CHAPTER XIII 



INTERMEDIATE ORGANISMS 



ONE of the interesting findings of Biology is that it is sometimes 

 impossible to determine whether certain types of lower forms 

 are plants or animals. The classic example of this is the plant- 

 animal Haematococcus consisting of a single cell which moves about by 

 means of flagella. 



It will be remembered that Euglena viridis has chlorophyl in the 

 body but is classified as an animal. One of the great and outstanding 

 characteristics of plants is that most of them possess chlorophyl if they 

 grow in the light, and that they are capable of manufacturing their own 

 food by virtue of this fact. (See Chapter on the Chemistry of Living 

 Matter.) 



Pleurococcus (Fig. 85), commonly 

 studied in the laboratory, is a close rela- 

 tive of Haematococcus. It is a one-celled 

 organism found commonly on the north 

 side of trees, moist rocks, and wooden 

 fences, dull green in color, and powdery 

 when dry. When moist it becomes 

 brighter in color and slimy to the touch. 

 It is found in practically every part of the 

 world on the shady and moist sides of the 

 objects mentioned above. 



Under the microscope it is found 

 that this substance consists of thousands 

 of tiny single-celled organisms to which 

 the name of Pleurococcus has been given. There is a definite cell wall 

 and a nucleus. The chloroplast, however, obstructs a view of the nucleus 

 in the unprepared cell. The organism reproduces by simple fission and 

 has a tendency to form clusters or colonies, usually of from two to ten 

 or twelve cells. When this occurs, the cells assume a more or less 

 irregular shape due to the pressure of the adjoining cells. The nucleus 

 lies near the center of the cell and contains one or more nucleoli. The 

 network of the nucleus can also be distinguished. 



In the cytoplasm will be found the chlorophyl-bearing organ or 

 region called the chloroplast. Due to the chlorophyl this will appear 

 bright green, but, if the cell be placed in alcohol, the chlorophyl will be 

 dissolved, leaving the chloroplast grayish. It is important to note the 

 distinction between the chloroplast, which is a living organ of the proto- 



Fig. 85. 



A. Pleurococcus. 



B and C. Haematococcus Cells. 

 (Greatly magnified.) 



