290 
NATURE 
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[Fuly 30, 1885 
known, and already occupied an important position in 
Carboniferous floras. Though they are now completely 
extinct, the Cycads have to some extent preserved their 
characteristics. They retained many characteristics of 
the cryptogamic stock whence they originated, which are 
completely lost to remote descendants of the present day. 
As the earliest connecting links between Cryptogams and 
Phanerogams their morphology is peculiarly interesting, 
and the exquisite preservation of many of their silicified 
or calcareous stems permits the minutest details of this 
part of their structure to be studied. 
That Cryptogams reached a far higher stage of deve- 
lopment in the Palzozoic time than exists in any living 
representative is one of the few facts that has not been 
disputed. One of the best-known of these is Lepidoden- 
dron, a tree-like plant allied to the Lycopodiacee. Its 
structure has frequently been described, and presents 
nothing unusual to Cryptogams. But in Sigillaria, a plant 
strongly resembling it in nearly every other respect, we 
find a radiating vascular cylinder or woody zone in the 
cellular stem, with unmistakeable exogenous growth. It 
is richly supplied with medullary rays, and, Prof. William- 
son allows, presents clear evidence of interruptions to 
growth, succeeded by periods of renewed vital activity. 
The same writer also describes the prosenchymatous and 
the parenchymatous structure investing the woody zone 
as a bark, and remarks that, although not divisible into 
three layers, the enormous development of the elongated 
prosenchymatous fibres or bast-tissue in the inner layers 
of the epidermis of the fossil stems is a manifest fore- 
shadowing of the presence of that same tissue in the 
bark of living exogens, especially the Cycads. In Di- 
ploxylon there is a further development, the woody zone 
being made up of an inner or medullary vascular cylinder, 
either interrupted or continuous, composed of large 
scalariform vessels without definite order, and an outer 
cylinder of scalariform vessels of smaller size arranged in 
radiating fasciculi. There is no difference of opinion as 
to the exogenous nature of the woody zone, which bears 
a relatively small proportion to the diameter of the stem, 
and as to the presence of medulla or pith and bark; but 
while Adolphe Brongniart and our authors class Sigillaria 
in consequence as a low form of exogen, a progymno- 
sperm, Prof. Williamson and some of the German authors 
piefer to regard it as a highly-developed Cryptogam. He 
possesses specimens which conclusively prove to him that 
the exogenous wood is undeveloped in the young stages, 
and that young stems of Sigillaria are indistinguishable 
from Lepidodendrons ; but though there is a gradual pass- 
age from one to another, the typical Lepidodendron never 
produced a ligneous zone. Sir J. W. Dawson, who 
has done much to elucidate this subject, believes that 
even some Lepidodendrons are exogenous, and that 
all Sigillarias are so. The evidence goes to prove 
that unquestionable Lepidodendrons in youth gradually 
acquire the internal features, notably the exogenous 
ring, characteristic of Brongniart’s gymnospermous 
family of Sigillaria. So far as its bearing on evolu- 
tion is concerned, the differences of opinion scarcely 
affect the question. Whether they are looked upon as 
Cryptogams with exogenous growth, or exogens with 
cryptogamic characters, they are equally valuable as 
connecting-links, and if we agree with Prof. Williamson 
that they pass direct into true Lycopodiacez, the chain 
only becomes so much the more direct and complete. 
During growth the woody or exogenous zone increased for 
a certain period, but this was quickly arrested by the 
absorption or destruction in some way of the Cambium 
layer. The subsequent increase in diameter took place 
mainly in the cortical system, and to it the growth and 
solidity of the stem was principally due. The exogenous 
element in the oldest known trees is thus seen to have: 
been transitory and subordinate, for had it persisted in- 
definitely, the continued generation of fresh layers or new 
rings of growth would have produced true dicotyledonous 
stems. It is suggested that until seasons, or alternations 
of activity and repose, replaced the earlier uniformity of 
climate, an exogenous growth would have been of relatively 
little use to the plant. 
Sir J. W. Dawson has observed specimens of Sigil- 
larian stems possessing still more definite exogenous 
characters, and in Poroxylon-M. Rénault finds that the 
wood is dotted with areolated puncte similar to those 
distinguishing the spiral vessels of Cycads and Arau- 
cariz. Still the structure of their stems agree in so 
many respects with those of the highest heterosporous | 
Cryptogams, the Lepidodendrons, that the difference 
between them remains almost insensible. Moreover, the — 
Sigillarias are not supposed to be in the direct line of 
the evolution of Gymnosperms, but an offshoot which was 
quickly extinguished, and even in the Carboniferous time 
exogenous trunks were growing side by side with them. 
The construction of their stems was greatly varied, and it 
is evident that their plan of growth was susceptible of 
very considerable modification and development. It is | 
now universally acknowledged that some Stigmarias are 
the roots of Sigillaria, yet here again we find a remarkable 
divergence of opinion, for while our authors regard them 
as rhizomes capable of bearing leaves as well as roots, 
Prof. Williamson contends that they are merely roots 
with rootlets. The Stigmarian rhizomes were procumbent 
and vegetated in the soft mud, Sigillarian stems budding 
from them occasionally, erect and cylindric, and crowned 
with a mass of long and linear leaves whose scars, im- 
pressed upon the bark, give rise to complex and beautiful 
tesselated designs. To how great an extent their fruiting 
organs preserved their cryptogamic attributes is unfor- 
tunately even yet imperfectly known. 
The next type of progymnospermous stems, that of 
Calamodendron, is more remarkable because more ab- _ 
normal, for it possessed a hollow fistular stem with verti- 
cillate leaves, closely resembling in appearance a gigantic 
Equisetum. Here again there is an irreconcilable 
divergence between the views of the French authors and 
those of Prof. Williamson. The former separate Calam-_ 
ites from Camalodendron, believing them to have been 
confounded simply because the casts of the interior of 
the hollow stems of both accidentally present the same 
grooved and articulate aspect, though morphologically 
they completely differ. Calamites they maintain to be a 
Cryptogam whose thin walls presented within and without 
the same structure as those of Equisetum. Prof. William- 
son urges that no such Calamite has ever been found, but 
that, however thin the walls of a specimen may be, they 
always show the Calamodendron structure if any is pre- 
served, and that the points of agreement are too remark- 
