64 PROTOPLASM AND THE AMCEBA. PROTOCOCCUS 



depend essentially on the maintenance of the proto- 

 plasm within a comparatively narrow range of mole- 

 cular aggregation, and on its power of passing backwards 

 and forwards within this range. 



Since the watery continuous phase of protoplasm is 

 an essential part of its structure, the equiUbrium of 

 the protoplasmic water must be maintained or the 

 structure of protoplasm would be destroyed. In 

 other words, the forces tending to drive water out of 

 the cell and those tending to draw water into the cell 

 must be balanced. This condition of balance is main- 

 tained by primitive organisms Uving in water and 

 consisting of naked protoplasm, when they are sur- 

 rounded by water. If such an organism is transferred 

 to air, evaporation from the surface into the air at once 

 destroys the water balance, the body loses water and 

 the organism dies. This is why terrestrial organisms 

 which live surrounded by air must have some means 

 of checking the loss of water by evaporation and of 

 renewing it as it is lost. We shall see in later chapters 

 that the history of evolution of land plants is very 

 largely a history of the appearance and development 

 of structures which have these results. 



The Amoeba. — The most suitable organism for pre- 

 liminary study is the simple minute animal called the 

 amoeba. There are various species which live in fresh- 

 water pools and slow rivers, where they creep about 

 upon the surface of the mud or upon the water plants. 



Each individual consists of a minute irregular mass 

 of naked protoplasm var5dng in different species from 

 about 100 fi to about 250 ^ in diameter.^ The surface 

 layer of the body (e^itoplasm) is clear under the micro- 



■ I.e. from about t^ to J of a millimeter, or about 1^5 to y^n of 

 an inch. 



