THALLOPHYTA. 653 



The chromatophores are many, disc-like, and angular, with no pyrenoids. The cell- 

 contents is usually much marked with hsematochrome. The zoosporangia and gamet- 

 angia are usually terminal, often swollen cells. The gametes and zoospores are 

 much alike. Trentepohlia lolithus, growing on damp stones, is known as the 

 " Violet-stone " from possessing a scent which recalls that of violets. T. umbrina is 

 often attacked by lichen-forming Fungi. T. spongophila inhabits the jelly of 

 Ephydatia (Spongilla) Jiuviatilis in a certain volcanic lake in Sumatra. The 

 situation is very advantageous to the Alga, but the Sponge seems to suffer from 

 the piercing of its tissue by the guest. Trichophilus is a nearly allied form which 

 lives in the hollow hairs of the Three-Toed Sloth. 



Mycoideacece. — These are epiphytic or parasitic forms nearly allied to the two 

 preceding families, and probably derived from one of them. They form regular 

 discs of cells often attached to the host by much-branched unicellular rhizoids. The 

 disc grows by regular divisions of its marginal cells. Zoospores and sometimes 

 gametes are formed in all or some of the cells. 



Chcetopeltis forms very regular discs of cells on fresh-water plants in Europe. 



Mycoidea (Mycoidea parasitica) forms discs of cells between the cuticle and 

 epidermal cells of Camellias, Rhododendrons, &c. in the East Indies and South 

 America. In this position it withdraws a good deal of water from the tissue of the 

 leaf, and this leads to the dying of the leaf-cells in a gradually increasing area 

 round the parasite. Eventually a hole is formed right through the leaf, and the 

 Mycoidea-thailns occupying a position all round the area of dead tissue continues 

 to increase. Zoospores are only formed in the wet season, at which time alone have 

 they any chance of swarming and germinating on the surface of the leaf. In this 

 position primary (embryonic) discs are formed, many of which die, and others are 

 attacked by Fungi to form Lichens, but some succeed in sending processes through 

 the cuticle and establishing themselves below. 



Coleoch<xtacece. — This family contains a single pmall genus, Coleochcete, which 

 forms radiating, dichotomously branching rows of cells, usually on the surface of 

 othet plants in fresh-water. If the cell-rows are in lateral contact, a close disc is 

 formed (G. scutata), if separate a looser one (C soluta), or the branching may be 

 rather irregular (C divei-gens). The chromatophore is parietal and disc-shaped, 

 and contains a single pyrenoid. 



Zoospores can be produced in all or only the end cells of the rows. A single one 

 is formed from each cell. In germination a new plant is directly formed. 



Coleochcete is oogamous, the plants being either monoecious or dioecious. The 

 oogonium is always formed from the end cell of a row. The cell swells and puts 

 out a narrow tube wliich opens at the end, and extrudes a drop of mucilage. The 

 protoplasm of the swollen basal part then rounds itself off. In the forms with a 

 disc-shaped thallus, the antheridia are produced by the division into four of the 

 members of certain cell-groups. Each daughter-cell (antheridium) then liberates a 

 single spermatozoid. In the branching forms certain end cells form flask-shaped 

 swellings (antheridia), which are cut off from the mother-cell by transverse walls. 



