Boston Sewer Dirision 



POPLAR ROOTS REMOVED FROM A SEWER 



Roots of willows, poplars and other plants have been known to grow hundreds of 

 feet in the direction of relatively abundant moisture in the soil. Some cities prohibit 

 the planting of poplar trees along the streets because they tend to fill the sewers 

 with their roots 



absence of something, can make the stem grow faster. Or is it possible that 

 the light actually restrains the plant's growth ? In any case, the unequal 

 growth of the two opposite sides of the stem would explain the turning of 

 a plant toward the light. 



Tropisms^ The bending of a plant in response to the action of some 

 external force has been called a tropism, from a Greek word meaning "to 

 turn".' The turning of a plant axis (or any other living organ or organism) 

 in response to illumination is called photo-tropism — that is, "light-turning". 

 Similarly, we apply the name hydro-tropism to the response of a plant or 

 plant part to water. The stem and the root are said to show geo-tropism, or 

 earth-turning, under the one-sided influence of gravity. A plant sometimes 



^See No. 2, p. 270. 



"The root of this word is the same as that of the word tropic used in geography. The 

 Tropic of Cancer and the Tropic of Capricorn were the astronomers' time points in the 

 calendar when the sun turned back in its apparent north-and-south migrations in the seasonal 

 cycle. The tropics are the regions between these two turning-points; that is, they are the lati- 

 tudes in which the sun is directly overhead at some time between one turning and the next. 



256 



