CERAMIALES 233 



After fertilization has taken place the auxiliary cell is cut off from 

 the apex of the fertile pericentral cell and in addition two branch 

 systems composed of nutrient cells appear. When the zygote nucleus 

 has divided the two daughter nuclei (only one of the two in 

 P. nigrescens) pass into the auxiliary cell which has become fused to 

 the carpogonium in the meantime, and there the two nuclei are 

 isolated from the carpogonium by a new wall. By this time the 

 carpogonium and its three lower cells have broken down. The 

 auxiliary cell then fuses with the pericentral cell and after the two 

 diploid nuclei have passed into it, it unites with the other support 

 and axial cells to give a large fusion cell. The diploid nuclei undergo 

 a number of divisions and the products pass into lobes that are 

 budded off from the fusion cell. Each lobe then gives rise to a 

 two-celled gonimoblast filament, the first cell acting as a stalk cell 

 whilst the end cell produces a carpospore. The wall of the cystocarp 

 is two-layered, the outer wall being formed from the lateral sterile 

 cells that are cut off from the support cell, whilst the inner lining is 

 formed from the axial cell of the fertile segment. The tetrasporangia, 

 which develop from pericentral cells, are protected by being 

 embedded in the thallus, a feature which results in the fertile 

 branch usually being much swollen and distorted. 



Ceramiaceae : Griffithsia (after Mrs Griffiths). Fig. 155. 



The monosiphonous ecorticate fronds are composed of large 

 muhinucleate cells connected to each other by a pore, although 

 this is often closed by a plug. In G. glohulifera the larger cells may 

 each have as many as 3000-4000 nuclei. Vegetative division is 

 brought about either by the cutting off of terminal segments from 

 the end cells or else by the delimitation of a small cell from the 

 upper edge, but as this grows very rapidly by mere swelling the 

 appearance of a false dichotomy is produced. In G. corallina 

 miniature shoots and also delicate colourless branched hairs de- 

 velop from the large cells of the main thallus. Regeneration can 

 occur in order to replace an old cell or one that has been wounded, 

 the process involving the two neighbouring cells which send out 

 tubes that meet and fuse. The sessile antheridia are borne on the 

 distal ends of much-branched dwarf shoots which surround the 

 nodes of the main thallus in tufts or dense whorls, each branch 

 arising as a protuberance that is cut off from one of the large axial 



