Fig. 70 Ectocarpus. A, E. confervoides, plant ( x 0-44) B, E. 

 tomentosusy unilocular and plurilocular sporangia ( x 100), C, 

 meisporangium, E. virescens. D, megasporangium, E. virescens. E, 

 microsporangium, E. virescens. (A, B, original; C-E, after Kniep.) 



creasing age, they become much shorter through truncation. The 

 erect filaments of some species have an intercalary growing region, 

 whilst in other species growth is diffuse, but the rhizoids increase 

 in length by means of apical growth. Each cell, which contains one 

 nucleus together with brown, disc or band-shaped chromatophores, 

 possesses a wall that is composed of three pectic-cellulose layers. 

 Generally two kinds of reproductive structures are present, the 

 plurilocular and unilocular sporangia, but some species possess a 

 third type, the meiosporangia. The unilocular sporangia always 

 occur on diploid plants and they give rise, after meiosis, to numer- 

 ous haploid zooids which may either function as gametes or else 

 develop without undergoing a fusion. The sporangia are sessile or 

 stalked and vary in shape from globose to ellipsoid, the mature 

 ones dehiscing through the swelling up of the centre layer in the 

 wall. The plurilocular sporangia, which are either sessile or stalked, 

 range from ovate to siliquose in shape and are to be found on hap- 

 loid or diploid thalli. In E. siliculosus they represent modified 

 lateral branches and arise as side papillae from a vegetative cell in 

 the filament. The plurilocular sporangia are divided up into a 

 number of small cells, each one of which gives rise to a zooid and, 

 when ripe, dehiscence takes place by means of a pore, the contents 

 either germinating directly or else behaving as gametes. The 

 gametes are usually aUke in size but the sex function becomes 

 weaker with age so that relative sexuality is induced, the older and 



129 



