210 



STRUCTURAL BOTANY 



the carpogonium. So far, therefore, as the act of fertili- 

 sation is concerned, the Florideae do not differ from other 

 sexual organisms. 



The result of fertilisation, however, is not the formation 

 of a single oospore, but the development of a whole fruit. 

 The fertilised carpogonium is cut off from the trichogyne 

 by a plug of cell-wall, and then 

 sends out short branches, which 

 come into close contact with the 

 auxiliary cells on either side ; 

 there is an actual union of the cell 

 contents of the carpogonium with 

 those of the two auxiliary cells, 

 but the nuclei do not fuse. Each 

 auxiliary cell now divides by a 

 transverse wall (see Fig. 91, c); 

 the upper of the two cells in each 

 case becomes a placenta which 

 gives rise to the spores. Calli- 

 thamnion is different from most of 

 the simpler red seaweeds in so far 

 as it regularly forms two fruits 

 from each procarpium, whereas its 

 near allies usually form one only. 

 Each placenta buds out into a 

 number of cells, which themselves 

 divide repeatedly, so that eventually two large groups 

 of cells arise, one on each side of the filament (see Fig. 

 88, c). The groups are really built up of a system of 

 very short and densely crowded unicellular branches, 

 those of each cluster all springing ultimately from the 

 placenta belonging to it. The whole fruit is enclosed in 

 a gelatinous cell - wall, but no cell - walls are formed 



FIG. 91. Callithamnion 

 corymbosum ; part of a 

 branch of a female plant, 

 showing the earliest 

 stage of development of 

 the cystocarps (c, c) after 

 fertilisation. They arise 

 by the growth and 

 division of the two 

 auxiliary cells. Magni- 

 fied 250. (After Thuret 

 and Bornet.) 



