SEXUAL Rl PRODUCTION IN DIATOMS 85 



PENNALES 



Auxospore formation is usually initiated by the coming together 

 of the mother cells. Of course, in cases of apomixis and automixis, 

 this may not occur. These cells are diplonts and usually considered 

 not to be sexually differentiated. However, in Naviciila balophila 

 (Subrahmanyan, 1945), Syjiedra iihia (Geitler, 1939), and Synedra 

 rwnpens var. fragilarioides (Geitler, 1952f), it has been reported that 

 one cell produces two passive gametes, and one produces two active 

 gametes. This might indicate that the two mother cells are sexually 

 differentiated. 



These cells may be about the same size as in Rhokosphenia 

 curvata (Geitler, 1952a) or may be very unequal in length as in 

 Ewwtia arciis (Geitler, 1951b). Sometimes more than two cells come 

 together for auxospore formation, as in GoiJiphonema parvzdwn var. 

 viicvopus, Achnanthes lajiceolata, and Navicula seininulum (Geitler, 

 1932). In Ajiovweoneis exilis (Geitler, 1949b), Navicula radiosa 

 (Geitler, 1952d), and Synedra uhia (Geitler, 1939), several cells 

 often come together. Usually these cells are not sister cells, but in 

 Navicida semimdinn they may be (Geitler, 1932). 



In most cases both cells are active and approach each other. In 

 GoiJiphonema parviduvi var. micropiis one cell is attached by a gelati- 

 nous stalk and only one is mobile (Geitler, 1932). These cells assume 

 various positions on contacting each other. The most common posi- 

 tion is for them to lie opposite and parallel with their girdle faces 

 in juxtaposition. The cells of Goijiphoiiema parvidinn var. iincropus 

 orient themselves so that the apical pole of one cell is opposite the 

 basal pole of the other. Owing to the curvature of the frustule of 

 Rhoicosphenia curvata, the cells may be in various positions (Geitler, 

 1952a). 



Jelly is produced by both cells in varying quantities. Liebisch 

 (1929) considered this jelly part of the hydrated pectin membrane 

 of the cell. Other research indicates that it has a different origin. It 

 is evident that more work needs to be done on this point. 



This jelly is usually homogeneous and varies in thickness accord- 

 ing to the kind of diatom. In Achnanthes longipes, Navicida didyma, 

 and Fleurosigina imbecida it is fairly soft, whereas in Frustulia rhom- 

 boides var. saxonica and Achnanthes laficeolata it is relatively stiff. 



