THE ALGAE in 



The Phaeophyceae are divided into a number of orders of which we shall 

 consider examples from five : — 



1. Ectocarpales {e.g., Ectocarpus). 



2. Cutleriales {e.g., C idler ia). 



3. Dictyotales {e.g., Dictyota). 



4. Laminariales {e.g., Laminaria). 



5. TucdXes {e.g.,Fuciis). 



Ectocarpales 



The Ectocarpales are a small group of relatively simple Phaeophyceae in 

 which the thallus is filamentous in its fundamental structure, though very 

 varied in form. In the simplest cases the filaments are entirely free, but 

 in some other forms the lower parts of the filaments are variously aggregated 

 together to form more or less solid bodies, the ends alone remaining free. 



The vegetative cells in Ectocarpales are shortly cylindrical, containing 

 irregularly shaped chromoplasts with pyrenoids. There is no special 

 apical cell. Growth takes place by the division of an intermediate group of 

 cells in the filament, which form a meristematic zone. Such a method of 

 growth, which is neither apical nor basal, is called intercalary. 



There are two characteristic reproductive structures. Firstly, unilocular 

 sporangia, which consist of single enlarged cells, the contents ot which 

 divide up, without the formation of any cell walls, to liberate a large number 

 of motile cells which are pear-shaped and have two flagella, one directed 

 forwards and the other backwards. Secondly, gametangia (plurilocular 

 sporangia), derived from the cells of a short side branch. These divide 

 up by transverse and longitudinal walls into a large number of small cubical 

 cells, each of which produces a biflagellated motile gamete, similar in 

 structure to the products of the unilocular sporangia. 



Meiosis takes place at the first nuclear division in the unilocular 

 sporangium, so that the spores from this type are definitely monoploid. In 

 general, they function as asexual zoospores, giving rise to haploid plants. 

 The plurilocular sporangia, on the other hand, are generally regarded as 

 gametangia, the cells from which fuse in pairs, and from the zygote thus 

 formed diploid plants arise. Variations of this behaviour are, however, known, 

 as explained below. 



We shall consider only one example of this order, Ectocarpus, but as 

 there is considerable variation in details between species, the description must 

 necessarily be a general one. 



Ectocarpus 



Species of the genus Ectocarpus are very common in sea water around the 

 British coast. Some are attached to rocks and stones, but many grow attached 

 to other Algae, and are therefore said to live as epiphytes. We must dis- 

 tinguish between an epiphyte, in which the plant obtains no nourishment 



