98 THE ALGAE 



besia threads produce zoospores that germinate into prostrate 

 filaments, and these later give rise to slender branched rhizoids 

 which, after eight months, produce the characteristic Halicystis 

 bladder. Some weeks after its development the bladder becomes 

 fertile and so the cycle starts once more. Although the cytology of 

 the two plants has not yet been worked out, the Derhesia generation 

 is presumably diploid and the Halicystis haploid. It also remains to 

 be ascertained whether the other species o^ Halicystis have a similar 

 life cycle. Since this first initial discovery some evidence has now 

 been forthcoming to indicate that the same kind of life cycle holds 

 for Halicystis parvula and Derhesia tenuissima. A species apparently 

 belonging to the genus Derhesia occurs in New Zealand but no 

 plants oi Halicystis have ever been found. This species may there- 

 fore represent a final stage in evolution in which the sexual genera- 

 tion has been lost. Growth of the Halicystis vesicles is very slow 

 and they become shed at the end of the growing season by abscis- 

 sion, new vesicles arising later from the perennating rhizoid, and in 

 this manner regeneration may go on for several years. Some workers 

 believe that this remarkable life cycle justifies the placing of the 

 family in a new order, the Derbesiales. 



*Bryopsidaceae: Bryopsis (bryo, moss; opsis, an appearance). 



Fig- 55 

 Most of the species of this genus are restricted to warmer seas, 

 though a few, of which B. plumosa is the commonest, occur in 

 colder waters. The principal axis, which is often naked in its lower 

 part, arises from an inconspicuous, filamentous, branched rhizome 

 that creeps along the substrate and is attached to it by means of 

 rhizoids. In one species the bases of the lower branches develop 

 additional rhizoids that grow down and form a sheathing pseudo- 

 cortex. The bi- or tripinnate fronds either have the branching 

 confined to one plane or are radially branched, the branches being 

 constricted at the point of origin, whilst the cell membrane is also 

 thickened at such places. The cytoplasm in the main axis and 

 branches frequently exhibits streaming movements. The fimction 

 of the rhizome, especially in warmer waters, is probably that of a 

 perennating organ, although vegetative multiplication can also 

 occur through abstriction of the pinnae, which then develop 

 rhizoids at their lower end. The only other known method of re- 

 production is sexual. The plants are dioecious and produce aniso- 



