LEAVES 209 



noticed that they not only contain water, but also have great 

 power of retaining it. 



The need of protection against drought and intense light 

 is probably obvious to every one, but that leaves need to be 

 protected against rain is not often considered. Rain falling 

 upon plants is felt to be a blessing rather than a menace, and 

 so it is if the leaves are protected properly. If the water 



FIG. 175. A globular cactus. Photograph by LAND near Tehuacan, Mexico; note 

 the coin (a dollar) fastened to the base, to indicate the size of the plant. 



soaks into the leaves and fills up the air-spaces, replacing the 

 internal atmosphere, the communication of the working cells 

 with the external atmosphere is cut off. This would be as 

 dangerous to the leaf as if water should replace the internal 

 atmosphere in a man's lungs ; in other words, the leaf would be 

 drowned. The impression that leaves take in water when it 

 rains is doubtless due to the common observation that when 

 wilted leaves are sprinkled they " revive," the inference being 

 that the sprinkled water has passed into the leaves. The 



