68 



Class 8. Phaeophyceae (Olive-Brown Seaweeds). 



The Phaeophyceee are Alga?, with chromatophores in which the 

 chlorophyll is masked by a brown colour (phycophasin). The pro- 

 duct of assimilation is a carbohydrate (fucosan), never true starch. 

 In the highest forms (Fucacece), the thallus presents differentiation 

 into stem, leaf, and root-like structures. The asexual reproduction 

 takes place by means of zoospores. The sexual reproduction is 

 effected by the coalescence of motile gametes, or by oogamous 

 fertilisation. The swarm-cells are monosymmetric, each moved 

 by two cilia which are true protoplasmic structures, and generally 

 attached laterally (Fig. 65). The Phasophyceee are almost entirely 

 salt-water forms; a few species of Lithoderma live in fresh water. 



The class is divided into two families : 



1. PMIOSPORE^; : 1 Sub-Family, Zoogonicaa; 2 Sub-Family, 

 Acinetae. 



2. CYCLOSPORE^; : Fucaceoe. 



Family 1. Phaeosporeae. 



The family consists of multicellular plants, whose cells are 

 firmly united together to form a thallus; this, in the simplest 

 cases, may be a branched filament of cells (Ectocarpus), or, in the 

 highest, may resemble a stem with leaves (Laminariacece} , while 

 all transitional forms may be found between these two. The 

 thallus grows by intercalary divisions (e.g. Ectocarpus], or by an 

 apical cell (e.g. Sphacelaria) ; pseudo-parenchymatous tissue may 

 sometimes be formed by cells, which were originally distinct, 

 becoming united together. The size of the thallus varies ; in 

 some species it is quite small almost 

 microscopical, while in the largest it 

 is many metres in length. 



The vegetative cells in the lower 

 forms are nearly uniform, but in those 

 which are more highly developed 

 (Laminariacece and Fucaceic\ they are- 

 FIG. 65. Swarmspore of CMeria sometimes so highly differentiated that 

 mechanical, assimilating, storing and 



conducting systems may be found ; the last named systems are 

 formed of long cells with perforated, transverse walls, which bear 

 a strong resemblance to the sieve-tubes in the higher plants. 



The colouring niatter in the living cells (" phoeophyl ") contains 



