THE LIVING CELL 19 
with nutrient substances in solution), are called ** sap-vacuoles.”” 
The vacuoles appear as tiny droplets in young cells but grow with 
the cell, becoming larger as the cell increases in size, then fusing 
with each other until, in a fully mature old cell, one large vacuole 
is present in the center of the cell with protoplasm and nucleus 
pushed up against the cell wall. 
Fic. 10.—Portion of the inner epidermis of an Onion bulb scale, showing cells 
at various stages of maturity. Young cell (1); old cell (3); cell intermediate in 
age between 1 and 3 (2); cell wall (c); outer plasma membrane (/); middle lamella 
(ml); nucleus (n'); nucleolus (n?); nuclear membrane (n*); cytoplasm (cy); vacuole 
(v). Note that the young cell (1) shows numerous small vacuoles and a spheroidal 
nucleus near center of cytoplasm. In 2 (cell of intermediate age) the cell has 
enlarged, larger vacuoles have formed through the bursting of films of cytoplasm 
separating smaller ones, and the nucleus has moved toward the cell wall. In 3, 
the films have all burst, the cytoplasm and nucleus have been pushed up against 
the cell wall, the nucleus is flattened out, and a large vacuole appears in the center 
of the cell. 
Protoplasm is in intimate relation to water. The reaction of 
the cytoplasm to a bounding film of water between it and the cell 
wall forms the outer plasma membrane or ectoplasm, a clear, homo- 
geneous, outer band of cytoplasm; the reaction of-cytoplasm to 
the water within the sap vacuoles forms the vacuolar membranes; 
the reaction of the dense protoplasm of the nucleus to the water 
in the cytoplasm around it forms the nuclear membrane. All of 
these membranes are bounding films of protoplasm belonging to 
the region in contact with water and are collectively termed 
plasma membranes. Upon mounting another portion of epidermis 
in iodine solution, removing the excess of stain and adding a drop 
of sulfuric acid and then examining under high power, we note 
