284 



TYPICAL FLOWERING PLANTS 



along the sides of the stem to help the plant cling to its 

 support. Roots which grow in the air are called aerial 

 (Latin, aer, air) roots (Figure 281). 



Sometimes roots arise from the bottom of a stem which 

 has been cut or broken from the main plant, as in the case 

 of a geranium slip. Such roots are called adventitious. 



The willow is a tree which is 

 easily grown from a twig, be- 

 cause it readily forms adventi- 

 tious roots. 



Most roots grow downward 

 in soil which is well cultivated. 

 The stimulus which causes them 

 to take this direction is gravity, 

 or as scientists say, they are show- 

 ing geotropism (ge-6t'r5-pizm : 

 Greek, ge, earth ; tropos, a turn) . 

 Other influences governing 

 the direction in which roots 

 grow are the presence of water 

 and obstacles. When a root 

 turns in the direction which will give it the best supply 

 of water, it is exhibiting hydrotropism (hi-drot'ro-pizm : 

 Greek, hydro, water; tropos, a turn). When a root 

 turns aside to avoid an obstacle it acts in response to 

 the stimulus of touch or contact, showing thigmotropism 

 (thig-mot'ro-pizm: Greek, thigmos, touch ; tropos, a turn). 

 The roots of poplar, willow, and soft maple trees, in seek- 

 ing water, often clog sewer pipes by filling them with 

 rootlets after they have gained an entrance through a 

 joint, a habit which renders them objectionable as shade 

 trees. 



In agriculture, the soil is made fine and porous to help 

 the roots of plants get food and moisture. 



Figure 281. — Aerial Roots 

 of Ivy. 



