EUPHYCOPHYTA IO5 



* CONJUGALES 



The members of this order are somewhat distinct from the other 

 members of the Chlorophyceae. The plants are either filamentous 

 or else single cells with a wall of one or two parts. There is con- 

 siderable variation in form of the plastids. The species are all 

 characterized by a pecuUar mode of sexual reproduction in which 

 fusion takes place between masses of protoplasm which do not 

 possess organs of locomotion. In the process cells come together 

 in pairs and conjugate (see the different examples). Their pecuUar 

 reproduction suggests that they were evolved at a very early stage 

 from one of the simpler orders, i.e. Ulotrichales. The order is sub- 

 divided into four major famiUes, the Zygnemaceae, which are fila- 

 mentous, the Gonatozygaceae which fragment into short lengths or 

 single cells, and the Mesotaeniaceae and Desmidiaceae, most of 

 which are single-celled. The Mesotaeniaceae are sometimes known 

 as the saccoderm desmids and, together with the Zygnemaceae, 

 are placed in the Euconjugatae. The Desmidiaceae are also some- 

 times known as the placoderm desmids. Both desmid families have 

 probably been derived from filamentous ancestors by over- 

 specialization of fragmentation. 



* Zygnemaceae: Spirogyra (spiro, coil;^3;m, curved). Figs. 59, 60 



The unbranched filaments are normally free-living although 

 attached forms are known, e.g. S. adnata^ and they form slimy 

 threads which are known as *Water-silk' or *Mermaid's tresses'. 

 They grow in stagnant water and are most abundant in either the 

 spring or autumn, the latter phase being due to the germination of 

 a percentage of the spring zygospores. Each cell contains one or 

 more chloroplasts possessing either a smooth or serrate margin and 

 arranged in a characteristic parietal spiral band. The single nucleus 

 is suspended in the middle of the large central vacuole by means of 

 protoplasmic threads that radiate out to the parietal protoplasm. 

 The chloroplasts, which may occasionally be branched, are T- or 

 U-shaped in cross-section and contain numerous pyrenoids which 

 project into the vacuole on the iimer side, the majority of the 

 pyrenoids arising de novo at cell division. The cell wall is thin and 

 composed, according to some investigators, of two cellulose layers, 

 whilst others maintain that there is only an inner cellulose layer 

 with an outer cuticle. The whole filament is enclosed in a mucilage 



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