Notes. 
776 
question must remain at present undecided, although the mature 
structure certainly gives a strong impression of centripetal develop- 
ment. Potonid 1 has established a comparison between the secondary 
vascular tissues of the Calamarieae and the Sphenophyllaceae by men- 
tally doing away with the central mass of primary xylem that exists 
in the latter. By inverting this procedure, and considering it possible 
that the ancestors of the Equisetums may have possessed a xylem that 
extended to the centre of the stem, one is led to derive their structure, 
as it exists at present, from the modification of a stele with a solid 
central mass of centripetal xylem such as that of Sphenophyllum , or of 
certain Lepidodendreae. To illustrate the nature of the modifications 
that such a stele would have to undergo, a series of parallel de- 
velopments may be pointed out within the latter group ( Lepido - 
dendron Rhodumnense , selaginoides, Harcourtii , Sigillaria spinulosa , and 
Menardi ), in which parenchyma appears in the xylem, and gradually 
increases in quantity until only an attenuated peripheral ring of 
xylem remains, which then becomes more or less broken up into 
separate strands. 
It is suggested that the lateral xylem-strands in the vascular bundles 
of the existing Equisetums may perhaps be taken to represent the last 
remnants of a primitive central mass, and that this would be entirely 
in agreement with their apparently centripetal development, and in 
particular with their cauline course. 
D. T. GWYNNE- VAUGHAN. 
Glasgow. 
SOME OBSERVATIONS UPON THE VASCULAR ANA- 
TOMY OP THE CYATHEACEAE 2 . — In a number of Dicksonias 
with creeping or prostrate stems it is shown that the vascular system is 
solenostelic, the leaf-traces departing as a single strand curved into the 
form of a horseshoe, with its concavity facing towards the median line 
of the rhizome — Dicksonia adianioides , cicutaria , davallioides , apiifolia, 
and punctiloba. 
In D. apiifolia it is found that along the free margin of the leaf-gap 
there is a considerable increase in the amount of xylem in the 
1 Pflanzenpalaeontologie, p. 205. 
3 Abstract of paper read before the Botanical Section of the British Association, 
Glasgow, September, 1901. 
