PSILOTOPSIDA 43 



Islands, while another species, T. Vieillardi, is probably con- 

 fined to New Caledonia. (Some workers recognize a further 

 four species, of restricted distribution, although it is possible 

 that they warrant no more than subspecific status.) T. 

 tannensis most commonly grows as an epiphyte on the 

 trunks of tree-ferns or, along with other epiphytes, on the 

 trunks and branches of forest trees, in which case its aerial 

 axes are pendulous, but occasionally it grows erect on the 

 ground. By contrast, T. Vieillardi is more often terrestrial 

 than epiphytic. It may further be distinguished by its 

 narrower leaves and by certain details of its stelar 

 anatomy. 



Like PsUotum, Tmesipteris is anchored by a dichotomous 

 rhizome with rhizoidal hairs and mycorrhizal fungus hyphae. 

 The aerial axes, however, seldom exceed a length of 25 cm 

 and seldom branch or, if they do so, then there is but a 

 single equal dichotomy. Near the base, the aerial axes bear 

 minute scale-hke leaves very similar to the leaves o^ PsUotum, 

 but elsewhere the branches bear much larger leaves, up to 2 cm 

 long, broadly lanceolate and with a prominent mucronate 

 tip (Fig. 6E). Their plane of attachment is almost unique 

 in the plant kingdom, for they are bilaterally symmetrical, 

 instead of being dorsiventral. They are strongly decurrent, 

 with the result that the stem is angular in transverse section 

 and they each receive a single vascular bundle which extends 

 unbranched to the base of the mucronate tip, but does not 

 enter it. In the distal regions of some shoots, the leaves are 

 replaced by fertile appendages which, like those oi PsUotum, 

 may be regarded as very short lateral branches, each bearing 

 two leaves and terminating in fused sporangia (normally 

 two) (Figs. 6F-6H). 



The internal anatomy of the rhizome is so similar to that 

 of PsUotum that the same diagram (Fig. 6C) will suffice to 

 represent it. In the transition region of Tmesipteris tannensis 

 (Fig. 61), the central rod of tracheids becomes medullated and 

 sphts up into a variable number of strands which are mesarch 



