DIV. I 



THALLOPHYTA 



423 



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, and in some cases a blue pigment (PHYCO- 

 CYAN) in addition. They are developed from colourless, spindle- 

 shaped leucoplasts in the apical cells and germ cells. 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 



FIG. 364. Gigcrtina mamttlosa. s, Wart-shaped 

 cystocarps. (| nat. size.) 



sexually by the fertilisation of 

 female organs by male cells. 



The asexual SPOKES are of two kinds. 

 In the first case they are non-motile, 

 have no cilia, and are simply naked 

 spherical cells. They are produced, 

 usually, in groups of four, by the division 

 of a mother cell or sporangium. The 

 sporangia themselves are nearly spherical 

 or oval bodies seated on the tlialloid 

 filaments or embedded in the thallus. 

 The spores escape by a transverse rup- 

 ture of the wall of the sporangium. In 

 consequence of their usual formation 

 in fours, the spores of the Florideae 

 are termed TETRASPORES (Fig. 365). 



They are analogous to the swarm-spores of other Algae ; similar spores are found 

 also in the Dictyotaceae among the Brown Algae. The tetrasporangiuni as a rule 

 has to begin with a single nucleus, which divides to give rise to the nuclei of the 

 four spores. In some cases (Martensia, Nitophyllum), however, they are to begin 

 with nmlrinucleate, but all the nuclei except one degenerate. The monosporangia 

 of the Nemalionaceae, which liberate only a single spore, and the polysporangia of 

 the Ceramiaceae, which form a number, are equivalent to the tetrasporangia. 



The second form of asexual spore in the Red Algae is represented by the 

 CARPOSPORES (cf. p. 424), which are liberated singly from terminal carposporangia 

 as spherical and, to begin with, naked, non-ciliate protoplasts, and thus resemble 

 the monospores. 



In the construction of the sexual organs, particularly the female, the Rhodo- 

 phyceae differ widely from the other Algae. atrachospermum -monilifonne, a 

 fresh-water form, may serve as an example to illustrate the mode of their 

 formation. This Alga possesses a brownish thallus, enveloped in mucilage, and 

 consisting of verticillately- branched filaments. The sexual organs appear on the 

 branching whorls seated on closely-crowded, short, radiating branches. 



The antheridia, also known as spermatangia (Fig. 366 A\ are produced 



2 E2 



