PLASMA MEMBRANE 3 



size and its network in consequence becomes broken in places, 

 allowing the small meshes to coalesce into larger ones known 

 as vacuoles. Both the meshes and the vacuoles are filled with 

 cell-sap. The cell-sap is largely water containing in solution 

 salts that have come up from the soil, and food substances together 

 with various other compounds manufactured within the plant. 



After this general view of the several parts of the protoplast 

 we are ready to examine into the character and uses of each 

 more thoroughly. In doing this let us not lose sight of the fact 

 that the protoplasts taken collectively constitute the living body 

 of the plant and whatever the plant does as a living organism is 

 accomplished by them. They put to use various forces of the 

 external world; they compound the plant's food and a multitude 

 of other products, which are of various uses to plants themselves 

 as well as to man; they make the cell- wall framework of the 

 plant; they multiply and increase in size so that the plant as a 

 whole is made to grow; they differentiate the various tissues, each 

 suited to perform a particular service; they are sentient to gravity 

 and light, moisture and temperature, and to the general state 

 of the whole organism of which each protoplast forms a part, and 

 they are capable of responding to these things in a definite and 

 useful way. With this knowledge we might anticipate, and it is 

 not surprising to find that each protoplast is a complex thing 

 with visibly distinct parts, and that each part has its own physio- 

 logical significance. We shall now take up the parts of the 

 protoplast in the following order: plasma membrane, general 

 cytoplasm, nucleus, plastids. 



Plasma Membrane or Ectoplasm. — At the exterior of every 

 protoplast is a very thin, hyaline membrane, which, as has been 

 said, is morphologically a part of the cytoplasm, for when a 

 protoplast is torn or cut in two a membrane is produced from 

 the cytoplasm over the wounded surface. This membrane is 

 known as the plasma membrane or ectoplasm (Fig. i,A,d). It is 

 approximately .0003 mm. thick, or about ^Is of the thickness of 

 this page. After the cell-wall has been formed it is very difficult 

 to distinguish the plasma membrane because of its extreme thin- 



