144 THE ALGAE 



sexual hormone production (see p. 415) is involved it must be as- 

 sumed that it is only secreted for a relatively short period of time. 



Upon germination a small columnar structure is first formed and 

 then a flat basal expansion grows out from its base to form the adult 

 sporophyte, which is a prostrate expanded thallus attached to the 

 substrate by means of rhizoids. It differs so very much from the 

 gametophyte that when first found it was thought to be a separate 

 genus and was given the name of Aglaozonia. It sometimes happens 

 that the ova do not become fertiHzed, and when this happens they 

 germinate parthenogenetically to give haploid Aglaozonia plants, 

 but these do not bear any reproductive organs. The sporophytic 

 thallus is composed of large cells in the centre with superficial 

 layers, both top and bottom, of small cells. The sessile unilooilar 

 sporangia, sometimes accompanied by deciduous hairs, are borne 

 in paHsade-like sori or else are scattered irregularly on the upper 

 surface of the thallus. Each superficial cell first divides into a stalk 

 cell and sporangium initial, then meiosis occurs and eventually 

 eight to thirty-two zoospores are formed in each sporangium. The 

 zoospores on germination give rise to new Cutleria plants. This life 

 cycle was first worked out by Yamanouchi (i 912) for the common 

 species Cutleria multifida and its sporophyte Aglaozonia reptans. 



REFERENCE 

 Hartmann, M. (1950). Publ. Staz. Zool. Napoli, 22 (2), 120. 



* DiCTYOTALES 



This order is characterized by species possessing a well-marked 

 regular alternation of two identical generations. Morphologically 

 the plants are parenchymatous and consist of one or more medul- 

 lary layers of large cells and a cortical layer of small cells. The 

 species are also characterized by apical growth and usually by 

 dichotomous branching in one plane. In Dictyota there is a single 

 apical cell but in other genera, e.g. Padina, Taonia, it is replaced by 

 a marginal meristem. In the tropical genus Padina there may be 

 considerable deposition of Ume on the thallus. It is perhaps worth 

 noting that Ume deposition occurs with much greater frequency in 

 the Chlorophyceae (pp. 92, 96) and Rhodophyceae (pp. 233, 



