FLORIDE^. 125 



motionless male bodies called pollinoids, that become 

 attached to a trichogyne : after fertilization the oogonium 

 usually becomes enclosed in a pericarp. 



With very few exceptions the species are marine, and 

 grow in deep water below low-water mark, or in rock-pools 

 that retain their water at low-tide. Their colour is always 

 some shade of red or purple, when flourishing under normal 

 conditions ; but when growing in positions where they are 

 much exposed to light they become green, as is also the 

 case when stranded on the beach, and especially after a 

 shower of rain. The red colour is due to a complex 

 pigment called rho'dospermin, that can be extracted by 

 immersing the plant in fresh water. 



The structure of the thallus presents a sequence in the 

 various groups forming the Florideae, from primitive filaments 

 of superposed cells, to the most complex types of structure 

 met with in the Algse. In the simpler species of Calli- 

 thamnion as C. Rothii, the thallus consists of minute, 

 sparingly-branched filaments, consisting of a single row 

 of cells, that form broadly-spreading velvety patches on 

 rocks at half-tide level. In another group, represented in 

 the British seas by the genus Melobesia, the thallus forms 

 flattened or irregularly rugulose crust-like expansions, at- 

 tached to the larger algae or to rocks, and of a whitish 

 colour, due to the cell-walls becoming encrusted with 

 carbonate of lime. In the genus Porphyra the thallus 

 consists of a large flat plate of cells. In Xh^ polysiphonous 

 type of growth, illustrated by the species of Polysiphonia, 

 there is a large central axis or filament, composed of a 

 single row of superposed cells, surrounded by a varying 

 number of smaller filaments known a.s periceiifral-cells. This 

 arrangement originates as follows : The species are fila- 



