OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
207 
Sivpplementa)nj Ohservations. 
Strong objections have been offered to the supposition contained in the foregoing 
paper that any translation of position in the elements constituting any p)ennanent 
tissue wa.s possible. Since the paper was written I have devoted much time to this 
matter, which has an important bearing upon the chief subject dealt with in the 
present Memoir. The further I carry my enquiries into the question of the origin 
and growth of the medulla, and the contenq)oraneous expansion of the investing 
vascular medullary cylinder (the “etui medullaire ” of Brongniart), the more clear 
the evidence becomes that in these primaeval vascular Cryptogams we are brought 
face to face with important histological and physiological phenomena to which no 
exact parallels are to be found amongst living plants. Yet these phenomena must be 
as capable of explanation as the many similar ones to which the present race of philo¬ 
sophic botanists have given so much attention ; and, as the phenomena in question 
must have an important bearing upon the problem of evolution, they demand a similar 
amount of careful study. 
Though, as already observed, living plants present no exact parallels to the con¬ 
ditions which I have discovered amongst the Carboniferous Cryptogams, we can 
scarcely suppose that those conditions are the results of vital agencies of which no 
traces have descended to living plants. Some further important conclusions arrived 
at by the late Professor de Bary seem to me to have a practical bearing upon the 
subject. Speaking of the origin of intercellular spaces, he says, “ These arise in two 
ways in the original masses of cells, which, at least when in the state of meristem, are 
always in uninterrupted connexion. First, by separation of permanent* tissue elements, 
as the result of their unequal surface-growth in different directions, the original 
common walls splitting, while the common limiting layer, which was originally 
present is—perhaps always—dissolved. Secondly, by disorganisation, dissolving, or in 
many cases rupture of certain transitory cells, or groups of cells, which are surrounded 
by permanent tissues. We may call the first mode of origin scldzogenetic, the 
second lysigenetic” (‘Comparative Anatomy of tlie Vegetative Organs of the Phanero¬ 
gams and Ferns,’ p. 200). 
In the above passages de Bary had chiefly in view the formation of cavities 
destined to be occupied by gums, resins, and various other unorganised secretions ; 
whereas, in the case of our fossil plants, though we have to deal with similarly 
enlarged cavities, these latter are destined to be occupied by an organised cellular 
parenchyma ; but these differences in their ultimate purpose do not materially affect 
the way in which the newly-formed and expanding cavities are brought into exist¬ 
ence. The essential points established by de Bary are ; first, that vertically elongated, 
more or less cylindiical canals can be developed even m p>ermanent tissues where such 
hollow spaces had no previous existence ; and, secondly, that such cavities may be 
* The italics are Profe.ssor de Bart’s. 
