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. squarrosa, 

 antarctica, and Schiedei, appears to be the same as that in D. Barometz, 

 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 



FIG. 331. 



Dicksonia Karometz. 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, Barometz ; 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 Thyrsopteris and 

 the Dennstaedtiinae, while in soral condition, though not in spore-output, 

 the family shows analogies with the larger species of Hymenophyllum. 



DENNSTAEDTIINAE. 



This sub-tribe was founded by Prantl, 1 to include the genera Dennstaedtia, 

 Microlepia, Leptolepia, Saccoloma^ and Hypolepis. The most important of 

 1 Arbeiten Kbnigl. Bot. Garten zu Breslau, vol. i., p. 18. 



