222 



A YEAR IN SCIENCE 



It is now known that these substances are frequently 

 injurious to plants. It is probable that the decrease 

 in the fertility of soil is frequently due to the presence 

 of these excretions rather than to the lack of the proper 

 mineral materials. Most of these excretions appear 

 harmful to the species of plants which produced them, 

 but they are not necessarily so to other plants. This 

 may help to explain the benefits of rotation of crops. 



Fig-. 92. Soil is washed away fcy the water, but large roots serve 

 as an effective means of anchoring this tree. 



Anchorage. Roots usually serve as a very effective 

 means of holding plants in place. Many plants are 

 so well anchored that during a heavy wind a plant 

 will be broken off, but it will not be uprooted. The 



