THALLOPHYTA— ALG.E 



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 

 jolisia. (2 and 3 after Kny.) 



