ROOTS. 



93 



through the holes, so that the seeds rest on the paper, and 

 the root tips hang through the holes. If the paper is kept 

 moist germinatiun will continue, but geotropism will direct 

 the root tips downwards, and hydrotropism (response to 

 the moist paper) will direct them upwards. In this way 

 they will pursue a devious course, now directed by one 

 response and now by the other. 



If a root system he examined it will be found that when 

 there is a main axis {tap 

 root) it is directed 

 steadily downwards, 

 while the branches are 

 directed differently. 

 This indicates that all 

 parts of a root system 

 are not alike in their 

 response to these influ- 

 ences. Several other 

 influences are also con- 

 cerned in directing soil 

 roots, and the path of 

 any root branch is a 

 result of all of them. 

 How variable they are 

 may be seen by the 

 numerous directions in 

 which the branches 



travel, and the whole root system preserves the record of 

 these numerous paths. 



(2) The pull on the stem. — Another root property may 

 be noted in connection with the soil root, namely the pull 

 on the stem. When a strawberry runner strikes root at 

 tip (see Fig. 47), the roots, after they obtain anchorage in 

 the soil, pull the tip a little beneath the surface, as if they 

 had gripped the soil and then slightly contracted. The 

 same thing may be observed in the process known as 



Fig. 88. A section through the stem of a water- 

 wort (Elatine), showing the remarkably large 

 and regularly arranged air passages for root 

 aeration. The single reduced vascular bundle 

 is central and connected with the small cor- 

 tex by thin plates of cells which radiate like 

 the spokes of a wheel. — After Scuexck. 



