CYATHEAE 609 
with the main stele, except sometimes by a small branch near their point 
of origin. 
It seems, therefore, that the internal vascular strands of Alsophila 
excelsa owe their existence to the same initial phenomena as do those of 
Dennstaedtia rubiginosa; that is to say, they are probably derived from 
the elaboration of a local thickening of the xylem-ring at the margins of 
the leaf-gaps in the ordinary stelar cylinder; but they do not appear at 
all until the ordinary stelar cylinder has become dictyostelic. 
The ontogeny thus disclosed for a complex Tree-Fern may be held as 
a valid suggestion of the way in which the mature condition was achieved 
in descent. It starts from a protostelic state, which is, however, brief, and 
passes to the solenostelic by intrusion of outer-lying tissues into the xylem- 
core ; but this again passes into the dictyostelic by reason of the overlapping 
of the leaf-gaps: and lastly, by intrusion ‘of vascular growths from the 
margin of the leaf-gaps, the medullary system is produced. All these 
steps, so quickly passed over in the individual life, are readily intelligible, 
and even probable, in the evolutionary story of plants with a massive 
axis, bearing large and closely disposed leaves. 
The protostelic state, here so short, is the permanent condition in 
most of the Gleicheniaceae. But the most advanced species of Gleichenia 
(G. dichotoma and fectinata) show signs of solenostely, while in Adsophila 
pruinata the solenostelic state appears to be permanent. But in other 
species of Alsophila it also is a phase quickly passed through to the 
dictyostelic state, which is then permanent. Finally, the medullary 
system absent in A. pruznata, as it is also in Dicksonia, but developed 
in Cyathea, is clearly a late accessory, probably consequent upon the 
enormous distension of the pith in relation to the wide leaf-bases. 
The leaf-trace also presents features of comparative interest: in the 
young plant it consists of a single strand, as it is in all the G/eichenzas: 
in A. prutnata it appears to be so at the base even of the mature leaf, 
though it soon breaks up into separate strands as it passes up into the 
leaf-stalk; but in most of the Cyatheae the leaf-trace in the mature 
shoot is from the first composed of a number of distinct strands. These 
successive steps again indicate a probable phyletic progression, the young 
plant showing a condition similar to that seen in simpler types, and 
especially in the Gleicheniaceae. 
If the facts derived from the characters of the sorus be put into 
relation to these from anatomy, a substantial parallelism emerges, point- 
ing in both cases towards the Gleicheniaceae as a probable indication 
of the genetic source. In soral characters A/sopji/a is the nearest to 
Gleichenia, and especially to those species in which the sorus is no 
longer uniseriate, but consists of a large number of relatively small 
sporangia (G. dichotoma and pectinata). It is in these very species that 
there is a definite advance towards a state of solenostely not very far 
removed from that actually seen in A. pruinata. From such a 
2Q 
