. A manual of botany. Botany. THALLOPHYTA— 53 periphery of this cell. The oystocarp is surrounded by the wall of the eonceptaole. In the last type, that of the Bangiace^e, the formation is simple and direct, as in the NemalionaceEe. The carpogonial cells are very little specialised, and only differ from the ordinary cells of the thallus by being a little larger. They grow outwards and give rise to a very rudimentary trichogyne, which becomes fused with a poUinoid as in the other groups. The fertilised oarpogonium does not put out a gonimoblast, but either becomes at once a carposporangium
. A manual of botany. Botany. THALLOPHYTA— 53 periphery of this cell. The oystocarp is surrounded by the wall of the eonceptaole. In the last type, that of the Bangiace^e, the formation is simple and direct, as in the NemalionaceEe. The carpogonial cells are very little specialised, and only differ from the ordinary cells of the thallus by being a little larger. They grow outwards and give rise to a very rudimentary trichogyne, which becomes fused with a poUinoid as in the other groups. The fertilised oarpogonium does not put out a gonimoblast, but either becomes at once a carposporangium, or divides into a number of cells, each of which may be regarded as one. The cystooarp consists merely of the cluster of sporangia, no wall being formed. The asexual cells of the gametophyte are usually produced in groups of four in a gonidangium. They are variously arranged {fig. 801), sometimes being formed in tetrads, some- times in rows, and sometimes being quadrants of a sphere. They may occur within the cortical region of the thallus, or may be produced upon special hair-like outgrowths. The gametophyte in which they occur is very frequently a potential one, and when they germinate each gives rise to another potential individual. There is thus a succession of potential gametophytes, before an individual appears which bears sexual organs. This kind of alternation of generations has already been alluded to as homologoits alternation. The gonidia differ from the carpospores in not being clothed with a cell-wall on their liberation. From their occurrence in groups of four they are usually termed tetraspores, or tetra- gonidia. Sub-Class v.—Chlorophyoe^. In many respects this group may be considered to approach most nearly the next great division of plants, the Bryophyta, as in the higher forms the oosphere is fertilised in the oogonium apd not after extrusion from the plant. Though as a rule both. Fig. 801. Tetraspores of RhodophyceEe 1. PJocamium. 2. Dudresnaya. 3. Le
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbotany, bookyear1895
