58 



THE SMALLEST LIVING THINGS 



after which it divides into a definite number of cells, each of 

 which acquires two flagella and escapes as a swarmer (free- 

 swimming cell). This soon loses its flagella and repeats the 

 process. Many of these simpler forms acquire a saprophytic 

 mode of life, one outcome of which is the formation of asso- 

 ciations with other organisms in symbiosis.* Thus the "green" 



Fig. 29— SCENEDESMUS 



The creature with four horns is a 

 very common cause of the green coat- 

 ing on water pitchers and in glasses 

 which have stood in the light for 

 some days. The other creatures in 

 the field are perhaps the most common 

 type of flagellates in stagnant water, 



CHILOMONAS PARAMECIUM 



From a photomicrograph by the author 

 Magnification, 600 



cells of the protozoan Paramecium bursaria and of fresh-water 

 sponges are really the cells of the protophyte Chlorella which 

 live and multiply in these novel environments, to the mutual 

 benefit of both members of the association. 



Many of these forms, just as in the Volvox group, are asso- 

 ciated in colonies of different kinds. One of the most common 

 of these is the four-cell colony, Scenedesmus (Fig. 29) ; others 

 contain from two to thirty-two cells, and still others many cells. 

 These are all very common in drinking water and account for 

 the green color in pitchers, vases, etc., containing standing water. 



The Desmid Group 



The desmids, which are exclusively fresh-water plants, are 

 widely represented in the ordinary pools and lakes of all regions. 

 In 1902 there were known about 2000 species distributed in 

 thirty-one genera, and many more have since been added. They 

 vary in size from 8n to 1200[i in length and are peculiar in being 

 * See the Glossary of this book, page 123. 



