EUPHYCOPHYTA 8l 



gation, for if it is correct it may mean that there is an alternation of 

 two unlike generations, an unusual phenomenon in the Chloro- 

 phyceae. Under certain conditions the cells will also produce 

 aplanospores. The relation of this genus, with its advanced oogamy, 

 to the other green algae is by no means clear, and although in many 

 of its features the sexual reproduction is akin to that of the Rhodo- 

 phyceae, it is commonly regarded as parallel evolution rather than 

 as indicating a more direct relationship (cf. p. 326). 



* SiPHONOCLADALES 



Until 1935 this represented a well-estabHshed order, but in that 

 year Fritsch placed most of the genera as septate members of the 

 Siphonales but retained the Cladophoraceae as a separate order, 

 the Cladophorales, with affinities to the Ulotrichales. In 1938 

 Feldmann re-estabhshed the order and suggested a relationship on 

 the one hand with the Siphonales via Valonia and Halicystis and 

 on the other hand with Chaetophora and Ulothrix. More recently 

 Egerod (1952) has accepted the order but, imlike Feldmann, has 

 continued to exclude the Cladophoraceae from it. There seems to 

 be inadequate justification for this exclusion, and in fact the famiHes 

 in the order present a more or less orderly sequence of evolution, 

 commencing from the Cladophoraceae. The members of the order 

 are all septate at some stage of their existence, the cells or segments 

 being multinucleate and possessing reticulate chloroplasts. Such 

 plants are said to be partially coenocytic. A number of the more 

 advanced genera have a speciaHzed mode of cell division known as 

 segregative cell division. In this process the contents of a single cell 

 become separated by a nimiber of cell walls so that one parent cell 

 gives rise to a number of new cells which then enlarge. 



Morphologically the order can be subdivided into the fila- 

 mentous and vesicular forms. The former comprise genera such as 

 Lola, Rhizoclonium, Chaetomorpha, Cladophora and Microdtctyon, 

 whilst the latter contains SiphonocladuSy Valonia, Boodlea and 

 Dictyosphaeria. It is the vesicular forms that mostly possess 

 segregative cell division. 



Cladophoraceae: Chaetomorpha (chaeto, hair; morpha, structure- 

 like). Fig. 43 

 The plants consist of simple unbranched filaments which are 

 either attached or free-living. The multinucleate cells are large and 



