26 
D. H. Scott. 
the Flowering Plants the latter increases at the expense of the 
former. Some of the steps of the transition have been acutely 
traced by the French observers; if their inferences as to the 
course of descent do not hold good, it is because they did not 
sufficiently realize that a similar gradation was followed in several 
distinct phyla. 
It need scarcely be pointed out that the development of the 
primary wood in the stems of Cryptogams is not necessarily 
centripetal; the reverse holds good in various simple types of stem, 
e.g.: the Hymenophyllacese. Whether the centripetal primary 
wood was the most primitive or not is immaterial to our purpose; 
in the series of stems with which we are concerned, the primary 
wood is, as a matter of fact, wholly or largely centripetal, a fact 
which facilitates its distinction from the secondary wood superadded 
in the centrifugal direction by the cambium. Such part of the old 
or Cryptogamic wood as was from the first centrifugal could 
easily accommodate itself to the new conditions, and merely tends 
to become merged in the secondary tissues. The centripetal part, 
on the other hand, became more and more superfluous as the 
possibilities of indefinite extension in the opposite direction came to 
be realized. Here no compromise was possible; the old wood was 
becoming a useless encumbrance and had to go. 
We begin our series with the fern-like genus Heterangium 
(Lower Carboniferous to Permian). Here the old wood is still 
dominant, occupying the whole interior of the pithless protostele. 
Only the peripheral groups of primary tracheides are in /direct 
continuity with the leaf-traces; the central wood is purely cauline. 
This may indicate a very primitive condition, if leaf-trace wood and 
central wood had a distinct origin, as suggested by Tansley and 
Chick. 1 The leaf-trace strands have a little primary centrifugal 
xylem, but this is insignificant in comparison with the great centri¬ 
petal mass. Secondary growth began late, and did not usually 
attain any very great extent; the new wood is an auxiliary to the 
old, but does not yet attempt to displace it. 
Mcgaloxylon (Coal-measures). In this genus, discovered by 
Mr. Seward, the primary wood, as in Heterangium , still occupies 
the whole interior of the stele (here of large dimensions) but shows 
a striking differentiation. The exarch leaf-trace strands are very 
distinct and consist of ordinary elongated tracheides, but the 
'Ann. of Bot., vol. xv., p. 35. 1901. 
