374 



BOTANY 



latitudes. A few genera (e.g. Batrachospermum, Lemanea) grow in 

 fresh-water streams. 



The thallus of the Red Algae exhibits a great variety of forms. 

 The simplest forms are represented by branched filaments consist- 

 ing of single rows of cells (e.g. Callithamnion). In other cases the 

 branched filamentous thallus appears multicellular in cross-section. 

 In many other forms the thallus is flattened and ribbon-like (e.g. 

 Chondrus crispus, Fig. 310; Gigartina mammillosa, Fig. 311); while 

 in other species it consists of expanded cell surfaces attached to a 

 substratum. 



One of the more complicated forms is Delesseria sanguined 

 (Hydrolapathum) (Fig. 9), which occurs on the coasts of the Atlantic. 



The leaf -like thallus which 

 springs from an attaching disc 

 is provided with mid ribs and 

 lateral ribs. In the autumn the 

 wing-like expansions of the thallus 

 are lost, but the main ribs persist 

 and give rise to new branches in 

 the succeeding spring. All the 

 Florideae are attached at the base 

 by means of rhizoidal filaments 

 or discoid holdfasts. The thalli 

 of the Corallinaceae, which have 

 the form of branched filaments 

 or of flattened or tuberculate in- 

 crustations, are especially char- 

 acterised by their coral-like ap- 

 pearance, owing to the large 

 amount of calcium carbonate 

 deposited in their cell - walls. 

 The calcareous Florideae are 

 chiefly found on coasts exposed to a strong surf, especially in the 

 tropics. 



The Rhodophyceae are usually red or violet ; sometimes, however, 

 they have a dark purple or reddish-brown colour. Their chromato- 

 phores, which are flat, discoid, oval, or irregular-shaped bodies and 

 closely crowded together in large numbers in the cells, contain a red 

 pigment, PHYCOERYTHRIN, which completely masks the chlorophyll 

 and appears to be chemically combined with it (p. 62). True starch is 

 never formed as a product of assimilation, its place being taken by 

 other substances, very frequently, for example, by Floridean starch, 

 in the form of spherical stratified grains which stain red with iodine. 

 Oil-drops also occur. The cells may contain one or several nuclei. 



Reproduction is effected either asexually by means of spores, or 

 sexually by the fertilisation of female organs by male cells. 



FIG. 311. Gigartina mammillosa. s, Wait->haped 

 cystocarps. (} nat. si/r.) 



