reproduction; evolution 305 



In spite of this it is very probable that the leafy Nematothallus was 

 the photosynthetic lamina of the stem-like Nematophyton and may 

 also have functioned as the reproductive organ. In the lowest strata 

 the plants are to be found associated with remains of marine 

 animals, thus suggesting their power to grow under marine or 

 brackish conditions, whilst in the higher strata they occur in beds, 

 which are regarded as fresh water or continental, where they are 

 associated with plants that were undoubtedly terrestrial. The 

 presence of spores in Nematothallus is regarded as rendering it im- 

 likely that they were algal in nature, but the spores may be com- 

 parable to the hard-walled cysts such as are to be found in 

 Acetabularia, 



The genus Nematophyton is found in the Silurian and Devonian 

 rocks where it was first described under the name of Prototaxites 

 and referred to the Taxaceae, but subsequently it was accepted as 

 an alga and renamed Nematophyton or Nematophycus. Later the 

 name Prototaxites was revived and it was placed in the Phaeo- 

 phyceae, whilst Krausel (1936) has stated that it must have 

 had the appearance of a Lessonia (cf. p. 182) and also that it existed 

 in aquatic habitats which may have been marine, brackish or fresh. 

 The valid name is therefore Prototaxites^ but as this tends to convey 

 a false impression of the plant's affinities it would seem more satis- 

 factory to retain the better known name of Nematophyton. The 

 largest specimen is a stem up to two feet in diameter, but whatever 

 the size of the stem it is usually composed of two kinds of tubes, 

 large and small. The large tubes have no cross partitions, but in 

 some species they are interrupted in places by areas, regarded as 

 medullary rays or spots by some authors, which are wholly occupied 

 by small tubes that in other parts of the thallus simply take a 

 sinuous course between the large tubes. The wide tubes, in the 

 latest specimens described by Lang (1937)? show no markings in- 

 dicative of definite thickening, though striations have been seen in 

 specimens from other locaUties. Around the outside of the central 

 tissue there is a cortex, or outer region, composed of the same tubes 

 where they bend outwards towards the periphery and eventually 

 stand at right angles to the surface. The outermost zone of all is 

 apparentiy structureless and may well have been a mucilaginous 



layer during life. 



Nematothallus is a genus composed of thin, flat, expanded in- 

 crustations of irregular shape and up to 6\ cm. long by i cm. broad. 



