EXCRETION 
145 
can take place, for the plant is digested in it. Digestion is 
accomplished by means of an enzyme. The nutritious parts 
are absorbed into the protoplasm, the undigested parts are 
cast from the cell, and the food vacuole disappears. 
Food vacuoles are not always round (Figure 135), but take 
their shape from the form of the plant eaten. If a filament of 
alga, page 304, is taken as food, the food vacuole is much 
elongated. 
115 . Respiration. — From the air dissolved in the water, 
the amoeba obtains by osmosis the oxygen necessary to its 
life, and it gives off carbon dioxide from the cell. 
116 . Excretion. — The term, contractile vacuole, is given 
to the vacuole which is always present in the protoplasm of 
amoeba. This vacuole can be 
seen to increase slowly in size, 
then suddenly contract. As it 
contracts, the fluid in it is forced 
to the outside of the body of the 
amoeba. The filling out of this 
vacuole is due to the collection 
of excretory wastes from the sur¬ 
rounding protoplasm. It is called 
a contractile vacuole because it contracts and expands, and 
an excretory vacuole because it collects waste products. 
117 . Reproduction and Encystment. — The chief method 
of reproduction in the amoeba is simple (Figure 134). The 
living cell divides into two equal parts, forming two new cells. 
This process is known as fission (fishTin : Latin, fissus, cleft). 
When the food or water becomes unsuited to supply the 
needs of the cell, in order to live the amoeba often secretes 
(makes for itself) a thick wall completely surrounding the 
protoplasm. This process is termed encystment (en-sist'- 
ment: Greek, en, in; kystis, bladder). After the wall has 
been formed, the amoeba is able, for a long period, to resist 
cold, the drying up of the pond, or the lack of food. 
Figure 134. —Three Stages 
in Fission of Amceba. 
