CHAETOPHORALES 65 



with rhizoids from one or two basal cells. The thallus, which is 

 often invested by a gelatinous matrix of pectins, possesses a main 

 axis composed of large barrel-shaped cells, each containing a small 

 entire or reticulate chloroplast and several pyrenoids. This axis is 

 primarily for support, and it bears much branched laterals that 

 normally grow out in tufts, the short cells composing the laterals 

 being almost wholly filled by one entire chloroplast containing a 

 single pyrenoid. The apices of these branches, which perform the 

 functions of assimilation and reproduction, are often prolonged into 

 a hair. In some species rhizoids develop at the base of the branches 

 and grow downwards thus clothing the main axis with a pseudo- 

 cortex, but normal growth is generally restricted to a few cells of 

 the thallus. When grown in culture with increased carbon dioxide 

 or additional nitrate the plants take on a form very like that of 

 Stigeoclonium (cf. below). Asexual reproduction is by means of 

 quadriflagellate macrozoospores, one to four being produced in each 

 cell. These, after swarming for a few minutes, settle, and germinate 

 into a short filament which already possesses a hair at the four- or 

 five-celled stage when it commences to put out rhizoids. Sexual 

 reproduction is secured by means of quadriflagellate microswarmers 

 or isogametes which fuse whilst in an amoeboid state, though these 

 gametes may also develop parthenogenetically. The behaviour of 

 the microswarmers demands further investigation as it does not 

 seem to be clearly understood, nor has it been determined whether 

 the plants are haploid or diploid. In Draparnaldia glomerata the 

 nature of the swarmer is controlled by the pYi of the medium, 

 microswarmers being formed under alkaline conditions and 

 macrozoospores under neutral or acid conditions. 



*Chaetophoraceae : Stigeoclonium (stigeo, sharp pointed; clonium^ 

 branch) (Myxonema). Fig. 46. 



Many species are heterotrichous and the plants are frequently 

 enclosed in a broad watery gelatinous sheath. The chloroplast is 

 band-like and often does not fill the entire cell, especially in the 

 older parts of the thallus. The aerial part bears branches that 

 terminate in a colourless hair, the degree and nature of the branch- 

 ing depending upon illumination, nutrition and the rate of water 

 flow. There is no localized area for cell division in the aerial 

 portion, but in the creeping system only the apical cells are 



CSA 5 



