14 TUE NATURE AND WORK OF PLANTS 



from biting the stems, althoiigli it is not known 

 whether they were formed for thcat purpose or not. 

 In some instances mineral substances are deposited 

 on the bodies of plants in the same way in which 

 they are accumulated on the bottom of a kettle in 

 which water is boiled, and are not only of no use 

 to the plant, but are a detriment to growth and 

 food formation. 



Certain species living in localities where there is 

 but little rainfall and where the air is very dry, 

 however, have a layer of salts on the surfaces of 

 the leaves, which attracts water from the air, thus 

 preventing these useful organs from drying out. 

 The mineral coating of a stem may also serve a 

 similar purpose. 



12. Aquatic species ivith mineral coatings. — Many 

 plants which live in the water cause the mineral 

 matter dissolved in it to be deposited on their 

 surfaces. This may be seen on the pond weeds 

 {Potamogetons), if specimens are taken from the 

 water and allowed to dry. Some of the small 

 threadlike algae are said to be " calcareous " because 

 they are found surrounded by masses of lime of 

 greater bulk than their own bodies. Many of the 



