DIV. i THALLOPHYTA 427 



The spores produced in the tetrasporangia (or in the corresponding mono- 

 sporangia or polysporangia) represent a second form of asexual spore, by means of 

 which an increase in numbers of the sexual plants is effected. They may be 

 entirely wanting in some cases (Nemaliori). 



In certain genera these sporangia occur only on the sexual plant itself. In the 

 majority of genera, on the other hand, plants are developed from the carpospores 

 which bear tetraspores only, and from these tetraspores the, usually dioecious, 

 sexual plants arise. The life -history then includes the three generations, 

 gametophyte, carposporophyte, and tetrasporophyte. A purely asexual generation 

 morphologically resembling the sexual generation has here been secondarily derived 

 from the latter. In some Red Algae with such a life-history a further complication 

 is introduced by the gametophyte bearing tetraspores, or equivalent monospores, 

 which again produce gametophytes. 



The behaviour^of the nuclei and their reduction division has been investigated 

 in a small number of species and has revealed noteworthy differences in the distri- 

 bution of the haploid and diploid phases among the generations ( &e ). 



In Scinaia the reduction division, according to SVEDELIUS, takes place in the 

 first division of the fertilised egg, so that this only is diploid while the carpo- 

 sporophyte, the carpospores, and the gametophyte proceeding from the latter 

 are all haploid. The gametophyte here reproduces itself asexually by haploid 

 monospores in place of tetraspores, which are wanting. Nemalion, according to 

 KYLIN, behaves similarly but has neither tetraspores nor monospores on the 

 gametophyte. 



It may be anticipated that in all Red Algae with this simple alternation of 

 generations the reduction division will follow the Scinaia type, which can be 

 regarded as the most primitive. 



In those Red Algae in which three generations occur in the life-history the 

 reduction division is relegated to the tetrasporangium, so that the gametophyte 

 proceeding from the tetraspores is haploid, while the carposporophyte, the 

 carpospores, and the tetrasporophyte developed from the latter constitute the 

 diploid phase. Polysiphonia, Griffithia, Delesseria, Nitophyllum, and Rhodomela 

 are known to behave in this way. If in such cases asexual spore-formation takes 

 place on the gametophyte this does not involve a reduction division. According to 

 SVEDELIUS the gametophyte of Nitophyllum punctatum is an example of this ; it 

 bears monospores in addition to the sexual organs. These monosporangia correspond 

 to the tetrasporangia of the tetrasporophyte, are at first multinucleate, but all 

 the nuclei except one degenerate. They produce, without any reduction, a single, 

 haploid monospore. 



Harveyella mirabilis ( 50 ), one of the Florideae occurring in the North Sea, is of 

 special interest. It grows as a parasite on another red seaweed, Rhodomela sub- 

 fusca, on which it appears in the form of a small white cushion-like growth. 

 As a result of its parasitic mode of life the formation of chromatophores has been 

 entirely suppressed, and thus this plant behaves like a true fungus. 



Economic Uses. Gigartina mamillosa (Fig. 364), with peg -like cystocarps 

 2-5 mm. in length, and Chondrus crispus (Fig. 363), with oval cystocarps about 

 2 mm. long, sunk in the thallus and tetraspores in groups on the terminal segments 

 of the thallus. Both forms occur in the North Sea as purplish-red or purplish- 

 brown Algae ; when dried they have a light yellow colour, and furnish the official 

 CARRAGHEEN, "Irish Moss," used in the preparation of jelly. AGAR-AGAR, which 

 is used for a similar purpose, is obtained from various Florideae ; Sphaerococcus 

 (Gracilaria} lichenoides supplies the Agar of Ceylon (also called Fucus amylaceus), 



