i64 PHAEOPHYCEAE 



epidermis of small assimilatory cells from which groups of mucilage 

 hairs arise. 



The male and female sex organs are borne in sori on separate 

 plants, the male sorus being composed of as many as 300 pluri- 

 locular antheridia surrounded by an outer zone of sterile cells. At 

 the formation of an antheridium a superficial cell divides into a 

 stalk cell and an antheridium initial, the final partition of the 

 antheridium initial into the individual antheridial mother cells 

 taking place only a few days before the antherozoids are to be 

 liberated. The mature antherozoid is pear-shaped with only one 

 cilium, and as each plurilocular antheridium liberates about 1500 

 antherozoids, a single sorus may generate as many as 450,000. The 

 number of ova produced are not so numerous, and it has been 

 estimated that there are about 6000 antherozoids available for each 

 ovum. The oogonial sorus is very similar to the antheridial sorus, 

 the large fertile cells, twenty-five to fifty in number, being situated 

 in the centre and surrounded by sterile cells on the outside. The 

 oogonia likewise arise from superficial cells that divide into a stalk 

 cell and oogonium initial, and each oogonium when ripe produces 

 one ovum. Liberation of both kinds of gamete usually commences 

 from the centre of a sorus and fertilization takes place in the water, 

 but during the process the eggs are not caused to revolve by the 

 activities of the antherozoids as they are in Fucus (cf. p. 197). If 

 the process is followed under a microscope it can be noted that only 

 some of the eggs appear capable of attracting antherozoids, whilst 

 the unfertilized ova develop parthenogenetically ; such plants, 

 however, always die in culture, though it is possible that in nature 

 they may persist. The sex organs are produced in regular crops, the 

 new sori appearing between the scars of the old, and when the 

 whole of the surface has been used up the plant dies. 



After fertilization the zygote develops into a morphologically 

 similar plant which reproduces by means of tetraspores that are 

 formed in tetrads in superficial sporangia. At sporangium formation 

 an epidermal cell swells up in all directions, and after a stalk cell 

 has been cut ofir the sporangium initial divides twice to give the four 

 tetraspores, during which the thirty-two diploid chromosomes are 

 reduced to the haploid number of sixteen. A tetraspore at the time 

 of liberation is an elongated body and grows at once into a new 

 sexual plant. In some cases, however, the tetrasporangium fails 



