490 
NATURE 
[ Oct. 19,1871 

a north-west storm passing slowly north of the city 
without bursting, and disappearing in the south-east. 
Great branched masses of cloud appeared suspended from 
a sheet of Pallio-Cirrus. Some resembled bunches of 
grapes (a), others stalactites (4) in a striking manner, 
and still others formed round balls (c) separated by the 
azure of the sky. These balls seemed to be formed of 
snow flakes, and approached the form of Cirro-Cumu- 
lus; one might say of masses of snow rolled upon 
themselves by the effect of electric currents deve- 
loped during the storm. This was accompanied by 
thunder and lightning at Washington, and by lightning 
only at Beloit. @ represents one of these balls de- 
tached, with two sorts of penumbra, darker in e and /, 
and a streak at g, the rest whitish. Somebody at Beloit 
told me he had seen this form of cloud two or three 
times. A slightly brilliant aurora borealis was seen at 
Beloit the same evening. The night of its appearance at 
Washington no aurora was visible, but I do not know 
whether there may not have been one in other parts of 

the United States. The same evening and the next day 
at Beloit the temperature fell several degrees. It is a 
general belief that the aurora borealis is followed by a 
decrease of temperature. We know that in higher 
strata of the air vapour of water floats constantly in the 
form of frozen needles, especially in the polar regions. It 
is not impossible that these ice needles may be drifted 
by the electric current which engenders the aurora 
borealis* into lower latitudes, and thence towards lower 
strata of the atmosphere by the winds and storms. 
Hence the cooling of the air which is said to attend the 
aurora. ANDRE Poky 

EXOGENOUS STRUCTURES AMONGST THE 
STEMS OF THE COAL MEASURES 
HE perusal of Dr. M‘Nab’s reply to my short article 
: on the existence of an exogenous process of growth 
amongst the cryptogamic stems of the coal measures, 
confirms my previous conviction that the discussion of 
the details of my proposition can lead to no beneficial 
results until the publication of my large store of new 
* * See my Memoir on the Development of Electricity during the Aurora 
Borealis in the “ Annuaire de la Société Météorologique de France,” 1861, 
vol, ix. p. 42. 


facts has been completed. Dr. M‘Nab’s article convinces 
me, as indeed is necessarily the case, that he has no con- 
ception either of the nature or of the extent of those 
facts. Were it otherwise, he would see ata glance how 
far his explanations are from accounting for them. He 
has given an exposition of a common process of exo- 
genous growth, which is true as far as it goes; but I can 
assure him that the modifications of that process, so far 
as we can infer from peculiarities of structure, have been 
much more varied in past geological ages than he is aware 
of. He is pleased to affirm two things which require 
proof: (1) that I have “ been led away by the mere super- 
ficial resemblance of the parts;” and (2) that I have 
“never tried to understand the homologies of these 
stems.” To the first of these charges I plead not guilty ; 
to the second I reply that I was “#yzzg to understand 
these things when he was a child at school. Whether or 
not I have succeeded remains to be seen, but as yet he 
has told me nothing new to me. 
In studying the relations of the several parts of a 
plant, we have to consider three things, of which Dr. 
M‘Nab has mainly dwelt upon one. These are— 
1. The relative positions of the tissues. 
2. The mode of their development. 
3. The functions they have to perform. 

FIG. I 
The first point where I shall differ from Dr. M‘Nab is 
in supposing that a correspondence on the first of these 
clauses invariably pre-supposes a similar correspondence 
on the second. I shall have to show on a future occasion 
that Nature has attained the same end in more ways than 
one ; and that she refuses to be shut up to that dicho- 
tomous arrangement pre-supposed by Dr. M‘Nab; but 
for the present I will limit my illustration to the par- 
ticular mode of growth upon which he rests his case. 
If we take a perfect Stigmaria, we find its centre (a, Fig. 
2, p- 491) to be occupied by an axis of ordinary cellular pa- 
renchyma unmixed with any vascular tissue. This is sur- 
rounded by a ligneous or vascular cylinder (4) which, in its 
turn, is invested by a thick bark (c) consisting of a mix- 
ture of parenchyma and prosenchyma arranged in definite 
positions. The central axis differs in no respect whatever 
from the celluiar piths of ordinary exogenous stems. The 
woody cylinder consists of vessels which, in the trans- 
verse section, are arranged in radiating lines (@) running 
from the pith to the bark; these lines are separated by 
intervening cellular tracts (e), which I, in common with 
Brongniart and Dr. Hooker, designate medullary rays. 
The radiating lines of vessels exhibit proofs of distinct 
interruptions to the process of growth, and afford clear 
evidence that the cylinder began as a thin ring of vessels 
surrounding the pith, and which grew, by successive 
concentric additions of vessels, to its peripheral surface 
where the cambium layer is found in ordinary exogens. 
We have here no trace of the limiting tissues of which 
Dr. M‘Nab speaks ; the growth has been free and prac- 
