210 



STRUCTURAL BOTANY 



the carpogonium. Probably it is so in Callithamnion 

 also, but more observations on the fertilisation of these 

 plants are much wanted. 



The result of fertilisation 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. It 

 is probable that there is an actual 

 union of the cell contents of the 

 carpogonium with those of the two 

 auxiliary cells. Each of the latter 

 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. Callithamnion is different 

 from most of the simpler red 

 seaweeds in so far as it regularly 

 forms two fruits from each pro- 

 car pium, 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.) 



