122 BACILLARIOPHYCEAE (DIATOMACEAE) 



shell are thrust apart by enlargement of the protoplast, which 

 becomes enveloped in a slightly silicified pectic membrane, the 

 perizonium. No nuclear division takes place, but fresh valves and 

 connecting bands are formed inside this membrane so that a new 

 and larger individual results. In the Pennatae a union takes place 

 between two naked amoeboid protoplasts that have arisen from two 

 distinct individuals which come together in a common muci- 

 laginous envelope. These are the gametes, and as meiosis occurs 

 during their formation the normal diatom cell must be regarded as 

 diploid (fig. 85). The zygote remains dormant for a time and then 

 elongates in order to form auxospores, the perizonium either being 

 the remains of the zygotic membrane or else formed de novo. 

 Isogamy is the normal condition but a few cases of physiological 

 anisogamy are known and also apogamy. In addition to auxo- 

 spores the Centricae also produce microspores, small rounded 

 bodies with flagellae, but whether these are true gametes has yet to 

 be established because their fate has not been fully studied. Some 

 diatoms are also known to produce resting spores but very little is 

 recorded about these bodies. 



CHRYSOPHYCEAE 



Fig. 86 



This assemblage is principally composed of uninucleate flagellate 

 forms although certain members do exhibit some algal character- 

 istics. Like the Xanthophyceae there is considerable morphological 

 parallelism with the Chlorophyceae indicating that evolution has 

 taken place along the same lines. Sexual reproduction is rare and 

 when it does occur is isogamous, the plants probably all being 

 haploid. They occur most commonly in both fresh or salt water 

 during cold weather. The colour is golden yellow or brown due to 

 the presence of the pigment phycochrysin which is contained in a 

 small number of parietal chromatophores that may also contain 

 pyrenoid-Hke bodies, although starch as a product of photo- 

 synthesis is replaced by oil or leucosin. The motile cells are uni- or 

 biflagellate, and in the latter event one flagellum is beset with fine 

 cilia ; one of the flagellae is said to provide forward movement and 

 the other rotation. When an individual has entered the amoeboid 

 state cysts may be produced endogenously and these have silicified 



