Cuapter VIII. 
‘** Weed’ is the term applied to a plant when 
of no use to man, but as soon as it is discov- 
ered that it possesses a merit, it ceases to be 
a weed,” 
THE PLANTS FOR THE AQUARIUM. 
Animal life in any locality can only then exist when a con- 
tinuous and liberal supply of oxygen is present. The oxy- 
gen inhaled by animals has the function to purify their blood; 
an insufficient supply of it will therefore cause disease and 
finally death. In nature, oxygen is supplied in various ways. 
Trees, shrubs and house-plants, as every one knows, purify 
the air in their immediate surroundings. This process of puri- 
fication is performed as follows: A plant under proper con- 
ditions grows, and as it grows it absorbs or inhales from the 
air of its surroundings a gas known by the scientists as car- 
bonic acid gas, a limited amount of this gas being required 
by plants to make a healthy growth. In turn, the plant ex- 
hales through its leaves a gas known as oxygen. The presence 
of the former in the air is as injurious to animal life as that 
of the latter is necessary to sustain it. 
Where no vegetation is at work on land, the current in the 
atmosphere, the wind and the sun purify the air; where these 
cannot penetrate, in caves, vaults, shafts, etc., foul air predom- 
inates and life is impossible. 
45 
