Sept. 28, 1871] 
or peculiar thickened bast-fibre-like cells from underneath, 
The stem thus consists of three sets of tissues: (1) the 
limitary tissues, including epidermis, periderm, &c. ; (2) 
the fibro-vascular bundles ; and (3) the primitive tissue or 
Grundgewebe of Sachs (see “‘ Mo. Mic. Journal,” vol. iii. 
p. 160). In an older dicotyledonous stem we find the 
limitary tissues becoming largely developed, cork-cambium 
and layers of cork being formed. The fibro-vascular 
bundles have also largely developed, the cambium cells by 
division, and the conversion of these new cells into per- 
manent tissue has formed a number of annual rings of 
wood-cells and vessels as well as layers of bast, while the 
primitive tissue only increases very slowly in the medullary 
rays, the pith not increasing, and the primitive tissue under 
the epidermis becoming lost in the rapidly-developing 
bark, Such is the structure of a dicotyledonous stem. 
In a monocotyledon we have the same tissues, limitary, 
fibro-vascular, and primitive. The primitive tissue is 
largely developed, forming the cellular tissue by which the 
fibro-vascular bundles are surrounded (Oliver, ‘“‘ Lessons,” 
p: 113, fig. 68). These fibro-vascular bundles differ quite 
as much in the nature of their cells and vessels as those 
of the dicotyledon, often one form being developed in 
excess of the other. The limitary tissues also develop 
cork and other cells. There is thus very little difficulty 
in comparing a very young dicotyledonous stem with that 
of a monocotyledon. In the monocotyledons the fibro- 
vascular bundles are closed, and therefore no annual 
layers are found ; but in such stems as Draczena, Aloe, 
Yucca, &c., we have the stem increasing in diameter. 
The outer cells of the primitive tissue divide and form not 
only new primitive tissue but new fibro-vascular bundles 
(Sachs, “Lehrbuch der Botanik,” ed. 2, p. 103, fig. 90). 
Prof. Williamson would probably call these Exogenous 
Endogens. 
When we come to the Lycopod and Fern stem, we find 
the same parts—limitary tissues, fibro-vascular bundles, 
and primitive tissue. In ferns the bundles are more or 
less scattered, like those of the monocotyledon, while in 
the Lycopods we either have them separate or else all 
joined together to form a central axis (see Sachs, of. 
cit., figs. 66 and 89). Round this central axis in Lyco- 
pods we have the primitive tissue, while outside we have 
the epidermis often with peculiar thickened cells under- 
neath, forming part of the limitary tissues. In Mosses, 
Charas, and Thallophytes we have only the primitive and 
limitary tissues, the fibro-vascular bundles being entirely 
absent. In some of the Thallophytes, however, as in 
Lessonia, we may have the primitive tissue increasing 
just as in Draczena. 
In Lepidodendron, as in some of our modern Lycopods, 
we have acentral axis of combined fibro-vascular bundles, 
and a large quantity of primitive tissue, no longer all 
parenchymatous, asin many of our recent Lycopods, but 
mostly prosenchymatous, as in L. chamecyparissus. This 
primitive tissue went on increasing year after year, new 
cells forming by division, these being soon changed into 
hard prosenchymatous cells. Outside we have the limi- 
tary tissue strengthened, as in some of our recent species, 
by remarkable prosenchymatous cells. In Lepidoden- 
dron the primitive tissue was capable of dividing in the 
same way as that of Draczena. The stem increased year 
after year, not by growth of the wood-cells, &c., of the 
fibro-vascular bundles, as in a dicotyledonous stem, but 
by additions to the primitive tissue. I never denied that 
the Lepidodendron stem increased in diameter, but pointed 
out that the increase takes place by multiplication of the 
cells near the periphery of the primitive tissue, the portion 
not likely to be often preserved in Lepidodrendon stems. 
This mode of growth is quite compatible with the state- 
ment that the fibro-vascular bundles are closed as they 
are both in Ferns and Lycopods. As Prof. Williamson 
admits that “the large vascular cylinder of the fossil 
forms is a development of what is seen not only in Zyco- 
NATURE 

427 

podium chamecyparissus, but in every one of the nu- 
merous Lycopods of which I have examined sections,” 
there is no difficulty .in settling the matter. The cylinder 
in L. chamecyparissus is part of the primitive tissue, not 
of the fibro-vascular bundles. Such being the case, the 
central axis of Lepidodendron is not a “vascular me- 
dulla,” but a series of closed fibro-vascular bundles, In 
Lepidodendron we have merely a_ pseudo-exogenous 
growth taking place in the primitive tissue, while in 
Gymnosperms and Dicotyledons we have true exogenous 
growth in the fibro-vascular bundles. In Ferns this 
pseudo-exogenous growth is not likely to take place, as a 
fern produces only a few large leaves, while ina Lycopod 
or Lepidodrendron, which produces numerous small leaves, 
water for purposes of transpiration would have to be 
rapidly supplied in yearly increasing quantities. This is 
provided for by the increase which takes place in the 
wood-cells of the primitive tissue, not as in Dicotyledons, 
by additions to the wood-cells of the fibro-vascular 
bundles. Prof. Williamson has been led away by the 
mere superficial resemblance of the parts, and has never 
tried to understand the homologies of these stems. He 
has mistaken the united closed fibro-vascular bundles in 
the centre of the stem for a vascular medulla, ze., for a 
portion of the primitive tissue ; and he has mistaken the 
woody cylinder surrounding this—which is a modified 
portion of the primitive tissue—for the united fibro-vascu- 
lar bundles of a dicotyledon. After making two such 
fatal errors, can his proposed new classification be con- 
sidered of any value? W. R. M‘NaB 

A NEW DYNAMETER 
2 need not be said that in astronomical observation it 
is always desirable, to say the least of it, to havea 
tolerably correct estimate of the magnifying power actually 
in use. This has hitherto been only attainable either by 
means of the maker’s valuation, or through the employment 
of the apparatus unfortunately termed a “ dynameter,” a 
word which every classical scholar would wishtoseeas soon 
as possible dismissed from circulation. The former alter- 
native is, I am sorry to say, often far from reliable; the 
latter involves an outlay not within the reach of every 
astronomical student. The Rev. E. L. Berthon, Vicar of 
Romsey, Hants, well known already for many ingenious 
and valuable inventions, has recently devised a little 
apparatus for attaining the same object, which deserves 
high commendation. Its very moderate price places it 
within the reach of all ; and its accuracy appears equal to 
that of instruments of more complicated construction and 
higher pretension. I have heard on excellent authority 
that very little dependence can be placed on the estimates 
of magnifying powers too frequently furnished to pur- 
chasers. Eyepieces are both constructed and rated too 
frequently by “rule of thumb,” and their real, if measured, 
will be found widely different from their nominal power. 
Some opticians, as, for instance, the celebrated reflector- 
maker Short, have had an unfortunate reputation for ex- 
aggerating the power of their instruments, and without 
any suspicion of misrepresentation: such has been the 
case even at the celebrated Optical Institute of Munich, 
as appears by the corrections made by W. Struve in the 
numerical values of the Dorpat oculars, 94, 140, 214, 
320, 480, 600, 800, 1,000, 1,500, 2,000, being respec- 
tively lowered by him on trial to 86, 133, 198, 254, 
420, 532, 682, 848, 1,150, 1,500. In this instance, it is pos- 
sible that some different mode of measurement may have 
led to the discrepancy. Uncertainty, it may be suspected, 
occasionally arises from this cause. I once undertook, at 
the special request of a friend, to verify with a double- 
image dynameter the power of some oculars constructed 
by a very eminent optician, whose name was an abundant 
guarantee for his good faith; but the results, on which I 
