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 



