euphycophyta i49 



Chordariales 



This order is not recognized by Fritsch (1945), the members 

 being placed in the Ectocarpales. The type of structure, though 

 obviously derived from a branched filament, is so much more 

 elaborate that segregation of the members appears justified. The 

 basic construction in this order is the cable type, in which one or 

 more erect parallel strands arise from a prostrate basal thallus. 

 These give rise to interweaving lateral branches and the whole is 

 often enclosed in a mucous matrix. In fully developed species there 

 are three principal zones that can be recognized in the plant thallus : 



(a) a medulla composed of one or more long threads accompanied 

 by off-shoots of the first order; 



(b) a subcortex composed of off-shoots from the medulla. 



(c) a cortex of peripheral assimilatory filaments and colourless 

 hairs; 



Growth is commonly trichothallic but apical growth occurs in 

 two famines. 



There is also a certain amount of seconardy tissue which in some 

 parts may be rhizoidal in character. Degenerate or modified 

 versions of this cable type of construction are to be found in some 

 genera {Leathesia, Elachista), whilst in epiphytic forms (Myrionem- 

 ataceae) there may be considerable reduction, though the structural 

 pattern remains clear. In nearly all the species investigated the basic 

 life cycle consists of a macroscopic diploid plant alternating with a 

 microscopic gametophyte. In some cases the diploid plant arises as 

 a lateral outgrowth from a microscopic plethysmothaUus. 



Chordariaceae : Mesogloia {meso, middle ;^/o/a, slime). Fig. 8i 



In this genus there is a single central strand terminating in a hair 

 and having a distinct intercalary meristem just below the apex. 

 The cortex is formed of short horizontal filaments with somewhat 

 globose terminal cells that are packed in a gelatinous material. The 

 hairs, which are frequently worn away in the older parts of the 

 thallus, occupy a lateral position, but owing to inequalities of 

 growth they may appear to be terminal. The unilocular sporangia 

 are ovoid and are borne at the base of the cortical filaments, but 

 the elongate plurilocular sporangia, which incidentally are only 

 known for M. levillei, replace the terminal portion of the assimila- 

 tory hairs and hence are always stalked. Meiosis takes place in the 



