HAEMATOCOCCtJS (SPHAEREtLA) . 263 



life-history of various Algae and Fungi may be controlled and modi- 

 fied at will by changing the conditions under which they grow, 

 especially as regards nutrition. They show also, in many cases at 

 any rate, that starvation may in itself induce sexual reproduction. 



HAEMATOCOCCUS (= SPHAERELLA). 



376. Sphaerella (Haematococcus) also occurs in rain- 

 water in gutters or puddles, in ditches, bog-pools, etc., often 

 giving the water a green or red colour from its abundance. 

 Like Chlamydomonas, it is unicellular, with an ovoid 

 body, a bell- shaped chloroplast, and two cilia. The chloro- 

 plast, however, is often flecked with red pigment, and the 

 cell has a curious and characteristic appearance owing to 

 the position of the cell-wall. The wall stands out from the 

 rest of the cell the protoplast so that a space is left 

 between the wall and the rest of the cell. This space is 

 traversed by fine protoplasmic threads, and through it 

 pass the two cilia on their way out through the wall at the 

 anterior end of the cell. 



(a) Mount in chlor- zinc-iodine, which will stain the 

 cell-wall, the starch, the general protoplasm, the cilia, 

 the protoplasmic threads crossing the space, and the 

 nucleus. 



(6) Some Sphaerella cells may be found which have passed into a 

 res ting condition, and have become rounded, covered by a thickened 

 cell- wall, and had the chlorophyll largely replaced by red pigment. 

 The general life-history is much the same as in Chlamydomonas. In 

 the resting condition, the cell may divide into two, four, or eight 

 daughter- cells (zoogonidia) which acquire cilia, escape from the 

 mother-cell, and become new plants. In sexual reproduction, the 

 cell divides into a larger number of zoogametes (32 to 64), naked 

 biciliate cells like the zoogonidia, but conjugating in pairs to form 

 a zygote which acquires a thick wall and passes for a time into a 

 resting-stage. 



PLEUROCOCCTTS. 



377. Pleurococcus vulgaris is far commoner than either 

 of the two preceding plants, and is, in fact, one of the com- 

 monest forms of plant known. It gives rise to the green 

 powdery deposit on wooden fences, walls, and tree trunks. 



