194 THALLOPHYTA. [CH. 



lays stress on the resemblance of some of the tubular elements 

 in the stem to the sieve-hyphae of the recent seaweeds Macro- 

 cystis and Laminaria. He concludes that the new facts he 

 records make it clear that Nematophycus "is an alga, and of 

 an alliance with the Laminarias." The recent evidence brought 

 forward by Penhallow is not entirely satisfactory ; the drawings 

 and descriptions of the supposed trumpet-shaped sieve-hj^hae 

 are not conclusive. On the whole it is probably the better 

 course to speak of Nematophycus as a possible ally of the 

 brown algae rather than as an extinct type of the Siphoneae, 

 but until our knowledge is more complete it is practically 

 impossible to decide the exact position of this Siluro-Devonian 

 genus. 



Solms-Laubach^ has suggested that the generic name Nema- 

 tophyton, used by Penhallow in preference to Garruthers' term 

 Nematophycus, is the more suitable as being a neutral designa- 

 tion and not one which assumes a definite botanical position. 

 In view of the nature of the evidence in favour of the algal 

 affinities of the fossil, the reasons for discarding Garruthers' 

 original name are hardly sufficient. 



Before discussing more fully the distribution and botanical 

 position of Nematophycus we may describe at length one of the 

 best known species, and give a short account of some other forms. 



1. Nematophycus Logani (Daws.). Fig. 39, A — E. The 

 stem possesses well marked concentric rings of growth due to 

 a periodic difference in size of the large tubular elements. 

 The tissues consist of two distinct kinds of tubular elements, 

 the larger tubes loosely arranged and pursuing a fairly regular 

 longitudinal course, and having a diameter of 13-35 yit; the 

 smaller tubes, with a diameter of 5-6 /i, ramify in different 

 directions and form a loose plexus among the larger and more 

 regularly disposed elements. Branching occurs in both kinds 

 of tubes ; septa have been recognised only in the smaller tubes. 

 Irregular and discontinuous radial spaces traverse the stem 

 tissues, having a superficial resemblance in their manner of 

 occurrence to the medullary rays of the higher plants. 



1 loc. cit. p. 83. 



