GENERAL 129 



{h) Erect filaments with cortication {corticated type, e.g. Spha- 

 celaria). 



(i) Simple or laminate parenchymatous thallus (e.g. Punctaria). 



(j) Improved parenchymatous structure with internal differentia- 

 tion of the tissues (e.g. Laminariales). 



(a) to {c) are generally of an ectocarpoid type (like Ectocarpus; 

 cf. p. 132) with a single central filament, or else of a mechanically 

 produced cable type when the central filaments are twisted to- 

 gether by wave action to form a rope or cable. Many of these, 

 whether reduced or not, exhibit the condition of heterotrichy 

 similar to that found in the Chaetophorales, but this is a feature 

 that will be discussed elsewhere (cf. p. 263). The thalli may also 

 reach a relatively large size and under these circumstances addi- 

 tional support is obtained as follows : 



(i) Increase in wall thickness (Stypocaulon) or the production of 

 a firmer cellulose material (Sphacelaria). 



(2) Twisting and rolling of the threads together. 



(3) Development of root branches or haptera. 



(4) The appearance of descending and ascending corticating 

 filaments. 



(5) Multiseptation takes place in a longitudinal direction. 



Branching may proceed from any cell, and it frequently takes the 

 form of a regular or irregular dichotomy, although sometimes a 

 spiral phyllotaxis may be found. 



The cells vary greatly in size but they always have distinct walls, 

 which are usually composed of cellulose, and although they are 

 uninucleate occasionally they become multinucleate. Plastids are 

 also present, but the green colour is masked by the brown pigment 

 fucoxanthin. This, how^ever, can be removed by boiling and the 

 thallus then takes on a green colour from the chlorophyll, the com- 

 position of which is not quite the same as that of the higher plants 

 because chlorophyll h is absent (cf. p. 290) whilst xanthophyll is 

 also missing in the higher members, e.g. Fucales. The products of 

 assimilation are alcohols, carbohydrates and oils but no true starch 

 is formed. Hyaline hairs occur in many forms and their function 

 has been variously ascribed as 



(i) shock absorbers, 



(2) respiratory and absorptive organs, 



CSA 



