272 THE ALGAE 



(Fig. 157). However, a long time elapses before this rejuvenation is 

 necessary and so auxospore formation is relatively rare.^ 



At auxospore formation in the Centricae the two halves of the 

 shell are thrust apart by enlargement of the protoplast, which be- 

 comes enveloped in a shghtiy silicified pectic membrane, the 

 perizonium. Nuclear division takes place, and fresh valves and 

 connecting bands are formed inside this membrane so that a new 

 and larger individual results. There is some evidence for meiosis 

 V followed by fusion of nuclei. In the Pennatae a union takes place 

 between naked amoeboid protoplasts that have arisen from two 

 distinct individuals which come together in a common muci- 

 laginous envelope. Each plant usually produces two gametes, and 

 as meiosis occurs during their formation the normal diatom cell 

 \^ must be regarded as diploid (Fig. 157). The zygotes remain dor- 

 mant for a time and then elongate at right angles to the axis of the 

 parent cells 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 auxospores 

 the Centricae also produce microspores, small rounded bodies with 

 flagella, and there is reason to beUeve that these are flagellate 

 male gametes though their fate has not been fully studied. 



Recentiy oogamy has been described in some detail for Melosira 

 varians, Cyclotella tenuistriata and Biddulphia mohiliensis (Cen- 

 tricae) together with a difference in size of the male and female fila- 

 ments. The species are diploid and reduction division takes place at 

 gamete formation. Some diatoms are also known to produce resting 

 spores but very littie is recorded about these bodies. 



Within the Chrysophycophyta there is plenty of evidence for the 

 relationship between the Xanthophyceae and Chrysophyceae. The 

 relationships of the Bacillariophyceae are rather more obscure, 

 though there is the basic biochemical one (cf. pp. 1-3). 



REFERENCES 



General 



Fritsch, F. E. (1935). Structure and Reproduction of the Algae, Vol. I^ 



pp. 564-643. Camb. Univ. Press. 

 Gross, F. (ig-iS).Philos. Trans. B. 3 228, 1. 



^ In a few cases where observations have been made it occurs once in 

 every 4-6 years. 



