ROOTS 



93 



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

 the root tips hang througli tlie holes. If tlie pajier is kept 

 moist germination will eontinne, but geotropisni will pull 

 the root tips downwards, and liydrotropism (tlie moist 

 paper) will pull tliem upwards. In this way tliev will 

 pursue a devidus eourse, now directed Ijy one influence 

 and now by the other. 



If a root .system be examined it will lie found that when 

 there is a main a.xis [taj) 

 runt ) it is d i r e c t e d 

 steadily downwards, 

 while the branches are 

 directed d i If e r e n 1 1 y . 

 This indicates that all 

 jiarts of a root system 

 are not alike in their 

 resi)(inse to these influ- 

 ences. Several other 

 influences are also con- 

 cerned in directing soil 

 roots, and the path of 

 any roiit brancli is a 

 result of all of them. 

 How variable they are 

 may be seen liy the 

 numerous directions in 

 which the liranches 

 travel, and the whole root system preserves the record of 

 these numerous paths. 



(2) TJie ^.)»// on the stem. — Another root property may 

 be noted in connection with the soil root, namely tlie 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 thev 

 had gripped the soil and then slightly contracted. The 

 same thing may be observed in the process known as 



Fn;. gs. A section through the etem of a water- 

 w ort {Elatiiit), showing the renmrkabiy large 

 ;ind regnlariy arranged air passages for root 

 aeration. The single reduced vascnlar bundle 

 is central and connected «ith the small cor- 

 tex by thin plates of cells whicli radiate like 

 tlie spokes of a wheel. — After Schenck. 



