Fig. 80 Dictyota dichotoma. A, portion of plant showing regular 

 dichotomy. B, apical cell. C, apical cell divided. D, group of 

 antheridia surrounded by sterile cells. E, single antheridial cell and 

 a sterile cell. F, sorus of oogonia. G, tetrasporangium. (A-D, F, G, 

 after Oltmanns; E, after Williams.) 



the formation of an antheridium a superficial cell divides into a 

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

 theridium 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 1,500 anthero- 

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

 of ova produced is not so numerous, and it has been estimated that 

 there are about 6,000 antherozoids available for each ovum. The 

 oogonial sorus is very similar to the antheridial sorus, the large 

 fertile oogonia, twenty-five to fifty in ntmiber, 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 fertihzation 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. 202). If 

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

 only some of the eggs appear capable of attracting antherozoids. 

 Unfertilized ova may develop parthenogenetically; such plants, 



146 



