XXXIII] COMPARISON 305 



give the type of Hcmitelia, and finally of Cyathea. It is a reasonable view 

 from comparison of the sori alone that the Ferns named form a natural 

 sequence with consistently superficial sori. That sequence should then 

 be tested by comparison in respect of other features. External morphology, 

 anatomy, and dermal appendages should be examined, together with spor- 

 angial structure, segmentation, and spore-output: while contributory evidence 

 may also be expected from the gametophyte. 



Theexternal morphology seems at firstsight unpropitious: for thedifiference 

 between the creeping Gleichenia, with peculiarly constructed leaves, often ot 

 unlimited apical growth, seems to differ widely from the Tree-Fern-habit. 

 But it has been shown how LopJiosoria bridges the difference by its runners, 

 which assume distally the upright pose. Alsopliila aciileata does the same, 

 though Metaxya, which has so often been included in the genus Alsophila, 

 retains the creeping habit permanently. Thus the series appears to illustrate 

 the transition from a prone to an upright stem. The branching is charac- 

 teristic: the bud at the leaf-base being constantly abaxial in Lophosoria, 

 Metaxya, and AhopJiila, with its vascular connections median, a fact that 

 strengthens the affinity. It differs from the somewhat similar branching in 

 the Dicksonieae, where the insertion is lateral, and the vascular connection 

 marginal (see Vol. I, p. 77, Fig. 71). The bifurcation of the axis seen so 

 commonly in Gleichenia is matched by that occurring occasionally in the 

 Cyatheaceae, while the other branchings may probably be in their origin 

 modifications of dichotomy (Schoute, Beitrdge ziir Blattstdliingskhre, ii, 

 Groningen, 19 14). 



The peculiar features of the leaf in Gleichenia depend upon varying 

 localisation of growth combined with interrupted apical activity, carried out 

 in a Pecopterid type of leaf The leaves both of Gleichenia and oi Alsopliila 

 of the Pecopterid type, and comparison of herbarium-series of Dicranopt 

 and Alsophila shows the essential similarity, but disturbed in the former by 

 intermittent intercalary growth of the rachis. In a minor degree this is also 

 habitual in certain Cyatheaceae. The so-called "aphlebiae" of Heinitelia are 

 merely basal pinnae left behind by intercalary growth localised above them 

 (Fig. 567). A similar condition is sometimes seen also in Alsophila ("Studies, 

 II," Fig. O). Thus the peculiarities which stamp the leaf of Gleichenia find 

 their occasional correlative in the Cyatheaceae. The venation in Gleichenia, 

 Lophosoria and Metaxya is always open, and readily referable to a dichoto- 

 mous source. In the Cyatheaceae it is also open as a rule; but in the more 

 condensed leaf-forms, and especially in Heniitelia and some Alsophilas 

 where webbing has produced broad leaf-expanses, occasional vein-fusion is 

 seen. There is never a pronounced reticulum, and the venation is always 

 readily referable to a Pecopterid source. Thus throughout the series there is 

 a general uniformity of the leaf-structure, and even in its modifications. The 



are 



ej'is 



20 



