276 REPRODUCTION, EVOLUTION, ETC. 



satisfactory to retain the better known name of Nematophyton. The 

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

 ever the size of the stem they are 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, re- 

 garded 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 indicative of definite thickening, though striations have 

 been seen in specimens from other localities. 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 apparently structureless and may well have been a muci- 

 laginous layer during life. 



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

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

 broad, and also constructed of the wide and narrow tubes. The 

 thallus is surrounded by a cuticular layer that exhibits a pseudo- 

 cellular pattern, and which includes within the cuticle and among 

 the peripheral tubes firm-walled spores of various sizes ; in N.pseudo- 

 vasculosa the spores were definitely cuticularized and so the 

 suggestion was made that these were land plants or parts of a land 

 plant. The wide tubes, which have thin pale brown walls, are 

 translucent in appearance and exhibit distinct characteristic 

 annular thickenings. The cuticle, which is apparently readily 

 detached, possesses distinct cell outlines that were probably made 

 by the ends of the wide tubes from the ordinary tissue where they 

 became fused together at the periphery, as in the living genera 

 Udotea and Halimeda. Another species Nematothallus radiata is 

 more imperfectly known. 



From the structure described above it can be seen that the 

 members of this group are strongly reminiscent of the Laminariales 

 and Fucales, and it is tempting to suppose that they represent land 

 migrants from one of these groups. Problems that have to be 

 solved are : (i) The cuticularized spores ; whilst no such spores with 

 hard outer walls are known from the brown algae they are recorded 

 from the Chlorophyceae, e.g. Acetabularia. However, the sug- 



