THALLOPHYTA— ALG.E 



53 



periphery of this cell. The cystocarp is surrounded bj- the wall 

 of the conceptacle. 



In the last type, that of the Bangiace.^, the formation is simple 

 and direct, as in the Nemalionaceie. The carpogonial cells are 

 very little specialised, and only differ from the ordinary cells of 

 the thalliis by being a little larger. They grow outwards and 

 give rise to a verj- rudimentary trichogyne, which becomes fused 

 with a pollinoid as m the other groups. The fertilised 

 carpogonium 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 cystocarp 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 

 Jiomologous 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 telra- 

 gonidia. 



Sub-Class V. — Chlorophyce^. 



In many respects this group may be considered to approach 

 most nearly the next great division of plants, the Brj^ophyta, 

 as in the higher forms the oosphere is fertilised in the oogonium 

 and not after extrus'on from the plant. Though as a rule both 



Fir/. 801. Tetraspores of Rhodophyceae 

 1. PJocamimn. 2. Dudresnaya. 3. Le 

 jolisia. (2 and 3 after Kiiy.) 



