SIPHONALES 87 



not appear to be any cellulose in the material composing the cell 

 wall. S warmers develop in the cytoplasm at the apex of the vesicle 

 in an area which becomes cut off by a thin cytoplasmic membrane, 

 the area thus cut off representing a gametangium. Macro- and 

 microgametes are formed and forcibly discharged in the early 

 hours of the morning through one or more pores. There are 

 several crops of these swarmers produced by successive migrations 

 of cytoplasm into the apical areas at bi-weekly intervals coincident 

 with the spring tidal cycles. Fertilization occurs in the water, and 

 the zygote in H. ovalis germinates into a branched protonemal 

 thread that in 3 months has developed into a typical Derhesia plant 

 with the erect aerial filaments arising from the basal rhizoidal 

 portion. 



It has been demonstrated only quite recently that both Halicystis 

 ovalis and Derhesia marina are simply two stages in the life 

 cycle of one alga, but in addition to the evidence from cultures 

 the two species have the same geographical distribution. The 

 mature Derhesia 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 of Halicystis 

 has a similar life cycle. Growth of the Halicystis vesicles is very 

 slow and they become shed at the end of the growing season by 

 abscission, new vesicles arising later from the perennating rhizoid, 

 and in this manner regeneration may go on for several years. The 

 genus, formerly regarded as a connecting link between Protosiphon 

 and members of the Valoniaceae, must now be removed into a 

 separate family because of this remarkable life history. This new 

 family must also include Derhesia in the same way that Aglaozonia 

 is now included in Cutleria (cf. p. 156). 



Phyllosiphonaceae : Phyllosiphon {phyllo, leaf; siphon^ tube). 

 Fig. 60. 



This is an endophytic alga that occurs in the leaves and petioles 

 of the Araceae, most of the species being confined to the tropics, 



