THALLOPHYTA: ALGAE 



49 



end of the cell, is a small group of calcium sulphate crystals that show 

 Brownian movement. In some desmids, the outer surface of the cell wall 

 displays warts, spines, ridges, or other markings, most of which show a 

 regular arrangement. 



Asexual reproduction occurs mainly by fission, rarely by apian ospores. 

 Zoospores have never been observed. In cell division the nucleus divides 



m 



J I 





ffii 





'•■^m* 



<* 



Fig. 36. Closterium, a common desmid. A, vegetative cell, showing nucleus at isthmus, a 

 large lobed c-hloroplast with a row of pyrenoids in each half of the cell, and at each end a 

 vacuole containing a few crjstals, X300; B and C, another species, showing two stages in 

 conjugation, X200. 



first and then a cell wall is formed across the isthmus. Each of the two 

 chloroplasts splits transversely. The daughter cells then separate and 

 each forms a new half similar to itself. In sexual reproduction two cells 

 come together and secrete a common mucilaginous sheath (Fig. 365, C). 

 Their walls generally break at the isthmus. Then the protoplasts escape 

 and fuse to form a zygote. In a few desmids each cell sends out a short 

 tube. These meet, become continuous, and the two protoplasts fuse in 

 the tube. The desmids are isogamous but their gametes, each represent- 



