EUPHYCOPHYTA 145 



238). Some of the genera, e.g. Dictyopteris, have a distinct mid-rib. 

 Asexual reproduction is brought about by means of tetraspores 

 produced in superficial tetrasporangia, whilst the sex organs are 

 represented by antheridia and oogonia borne in sori on separate 

 plants. All the organs of reproduction develop from surface cells 

 which enlarge and protrude above the surface of the thallus. In 

 some forms a stalk cell may be cut off. It seems that at meiosis in 

 the tetrasporangia, segregation of the sexes takes place so that two 

 of each group of four tetraspores will give rise to male plants and 

 two to female. In certain genera, e.g. Taonia, Padina, the tetra- 

 sporangia and sex organs are borne in zonate bands across the 

 thallus, and in some cases there is evidence of a correlation between 

 their development and with diurnal tidal cycles (e.g. Taonia) or 

 with spring tidal cycles, (e.g. Padina). It seems Hkely that Ught con- 

 ditions are involved in the case where single tidal periods are in- 

 volved. A period is evidentiy required between the initiation of 

 each new crop, probably so that the plant can accumulate the 

 necessary food material. In Taonia and Dictyota asexual plants are 

 more abundant than the sexual, at least in some locaUties. This is 

 partly accounted for by the persistence of a sporophytic rhizoidal 

 portion that regenerates new plants, but more commonly the con- 

 tents of the tetrasporangium fail to divide before they are Uberated. 

 These undivided contents give rise to new plants, which are more 

 resistant and vigorous than plants produced from normal tetra- 

 spores, possibly because of the larger amount of food material 

 available from a single sporangitun. The order is primarily a tropi- 

 cal one but some species, e.g. Dictyota dichotoma, are widely 

 distributed in temperate waters as well. 



*Dictyotaceae: Dictyota (Uke a mat). Fig. 80 



In the cosmopoUtan species, D. dichotoma, the flattened thallus 

 exhibits what is practically a perfect dichotomy because there is 

 always a median septation of the apical cell. Viewed in transverse 

 section the thallus is seen to be composed of three layers, a central 

 one of large cells and an upper and lower epidermis of small as- 

 similatory 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 



