V? D %.:^ C 



Fig. 157 Bacillariophyceae. A-G, auxospore formation by two 

 cells in the pennate diatom, Cymbella lanceolata. A, synaptic con- 

 traction. B, after first division. C, second division of meiosis 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.) 



masses and then spreads out posteriorly in the form of a fine thread 

 (Fig. 156). Two kinds of raphe occur. In the one it consists of two 

 median longitudinal slits following a straight or sinuous course, 

 whilst in the other (canal-raphe) it is a canal lodged in a crest or 

 keel. Each cell is surrounded by a cytoplasmic hning with a bridge 

 between the two halves of the shell in which the nucleus is com- 

 monly to be found. The chloroplasts are parietal, ohve green to 

 brown, the principal colouring matter being isofucoxanthin, whilst 

 pyrenoids may be present or absent. The product of photosyn- 

 thesis is a fatty oil. The pelagic forms frequently possess outgrowths 

 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 inside and then 

 the parent connecting bands separate. One individual 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 



/ "' / 



