OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
205 
clearly that this is a mistake, as is obvious from the fact that even in transverse 
sections these rays are seen to be very much more numerous than the foliar bundles. 
What gives an interest to this frequent occurrence of barred cells in the medullary 
rays of these exogenous Lycopods is the fact pointed out by De Bary (‘Vergleichende 
Anato m ic/ p. 505), that amongst living plants such cells are chiefly confined to the 
medullary rays of Gymnosperms and especially to the Abietince. 
General Conclusions respecting the Carboniferous Lycopods. 
I think I am justified in expressing my conviction that the present memoir confirms 
the views I have so long advanced, viz. : that at least many of the Lepidodendroid 
plants acquire, through advancing age, those characteristics that have hitherto been 
relied upon to distinguish the Sigillarian from the Lepidodendroid forms. In his 
latest memoir, already quoted, M. Renault combines his fossil Gymnospermous 
Phanerogams in one group, to which he gives the common name of “Diploxylees,” 
and in a foot-note ( loc. cit., p. 260) he says : “ Sous ce nom je reunis toutes les plantes 
qui, dans le faisceau libero-ligneux de la tige ou des feuilles, presentent un double 
accroissement, lun centripete, l’autre centrifuge.” My friend here reasserts the old 
Brongniartian hypothesis in all its definiteness. If the evidence produced in the pre¬ 
ceding pages is not sufficient to demonstrate the erroneousness of this hypothesis, I 
am wholly unable to conceive what kind or number of facts would accomplish that end. 
The enormous number of Lepidodendroid and allied plants that have now been 
described by M. Renault and myself seem to me to justify an attempt to ascertain 
what bearing our discoveries may have upon the problem of Evolution. In making this 
attempt we must not forget that, whilst our researches have been confined to carboni¬ 
ferous plants, the discoveries of Dr. Dawson have shown that the differentiation of 
the Gymnospermous Dadoxylons from the Cryptogamic Lycopods was as definite in 
the Devonian age as in that which succeeded it. Nevertheless, it seems to have been 
during the later portion of the Palaeozoic epoch that the great changes were effected 
which caused the arborescent Lycopods to be replaced by the oolitic Gymnosperms ; 
and it fortunately happens that the plants which have furnished so rich a morpho¬ 
logical harvest belong to that important period of transition. 
In endeavouring to ascertain possible genetic relationships between plants, Geologists 
have to consider: 1st, external form; 2nd, the successive appearances of different 
types of elementary tissue : 3rd, the arrangement of those tissues in the several 
vegetative organs ; 4th, the reproductive structures. 
The study of external forms has occupied, and still occupies, so many observers that 
I shall not now dwell upon this subject. Outward resemblances, taken alone, are very 
unsafe guides, though they are too often the only ones that we can obtain. I would 
make but two observations in reference to them : So far as external appearances are 
concerned, I am wholly unable to draw a line between Lepidodendron and Sigillaria, 
