PLEUROCOCCUS 



55 



when summer heat causes a thaw, they grow and divide, 

 the daughter-cells escape and swim in the snow-water 

 by means of flagella, each cell being furnished with a pair 

 of these motile appendages. So in " Red-snow," we 

 have a Chlamydomonas adapted to arctic conditions. 

 It is non-motile, and in a resting condition during the 

 winter; the summer sun awakes it into activity, and each 

 cell gives birth to a number of 

 flagellated motile cells. This 

 power of adaptation is highly 

 instructive, for it indicates how 

 a motile aquatic organism may 

 become adapted to terrestrial 

 conditions, and assume a form 

 and habit to meet their require- 

 ments. 



Further evidence of an ap- 

 proach of an aquatic one-cell 

 plant towards terrestrial exist- 

 ence is furnished by the green 

 Alga Pleurococcus vulgaris. This 

 occurs in vast numbers on 

 wooden palings, trunks of trees, 

 and wet walls, forming a very 

 familiar green powdery covering, particularly in situa- 

 tions not reached by the direct rays of the sun. Pleuro- 

 coccus flourishes best in damp weather. Fig. 19 illustrates 

 the story of this plant. The cell consists of protoplasm, 

 with a single nucleus, and a number of very small chloro- 

 plasts, and is invested by a thin wall of cellulose. The 

 cells occur singly or in little groups. When they cohere, 

 the sides in touch are slightly flattened by the pressure. 



Fig. 19. — Pleurococcus 

 vulgaris, x 540. 



A, An ordinary cell; ch, chlo- 

 roplast; n, nucleus; B, four 

 cells produced by division 

 and separating ; C, cells 

 grouped in contact, the two 

 to ,he left having just 

 divided ; D, tetrahedral 

 group. 



