DICKSONIEAE 595 
form of a broad ribbon of tissue, with its margins folded inwards (Fig. 331). 
Very shortly after its departure from the axial stele this ribbon breaks 
up into a number of isolated strands arranged in horse-shoe fashion as 
seen in the transverse section. The point of disintegration varies from 
one leaf to another, and sometimes it does not occur until the free petiole 
is reached. There are no accessory strands in the pith nor in the leaf- 
stalk, as are seen in some other related Ferns. 
The structure, even in the large dendroid species, such as D. sguarrosa, 
antarctica, and Schiedei, appears to be the same as that in D. Baromets, 
but on a larger scale. The relation of the whole to a solenostelic type 
is obvious: where a leaf is inserted a gap occurs in the solenostele; but 
the leaf-gaps are very small and close up rapidly: if these leaf-insertions 
be close together, as they are in an upright radial stock, the foliar gaps 
Fic. 331- 
Dicksonia Barometz. Portion of the vascular system of the stem, seen from within, 
and showing the departure of three leaf-traces. (After Gwynne-Vaughan.) 
will overlap, and give to the stele a reticulate character. On the other 
hand, the leaf-trace is originally a single strand, and is clearly seen to 
be so at its base in D. Barometzs; but as it passes up the petiole it is 
disintegrated so as to form a number of separate strands. It will be 
seen later that this structure is in principle the same as that seen in 
the genus Dennstaedtia, allowance being made for difference in size, and 
in the elongation of the rhizome with its isolated leaves. 
Both structurally and in the characters of the sorus and sporangium 
the Dicksonieae as now limited occupy a position between Zhyrsopieris and 
the Dennstaedtiinae, while in soral condition, though not in spore-output, 
the family shows analogies with the larger species of Aymenophyllum. 
DENNSTAEDTIINAE. 
This sub-tribe was founded by Prantl,! to include the genera Dennstaedtia, 
Microlepia, Leptolepia, Saccoloma, and HHypolepis. The most important of 
1 Arbeiten Konigl. Bot. Garten su Breslau, vol. i., p. 8. 
