96 THE SIMPLEST ORGANISMS 



call it a plant, because some animal cells appear to have small 

 green plastids within them ; and the microscope reveals the pres- 

 ence ot ehloroplasts within these animal cells. Also we might 

 suppose that movement is an indication of an animal form. Here 

 again we should find ourselves at fault, for there are many kinds 

 of motile plant cells, while some animal cells are immotile. 



The best indication of plant or animal cell structure seems to be 

 in the external layer of the cell. Plant cells usually have a cel- 

 lulose wall in addition to a cell membrane, while animal cells usu- 

 ally are surrounded by a membrane only. The internal structure 

 of the two kinds of cells also shows some differences. 



Bacteria as Simple Plants. — We have seen that perhaps the 

 simplest plants are the bacteria. They are so tiny that, whether 

 » o in the form of a ball (coccus) , a rod (haciVlus) , or 



'0 ^^c<^' a spiral (spiriVlum) the details of cell structure 



are not well understood. They may have a 

 ^^' ' wall of nitrogenous material, and sometimes 



1 -y 9 • ■//• ^^ addition there is a sheath of gelatinous mat- 



^^ ter. A nucleus probably is present, although 



"iliahd bacilli ^^® nuclear matter may be scattered through 

 the cell. Some bacteria have motile organs, 

 in the form of threads of protoplasm called 

 ''^"//A c r ' c^7'^a, or longer ones called flageVla, with 

 l\^ bacilli °''"^ which they move in fluids. They multiply 

 rapidly by simply dividing in the middle to 



Forms of bacteria. /? x n t r t_i j-x' 



lorm two new cells. In unfavorable conditions 

 they may form spores, which are resting cells with a heavy wall 

 secreted about them. Such cells may resist dryness or heat, — 

 even boiling, — for a considerable time. As we shall prove in 

 later studies, bacteria, like animals, need organic food and favor- 

 able conditions of the environment in order to grow. 



Pleurococcus. — A typical one-celled plant, however, would 

 contain green coloring matter or chlorophyll, and would have the 

 power to manufacture its own food under conditions giving it a 

 moderate temperature, a supply of water, oxygen, carbon dioxide, 

 and sunlight. Such a simple plant is the pleurococ'cus, seen on 

 the shady side of trees, stones, and city houses. This plant would 

 meet one definition of a cell, as it is a minute mass of protoplasm 





