in the coal measures. When we 
turn to the interior of these large trees, we find, as I have 
abundant evidence to prove, that they were enabled to 
sustain their huge bulk by an exogenous development of 
their outer cylinder of vessels, which were not mere modi- 
fications of the medullary vessels, but something super- 
added. This woody structure was amply provided with 
medullary rays, and each of the several layers of the thick 
bark increased #ar7z fassu with an increase of the ligneous 
zones, whilst a large cellular pith occupied the centre of 
the stem. So much forthe facts, which are very different 
from those recognised by either of my two opponents. 
Now as to opinions, Prof. Dyer says he thinks that this 
increase was “nothing more than an incident in the life- 
history of a particular race of plants, nothing more than 
an adjustment to an arborescent habit dropped when 
the arborescent habit was lost.” I am not sure 
that I understand all that Prof. Dyer means in this pas- 
sage. He appears, however, to imply that these exogenous 
conditions were merely adventitious growths assumed for 
a season, and thrown off at the earliest opportunity ; that 
they had no true affinity with the plants in which they 
were found. I confess I see no grounds for so remarkable a 
conclusion, especially remembering that, atleast, these con- 
ditions lasted throughout the vast duration of the Devonian 
and Carboniferous ages. That one object of the exogenous 
growth was to enable these trees to sustain a huge super- 
structure, is doubtless true, though we find that growth in 
myriads of plants that have no such ponderous super- 
structures ; but must we not say the same thing of the 
oak and the beech, as well as of the Lepidodendra? Isee 
no difference between the cases. We have no more reason 
for regarding these conditions as merely an incident in 
the life-history of a particular race in the one instance 
than in the other. 
I will not now discuss the value of the terms exogen 
and endogen, since the question has little importance in 
reference to the present object. I will only say that the 
mode of growth of a plant appears to me to have equal 
value with the mode of reproduction. There is a fashion 
in these matters—and in some circles there is now a 
tendency to elevate the reproductive at the expense of the 
vegetative, with which I do not agree, but I repeat this is 
not a question essentially important at present. My two 
great objects have been, first to demonstrate the existence 
of the exogenous structure in the trees in question ; and 
second, to show the absurdity of applying the term acrogen 
to trees so constructed. 
The value of my proposed classification is an indepen- 
dent question. I attach but a limited importance to the 
artificial boundary-lines introduced by systematisers, and 
do not wish to assign more to my own than to those of 
others ; nevertheless, such divisions are useful so far as 
they indicate affinities, and it is because I find such 
affinities in the plants before us, waecognised by existing 
classifications, that I have suggested a new one. Whatever 
value different minds may attach to the fact, there exists 

NATURE 


[ Oct. 19, 1871 
—$§ $$ $$$ ————————— 
the great vegetative difference upon which I have dwelt 
between the Lycopodiaceze and the Calamites on the one 
hand, and the Ferns on the other. There is certainly 
something more involvedin this fact than “ the old division 
of plants into trees and shrubs,” with which Prof. Dyer 
compares it. Such a division is merely one of size and 
duration, not of organisation. Herbs, if they belong to 
the exogenous group, are as truly exogenous in their type 
as the most gigantic trees of the same class. Size has 
nothing to do with the matter. The same uniformity of 
type, apart from size, exists amongst |my fossil cryptogams, 
True, the exogenous growth attains the fullest development 
amongst the large trees—but all the rudiments of this 
growth are equally to be found in the small ones, as my 
forthcoming memoirs will demonstrate. 
The outer exogenous growth must be distinguished 
from the primitive vessels of the central medullary axis. 
I have yet to publish a remarkable series of facts illus- 
trating this point. I have stated in a previous article that, 
in one sense, the exogenous vessels are a development of 
the vascular bundles of the living Lycopods. This is 
teleologically true rather than morphologically. Viewed in 
| the latter aspect the two groups of vessels are independent 
of each other. The medullary vessels may be, and often 
are, primitive tissues formed at the first growth of the 
plant or of its young branches. The exogenous ones are 
something added, furnished by a cambium layer. The 
two groups retain their independent positions permanently, 
just as in living exogens the medullary sheath remains 
distinct from the woody cylinder which encloses it. 
W. C. WILLIAMSON 


NOTES 
WE believe that the arrangements of the Eclipse Expedition 
are nearly all made, and that the numbers are now complete. 
The Expedition sails on Thursday next in the AZirzapore, arriving ~ 
at Point de Galle on the 27th November, if all goes well. M. 
Janssen, we believe, is already ex voyage. Prof. Respighi, of 
Rome, will accompany the English Expedition, 
Botu Mr. Hind and M, Stephan at Marseilles have obtained 
observations of Encke’s comet. Mr. Hind thus writes: ‘It is 
a large, faint, and very diffused nebulosity—a different-looking 
object from what I remember it in one or two former returns, 
when it has been drawing just within reach of the telescope. 
The last observation on the 12th of October gives the following 
place :—At 9» 16™ 18° mean time at Twickenham, right ascen- 
sion, 1" 7™ 37°85; north declination, 36° 47’ 38". The ephe- 
meris for this appearance, published in Wéanges Mathématiques, 
of the Academy of Sciences of St. Petersburg, and calculated 
by Herr von Glasenapp, of the Russian National Observatory at 
Pulkowa, required, according to the above observation, correc- 
tions of 36 seconds in right ascension and ten minutes in declina- 
tion, subtractive in both elements. The comet’s positions for the 
next few days will be nearly as follows :— 
for Midnight at Greenwich. 
R.A. Decl. N. 
heeeeaxtls Deg. Min. 
October 19 ‘ Oo 30°7 ». “Bsinray 
yeh : ee LO) tS - 38 45 
» = 23 5 5 ele PSH) 5 sto Le) 
p25 we 23 483. «38 «(59 
THE Expedition to Moab, which has been organised by Dr, 
Ginsburg, and goes out under the auspices of the British Associa- _, 
tion, will leave England in January. Its object is to explore » 
the geography, antiquities, and natural history of the region. 
Canon Tristram will accompany Dr. Ginsburg. 
Bulletin Astronomique de ? Observatoire de Paris is the title of 
an official circular, containing meridional observations of the sun, 
