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Oct. 5, 1871] 
NATURE 
445 

scalariform vessels, and that there is in fact nothing medullary or 
medulla-like about it. 
Outside this central structure is what Mr. Carruthers terms | 
the investing, and Prof. Williamson the vascular woody cylin- 
der. I believe that Mr. Carruthers is right in looking upon 
this as belonging to the central axis, which is therefore composed 
of two parts.* I find, which I did not sufficiently appreciate at 
the time, that Prof. McNab regards this investing cylinder as 
homologous with the cylinder of wood cells surrounding the 
central axis of fibro-vascular bundles which is met with in 
many recent Lycopodiaceze. From this I certainly dissent for 
two reasons ; (1) because I think its equivalent is to be found in 
the central axis itself, and not outside it ; (2) because it is not com- 
posed of wood cells but of scalariform vessels. 
Secondly, as to opinions. The terms Exogen and Endogen, 
as is pretty well known, were founded upon a mistake. A great 
deal too much has been made of the difference implied by them ; 
in fact, if we compare a one-year-old dicotyledonous shoot with 
-a monocotyledonous stem, we find that it does not exist. If 
Prof. Williamson will look at the stem of the common artichoke, 
he will find it difficult to convince himself that he is examining 
an ‘‘ exogenous” plant at all. 
The imagined characters which were implied by these terms 
are, nevertheless, as everyone knows, correlated with others, which 
_ vascular cryptogams. 
in the aggregate enable phanerogamic plants to be divided into 
two satisfactory groups ; but this is certainly not equally the case 
with the groups into which Prof. Williamson would divide the 
These groups, I think, most botanists will 
agree in considering in the highest degree unnatural, inasmuch 
as, assuming the vegetative distinction upon which they are 
founded to exist, it is a wholly artificial ground for classificatory 
purposes. Nor is it any argument that one vegetative character 
must be good because others are in use, since the simple answer 
is that these coincide with natural divisions, while Prof. William- 
son’s does not. 
I shall not dispute Prof. Williamson’s position that our living 
Ly copodiacez should be interpreted by the more complete extinct 
types. To dothis, however, the extinct types must be thoroughly 
understood ; when we are dealing with imperfect material, com- 
parison with the more perfect but less highly developed existing 
plants is not only justifiable but necessary. 
It is obvious that the great development of the stem in the 
Lycopodi2ceze of the Coal Measures was correlated with their 
arborescent habit. I am inclined to think with Prof. William- 
son that the stem increased in thickness ; it is certain that ZLefv- 
dodendron was branched, and not improbably also Szgi/laria. 
The branches as they were gradually developed must have been 
the cause of an increasing strain upon the stem ; it seems to me 
More congruous with known laws of. the response of structure to 
circumstances, to conclude that the stem was proportionately de- 
veloped as the strain increased, than that the stem should have 
been produced once for all of its maximum thickness without 
reference to the crown of branches that was finally to surmount it. 
I am quite prepared therefore to admit that the investing cylin- 
der may have increased by external additions, and probably did do 
so; this wou!d of course imply the existence of a cambium layer 
outside it. There is some analogy for this in the recent 
Tsoétes, where we have a ‘‘slight woody mass which occupies the 
longitudinal axis of the stem, but encloses 70 fith.” + Outside 
this we have a ‘‘bark-forming”” cambium (which also adds, but 
more sparingly, to the wood mass) ; in Sie//aria and Lepidoden- 
dron we might have had a cambium not merely renewing the 
bark but adding to the central axis. 
In whatever way the increase took place, it was, asI think, 
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, but showing 
a lingering ancestral tendency in /soc¢es.. Comparing a simple 
stemmed palm with Dracena, we have a parallel instance of the 
strengthening of the stem ari fassu with the continued deve- 
lopment of a system of branches; only in Dracena it is the 
circumferential part of the stem alone which developes. 
If I am right in regarding a stem gradually developing in size 
as the necessary correlate of a large system of branches, Prof, Wil- 
liamson’s view practically amounts to the old division of plants 
into trees and herbs. I cannot see how it can afford any safe 
ground for a re-arrangement of the vascular cryptogams. 
W. T. THISELTON DYER 
London, Sept. 26 
* Monthly Micro. Fourn., 1869, p. 169. 
+ Hofmeister, Higher Cryptogamia, pp. 356, 361 

The Solar Spectrum 
May I venture to suggest that quite possibly something of 
value might be obtained by observing the sun during totality with 
a spectroscope of reasonable dispersive power (say four or five 
prisms) wthout a collimator, or even simply with one of the so- 
called meteor spectroscopes. 
If the bright rays and rifts are really and simply (or even 
mainly) composed of the green-line-giving substance, they will 
give a well-defined green image ; if they are formed by reflection 
(either at the sun or in our atmosphere) of ordinary sunlight, they 
would be so dispersed as to be invisible or nearly so, and if 
formed by the reflection of chromosphere light they would give 
several images, the red (C) and blue-green (F) being most con- 
spicuous. C, A. YounG 
Hanover, N.H., U.S., Sept. 13 
* * Arrangements have already been made for carrying out a 
similar suggestion to this by the Eclipse Committee ; and the 
corona will also be observed with an open slit.—Ep. N. 
Eclipse Photography and the Spectroscope 
THE endeavour of the Eclipse Committee to secure some 
uniformity in the photographs from different stations next 
December does not appear to be duly appreciated, it being con- 
tended that immense ‘personality’? shown in various photo- 
graphers’ manipulation must frustrate the good intention. I 
submit that in this case the personality is greatly over-estimated ; 
that a number of competent photographers taking the same sub- 
ject would probably produce, under any ordinary circumstances, 
pictures bearing considerable resemblance ; while by using like 
apparatus and giving exposure of the same duration, we might 
safely predict a similarity of result amply sufficient for compara- 
tive purposes, and for the identification of structural peculiarity 
should it exist. 
Among others there is a possible advantage to accrue from 
uniform work by the philosophers which I have not seen or 
heard noticed. Supposing the outer corona, rays, streamers, or 
any portion of the apparently luminous matter be terrestrial, is it 
unreasonable to expect that photographs, taken at stations more or 
less widely separated, will, when properly combined in the 
stereoscope, give clear ocular proof of the sublunary situation of 
such luminous matter? Henry Davis 

Phenomena of Contact 
Mr. STONE can safely be left tomeet the arguments specially 
addressed to him in Prof. Newcomb’s letter; but as the subject 
relates to the only point of importance touched on in Prof. 
Newcomb’s criticism of my chapter on the sun’s distance, I crave 
permission to meet his general argument. 
I submit that he tries to prove too much. 
He admits that the phenomenon of irradiation exists in the case 
of adisc. The sun’s disc, then, must be to some extent enlarged, 
and the dark disc of Venus must be to some extent reduced by 
the effects of irradiation. Now this being so, what becomes of the 
cusps, when Venus is all but wholly on the sun’s disc? Either 
the irradiation is diminished near the cusps or it is not. If it is 
diminished there must be distortion, because the disc of Venus 
is then not uniformly reduced : if the irradiation is not diminished 
a ligament must appear. 
Let any one draw a large circle (say a foot in diameter) on 
paper, and a small one (say an inch in diameter) extending very 
slightly (say by the twentieth of aninch) beyond the boundary of 
the first ; and let him blacken the smaller circle as well as all 
the space outside the larger one. He has then a space represent- 
ing the disc of the sun with a very large Venus upon it near the 
time of internal contact. Now let him conceive the whole of 
this space (a sort of exaggerated crescent) slightly enlarged as by 
irradiation, the enlargement-fringe extending outside the boundary 
of the large disc and inside the boundary of the small black 
(incomplete) disc. He will find the conception of this enlarge- 
ment exceedingly easy everywhere save near the cusps ; but here 
there is a difficulty in determining how the fringe outside the 
larger disc is to be joined on tothe fringe inside the smaller disc. 
If he can conceive these two fringes meeting in such sort as to 
leave the reduced outline of the small disc completely circular up 
to the very points in which it meets the enlarged outline of the 
large disc, he will have done what Prof. Newcomb’s theory re- 
quires. But note, this must be done for the case when the fringe 
of enlargement is wider than the twentieth of an inch, by 
which the small disc overlaps the large one. When this is the 
