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 germination will continue, but geotropism will pull 
the root tips downwards, and hydrotropism (the moist 
paper) will pull them upwards. In this way they will 
pursue a devious course, now directed by one influence 
and now by the other. 
If a root system be 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 SCHENCK. 
