PLANTS DEFEND THEMSELVES 49 
and leaves up into the sunlight, and practically en- 
slaving all its neighbours by using them as props 
for its own progressive self. Like its cousin, the 
parasitic dodder, it not only uses but often kills 
other plants that come in its way. However, plant 
victories, like human victories, are frequently only 
temporary: in time the conquered species gradually 
regain their former positions and become the con- 
querors. 
The changing of a river bed, continuous 
droughts, even a severe storm that brings seed 
from other places, is all that is necessary to change 
entirely the flora of a small region. If one plant 
cannot successfully grow in a region, another kind 
soon drives it out and takes possession. 
Climatic conditions are not the least of the great 
forces against which plants must defend them- 
selves. These conditions are rain, cold, heat, wind, 
and lack of water. The instinct of self-preserva- 
tion is very clearly shown here. An evidence of the 
voluntary protective action of the plant against 
climatic conditions is that of the cacti of the desert. 
In an environment where the water supply is very 
scarce they have learned that, when a shower does 
come, special provision must be made for storing 
away, against the long months of drought that 
must follow, as much water as they are capable of 
