130 ROOTS 



Origin. Growing roots, unlike stems, are affected by the 

 force of gravity, which acts as a stimulus and causes the 

 root to grow into the soil. 



The radical or seed root develops into the primary or first 

 root. If the primary root continues its growth and becomes 

 the largest or predominating root, it is known as the taproot 

 and is the main axis of the root. Secondary roots develop 

 from the primary root in no regular order and the branches 

 of secondary roots arise in a similar manner. 



Nature. Simple roots are unbranched. Many plants have 

 only one simple root, which in this case is said to be solitary. 

 In the onion the roots are not branched but there are a great 

 number of simple roots which are fascicled or grouped in a 

 cluster. Branched roots, which are by far the most common, 

 show numerous branches and are usually very irregular. 

 Simple roots or branches may be straight, curved, twisted or 

 crooked. 



Types. Roots are vertical when they grow directly down- 

 ward into the soil, horizontal when they grow parallel to the 

 surface of the soil and oblique when they grow in an oblique 

 direction to the vertical root. 



Duration. Roots that live for one year are annual, those 

 that live for two years are biennial and those that live for 

 several years are perennial. 



Modification. Roots are occasionally modified as follows: 

 (a) as haustoria or absorbing roots of saprophytic or para- 

 sitic plants, (fe) as aerial roots. The aerial roots of orchids 

 function as water-condensing organs while the aerial roots 

 of Virginia creeper, which develop after the stem has become 

 quite large and mature, function as hold-fast organs, (c) as 

 root tubercles, which occur on the roots of all leguminous 

 plants, as the bean, pea and clover. 



