BACILLARIOPHYCEAE (DIATOMACEAE) 121 



bridge between the two halves of the shell in which the nucleus is 

 commonly to be found. The chloroplasts are parietal, olive green 

 to brown, the principal colouring matter being isofucoxanthin, 

 whilst pyrenoids may be present or absent. The product of photo- 

 synthesis is a fatty oil. The pelagic forms frequently possess out- 

 growths which must be regarded as adaptations to their mode of 

 existence. Cell division normally occurs at night time, and when 

 the nucleus and protoplast have divided new valves are formed 



Fig. 85. Bacillariophyceae. A-G, auxospore formation by two cells in the 

 pennate diatom, Cymhella lanceolata. A, synaptic contraction. B, after first 

 mitosis. C, second mitosis with functional and degenerating pairs of nuclei. 

 D, division of each protoplast into two uninucleate gametes. E, young zygotes. 

 G, zygotes elongated to form auxospores. H, microspore formation in Melosira 

 varians ( x 600), I, J, auxospore formation in Rhabdonema arcuatum. K, asexual 

 auxospores in M. varians. (A-H, K, after Smith; I, J, after Fritsch.) 



inside and then the parent connecting bands separate. One indi- 

 vidual thus becomes smaller and smaller because the size of the 

 new valve is fixed by the silica contained in the wall of the old 

 valve and in five months there may be a decrease of three-fifths to 

 two-thirds of the length until finally the shrinkage is compensated 

 for by auxospore formation (fig. 84 H). 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 



