226 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



roots of the Banyan tree. Huge root-caps of a series of scarious 

 layers will be seen on the ends of the thick adventitious roots of 

 Pandanus. , 



(e) Growth of Tcq^-root intluenced by Gravity. — We have 

 already seen that the tap-root grows vertically down into the 

 ground and that this is of great advantage to the plant. In 

 order to tind whether the tap-root always grows downward, we 



shall make an experiment: we 

 take a few soaked seeds in which 

 the roots have already appeared, 

 and put them in various positions 

 on moist soil covering them with a 

 tumbler. After twentj'-four hours, 

 we find the root-tips changed in 

 their direction and turned down- 

 w^ards. If we now disturb them 

 so that some point upwards, others 

 sideways, and so on, and examine 

 them after a time, they w^ill be found to have turned downward 

 again. This shows that each main root persists in growing down- 

 wards. If it meets with obstacles in its way, it resumes its 

 original course immediately after passing them. 



By experiment it has been found that the dow'nward direction 

 of the main root is determined more by gravity than by any other 

 influence, as light and moisture. We therefore say, the main root 

 is geotropic. 



In the same measure as the geotropism of the main root is 

 an advantage to the plant, it is good that side-roots grow in a 

 slanting or horizontal direction and are influenced by other forces 

 (sensitiveness to moisture). 



Fig. 210. — Geotropism of root. 



2. Roots as a means of Absorption. 



The main function of the roots is the absorption of water and 

 ]i(|uid food. In order to understand how roots absorb water from 

 the soil, wo must first know what the soil is and how w\atcr exists 

 in the soil. 



