SIMPLER PLANTS AND ANIMALS 



scarcely a pool or runnel that is not brightened by green alg£e. 

 They exist in a bewildering variety of sizes and shapes. Among 

 them are the minute single cells of the desmids, hundreds of 

 species fancifully shaped like crescents and stars and triangles, 

 slender interlacing strands of the filamentous algae like Spi- 

 rogyra, and swimming colonies such as Volvox and Euglena (Fig. 

 44), which are animal-like save that they are green with 

 chlorophyll and secure their food by the plant-like method. 

 These organisms are all astride the border between plants and 

 animals. Some of them appear like plants and others more 

 like animals; in classifications we find some of them placed 

 with the algae and some with the protozoans, their presence 

 in one or the other group depending a good deal on whether 

 a botanist or a zoologist is discussing them. Desmids are 

 single-celled like diatoms; but a living desmid is bright green 

 and a living diatom is golden brown. Many desmids are con- 

 stricted at the center, as if the cell were partly divided; others, 

 for example, Closterium, have a light band across the middle. 



Fig. 42. — Desmids: i, Closteriimi; 2, Micrasterias; 

 3, Tetmemorus. 



Desmids are always present in the plankton of lakes, and 

 abundant in clear sunlit ponds. A glass of water from sphag- 

 num pools (p. 65) will often be nile green with them and 

 crescents of the larger species of Closterium which grow in 

 such places can easily be seen with a hand-lens. 



49 



A 



