70 



are seen to attach themselves by one of the cilia, which by degrees 

 is shortened to form a kind of stalk (compare the upper gamete in 

 Fig. 66 II) ; these are the female gametes, which now become sur- 

 rounded by a number of males endeavouring to conjugate with 

 them, but only one succeeds in effecting fertilisation. The 

 protoplasm of the two gametes coalesces (Fig. 66 III), and a 

 zygote (e) is formed. The male gametes which do not conjugate 

 may germinate, but the plants derived from them are much weaker 

 than those produced by the zygotes. Strongly pronounced sexual 

 differences are found in the Cutleriaceae, in which order the male 

 and female gametes arise in separate gametangia (Fig. 67.4). The 

 male gametes (Fig. 67 D) are much smaller than the female 

 gamete (Fig. 67 0) ; the latter, after swarming for a short time, 

 withdraws the cilia, and is then ready to become fertilised (Fig. 

 67 J5, E\ thus we have here a distinct transition to the oogamous 

 fertilisation which is found in the Fucaceae. Alteruation of gene- 

 rations is rarely found. 



1. Sub-Family. Zoogonicae. 

 Reproduction by means of gametes and zoospores. 

 Order 1. Ectocarpaceae. The thallus consists of single or 

 branched filaments with intercalary growth, extending vertically 

 from a horizontal, branched filament or a disc, but sometimes it is 

 reduced to this basal portion only. Zoosporangia and gametangia 

 (for fertilisation see Fig. 66) are either outgrowths or arise by 

 the transformation of one or several of the ordinary cells. The 

 most common genera are : Ectocarpus and Pylaiella. 



Order 2. Choristocarpacese. Choristocarpus, Discosporangium. 

 Order 3. Sphacelariaceae. The thallus consists of small, 

 parenchymatous, more or less ramified 

 shoots, presenting a feather-like appear- 

 ance. In the shoots, which grow by 

 means of an apical cell (Fig. 68 ), a 

 cortical layer, surrounding a row of 

 central cells, is present. Sporangia and 

 gametangia are outgrowths from the 

 main stem or its branches. Sphacelaria, 

 Choetopteris are common forms. 



Order 4. Encoeliaceae. Punctaria, Aspero- 



coccus, Phyllitis fascia. 

 FIG. 68. Apex of the thallus 



otCha-wterisplumoBo. S Api- Order 5 - Stnanaceae. ptriaria, Phlao- 

 <! cell. spora. 



