256 THE ALGAE 



follows. Two cells are cut off from a cell in the middle of a branch 

 and these fimction as the auxihary mother cells. From one of them 

 the four-celled carpogonial branch is produced, whilst after fertili- 

 zation both auxihary mother cells divide and cut off a small basal 

 cell. The fertihzed carpogonium also divides into two large cells, 

 each of which cuts off a small sporogenous cell that fuses with the 

 adjacent auxihary cell. As a result of this fusion each auxihary cell 

 can receive a diploid nucleus which soon after its entry divides into 

 two; one daughter nucleus passes to the apex of the auxihary cell, 

 whilst the other, together with the nucleus of the auxihary cell, is 

 cut off by a waU. It is from the large upper cell that the gonimo- 

 blast filaments arise and so the mature cystocarp is produced. 



The sessile tetrasporangia arise in acropetal succession as lateral 

 outgrowths of the vegetative cells of young branches. In C. hrachi- 

 atum mature tetrasporangia and antheridia have been found on the 

 same plant, whilst other plants have been reported that bear both 

 tetrasporangia and cystocarps. 



Spermothamnion turneri is another plant in which sex organs have 

 been reported on normal tetrasporic plants, but as the procarp 

 branch in this case develops normally without meiosis the carpo- 

 goniimi is diploid. Fusion of male and female nuclei in the carpo- 

 gonium has been observed so that the carpospores are probably 

 triploid but unfortunately their fate is not known. Haploid plants 

 are also known that bear tetrasporangia but in such cases no divi- 

 sions occur in the sporangia. In 5. snyderae the tetrasporangia are 

 replaced by polysporangia which must be regarded as homologous 

 structures. The mother cells of each polysporangiimi contain two 

 to nine nuclei and they give rise to twelve, sixteen, twenty, twenty- 

 four or twenty-eight spores. 



Ceramiaceae : Plumaria (pluma^ soft feather). Fig. 146 



The filamentous thallus is much branched, the main axis, which 

 is monosiphonous throughout, being ecorticate near the apex but 

 corticate below. Branching is bilateral and typically pinnate. Corti- 

 cation is brought about by the activity of the basal cells of the 

 branches and of cells cut off from the main axis. In the old axis 

 three zones can be recognized : 



(a) outermost small-celled photosynthetic zone; 



{b) intermediate storage layer of large cells; 



(c) a conducting system of long narrow elements. 



