THE PTERIDOPHYTA : LYCOPSIDA, ETC. 607 



are not characters of any living Lycopod, but are both found in the fossil 

 Lepidodendraceae. In this group it is the tall, unbranched or sparsely 

 branched Sigilloria which seems to be the nearest type. Now SigiUaria was 

 the only Lepidodendroid to survive beyond the Carboniferous Period, and 

 it had a successor in the Mesozoic Period called Pleiiromeia, which retained 

 the outward morphology of SigiUaria with two very significant differences. 

 It became progressively shorter, so that instead of a 50-foot tree it formed a 

 plant only 2 or 3 ft. high, and the extensive Stigmarian rhizophore system of 

 SigiUaria was reduced to four, blunt, upturned lobes at the stem base, on which 

 the roots were produced. (See " Palaeobotany " in Volume III.) 



In hoetes the reduction of this rhizophore system has gone still further, 

 so that it no longer shows externally and is preserved only in the upturned 

 rhizophore-lobes at the base of the stele. Nevertheless the phylogenetic 

 connection is highly probable. 



It has been suggested that Pleuromeia represents a Sigillarian type adapted 

 to life in isolated marshes during the desert period of the Trias and hoetes 

 suggests a further stage of reduction in adaptation to a wholly aquatic life. 

 The morphology of hoetes indicates just such a reduction from a taller stock 

 by telescoping. In the Cretaceous Period there occurred another plant, 

 Nathorstiafia, which only reached a foot in height. It shows an even greater 

 resemblance to hoetes, although the rhizophores are still externally recogniz- 

 able as lobes at the stem base. As Walton says, " There seems little doubt 

 that the arborescent Lycopods of the Palaeozoic, and Pleuromeia, Nathorstiana 

 and hoetes form a series in which there is a progressive reduction in size and 

 in which the root-bearing region becomes simplified in external form." 



PSILOPSIDA 



The Psilopsida are Pteridophyta in which there are no true roots. The 

 organization is of a simple type, the branching being usually dichotomous. 

 Leaves are either absent or very much reduced. The plants are always 

 homosporous, the sporangia are terminal and are not aggregated into strobili. 

 The prothalli, where known, are saprophytic and subterranean. 



The series is divided into two orders, the Psilotales, which are living 

 plants, represented by the two genera PsUotiim and Tmesipteris, and the 

 Psilophytales, an order of archaic fossil plants which will be dealt with in 

 Volume III. 



Psilotales 



This is a very small order, of limited distribution, but interesting because 

 of its apparent connection with one of the oldest known groups of extinct 

 land plants, the Psilophytales, which flourished in Devonian times, some 

 1,500,000,000 years ago. 



There are only two genera, both small plants of peculiar morphology and 

 saprophytic habit : Tmesipteiis, which is limited to Australasia and the East 

 Indies, and Psilotwn, which is fairly common generally in the Tropics and 



